Interval Training

Interval training is a method of training that employs short to moderate duration intense efforts followed by short periods of active or complete rest. Interval training programs manipulate the intensity and duration of work intervals, and the length of the rest periods, to create the desired training responses (Walker, 2023). Interval training variables can be manipulated to achieve different training goals. The length of the work and rest intervals are determined by the training goal.

Interval training can be used to achieve both aerobic and anaerobic conditioning goals. Aerobic approaches typically utilise working repetitions that are longer in duration, at moderate/fast movement speeds and with rest periods shorter than the working interval. Anaerobic approaches use working repetitions of shorter duration, performed at higher speeds and with rest periods longer than the work intervals.

Much research has been devoted to the acute and chronic benefits of interval training. It appears that interval training can result in training improvements similar to those obtained through longer continuous forms of training (like LSD training). These include adaptations linked to improved sports performance like enhanced cardiorespiratory and cardiovascular fitness, increased blood volume, higher lactate thresholds, and enhanced lactate buffering in muscle (NSCA, 2017). The NSCA (2017) suggests that if similar adaptations in aerobic endurance performance can be achieved using interval training for 20 minutes versus LSD training for 45 to 60 minutes, then interval training is clearly a more efficient means of achieving those benefits.

While endurance athletes need to train their bodies to be able to produce continuous and efficient force over extended periods of time, the suggested efficiency benefits gained through interval training make it a very attractive training method for athletes who have a short window of time to work on cardio fitness, and for those that need to target multiple training goals at once (i.e. sports athletes).

While LSD and other continuous forms of training are more task-specific for endurance athletes (because CV and metabolic overload more closely mimic race intensity and duration), interval training is more closely aligned with shorter distance and intermittent sport athletes.

History of interval training

Interval training was first mentioned (in written form) in the 1500s but was never a popularised form of training until the 1900s when athletes started looking for training methods that would give them an advantage over their peers (What is Interval Training, n.d.). It wasn’t until the 1950s that interval training started to become widely used among athletes. The training method was popularised in the 1950s by the Olympic Marathon running champion, Emil Zatopek and since then middle and long-distance runners have used this technique to train at velocities at or above their competition race pace (Billat, 2001). See the following image of Emil Zatopek, Olympics 1952.

It was another 40 years until “HIIT fever” reached the wider training population in the 1990s. Publication of scientific studies on the effects of interval training started to emerge through the 50s and 60s and debate has raged ever since about the efficacy of this form of training compared to longer, more traditional forms of aerobic training.

Multiple studies have suggested that interval training approaches result in similar improvements in VO2 Max, muscle fibre mitochondrial density, and improved lactate threshold as bouts of lower intensity longer duration activity, but there is little consensus around which form of training is most appropriate for different types of athletes or the general population (What is Interval Training, n.d.). Research also suggests that interval training can improve both aerobic and anaerobic capacity simultaneously, versus LDS training which generally targets only aerobic capacity (What is Interval Training, n.d.).

Benefits of interval training

The following list of benefits comes from Shortsleeve (2020), What is Interval Training (n.d.) and Kilpatrick et al (2014):

Physiological benefits

Increases VO2 Max	Research shows this in both trained and untrained subjects. Results are comparable with LSD training studies. Interval programmes using longer work intervals appear to produce the greatest improvements in VO2 Max (even more than continuous moderate-intensity exercise of equal volume and time commitment).
Increases mitochondrial density in muscles	Studies have shown superior improvements in markers of muscle mitochondrial density compared with steady-state aerobic exercise, with meaningful improvements showing after just two weeks of training. Increased volume of mitochondria allows for more efficient conversion of fuel into ATP during aerobic exercise.
Improves metabolic health markers	increased depletion of muscle glycogen during interval training may facilitate enhanced glucose uptake by muscle and improve insulin sensitivity. This has also been shown to be the case when used with overweight subjects and those with or at risk of type 2 diabetes. The positive effects of interval training on glucose control can be seen in as little as two weeks of low-volume training. These effects have been shown in even low-intensity walking intervals, where walking intervals were more effective than continuous walking at improving glucose control and reducing abdominal visceral fat mass (despite no differences in total energy expenditure).
Great for your heart	Interval training improves endothelial heart function more effectively than all forms of continuous aerobic exercise. Endothelial cells are found within blood vessels and the walls of the heart. They release nitric oxide and have potent vasodilation (opening) effects that protect cardiovascular function and health. The proper function of these cells ensures efficient blood flow distribution and regulation of blood pressure. Research has clearly demonstrated greater improvements in endothelial cell function after interval training when compared with continuous exercise (consistent across a range of cardiovascular disease states also). It appears interval training elicits more elasticity in arteries and veins than continuous aerobic exercise modes by putting more pressure on these blood vessels during higher-intensity efforts.
May increase fat use during exercise	Higher intensity intervals are thought to increase levels of Free Fatty Acids (FFAs) in the bloodstream due to greater circulating levels of epinephrine. This is thought to drive greater aerobic metabolism in cells. However, types of interval training that elicit high lactate levels may impair this FFA response.

Other benefits

Higher calorie burn	Both during and in the hours after exercise. Interval training results in longer training periods at higher intensities which ensures more calories are burned during training (than continuous training of the same duration). Meaningful post-exercise calorie burn continues for up to two hours after an interval session (vs around 40 minutes in continuous training) making interval training useful in achieving body composition goals.
Variety and specificity	Interval training offers more variety than continuous forms of training which is thought to enhance motivation, thus increasing the likelihood of continuing the programme long term. Interval training is also a more usable method in sporting environments as intervals can include the movement patterns associated with sports (i.e. they don’t have to be in a single direction). This means intervals can be tailored to suit the athlete and their sport (e.g. interval shuttles vs straight-line running).
More enjoyable	Recent data suggests that when exercise participants were asked to compare interval training with continuous moderate-intensity cardio and continuous vigorous-intensity cardio, they rated interval training as the most enjoyable. It would seem that the interval duration that is deemed most enjoyable appears to be those under 60 seconds duration performed at high intensity. Enjoyment of exercise is a key factor in exercise adherence.
Time efficiency	Interval training has been found to elicit similar cardiovascular responses to training as continuous methods in nearly half the time. But before you decide to never do LSD training again, the majority of evidence surrounding interval training has come from studies involving well-training athletes suggesting a good aerobic base is necessary to get the best out of this form of training (improved recovery times for more repeated efforts). Also, not all forms of interval training are more time-efficient. The wide range of interval training approaches means that some work:rest ratio methodologies will produce sessions of a similar total time frame to continuous exercise sessions. This is particularly true in sessions performed by elite athletes who complete higher volumes of work during sessions.
Can improve both aerobic and anaerobic capacity	If a variety of interval durations and intensities are employed.
You can train groups with different levels of fitness at the same time	this is easier to manage the LSD training as work:rest ratios and pacing can be determined for each level of fitness before starting. If recovery between intervals is tracked using HR monitoring systems, work:rest intervals can be individualised to better suit fitness levels.
Great for sports specific conditioning	interval training can be adapted to match the work:rest ratios of different intermittent sports thus making it a great form of sports-specific conditioning
Allows for speed improvement	When compared with LSD training. This is important for endurance athletes and sports athletes alike.
May be suitable for a range of population groups (if well supervised)	Aerobic high-intensity intervals may be superior to moderate-intensity intervals for maximising health outcomes in a range of general population groups.
Great for maintaining aerobic condition in-season without having to do as many kms	Thus it fits nicely into the competition phase principle of maintaining intensity while reducing volume (for maintenance). The reduction in training volume can also benefit those who are at risk of overuse training.

As you can see, interval training elicits a raft of benefits, but most literature agrees that rather than replace one mode of cardiovascular training with another, there is a cumulative benefit to be had from using multiple CV training modes either concurrently or in a periodised sequence (depending on the athlete and their sport).

Safety considerations for interval training

Although interval training offers potent health benefits and is generally well tolerated by a wide range of populations, participation in this form of exercise requires trainers to consider cardiovascular risk stratification before prescribing this form of exercise to clients. Depending on the intensities used, this form of training can fit under the “vigorous exercise” umbrella which means that certain interval training approaches are appropriate for low-risk clients or moderate-risk clients who have been cleared for participation in vigorous exercise by a medical professional.

However, the wide range of approaches that can be applied when prescribing interval training, means that variables can generally be adapted to fit most client circumstances. As mentioned earlier, positive effects of interval training have been found using simple walking intervals. Those who need to perform submaximal exercise due to conditions can continue to be closely monitored during interval approaches.

Another safety consideration for some populations is the use of cardio equipment for interval training sessions. Most of the research data in the interval training space has been conducted on cycle ergometers as these pose less of a risk than some other forms of indoor training. Completing interval sessions on treadmills is common practice, but comes with increased fall risk. This is because treadmills are often left running at speed during rest intervals requiring clients to step onto fast-moving belts to start each subsequent interval. This requires a certain level of coordination, which can be too difficult for some populations. It is also worth noting that the faster a client runs, the more this risk increases with more than a few clients coming off the back of treadmills during this approach. One remedy to reduce this risk is to adjust the treadmill incline grade rather than speed for lower-intensity workouts.

Let’s take a look at each of the key interval training approaches in a little more detail. First up, aerobic (longer) interval training.

Aerobic interval training targets the improvement of the aerobic energy system. By definition, intervals that target the aerobic energy system would typically have to go beyond 120 secs. We know that the upper reaches of the anaerobic energy system are around the 2-minute mark, so any interval completed for longer than this time would target primarily the aerobic system. This isn’t to say that shorter intervals (e.g. 1-2 mins) wouldn’t translate to some aerobic improvement. There is also evidence that when performing repeated maximal sprints in a High Intensity Interval Training (HIIT) approach, the aerobic energy system plays a larger role in all subsequent sprints after the first.

For the purposes of this segment, aerobic interval training will refer to interval training work segments of above one-minute duration. This is because most experts agree that working intervals between 1-8 minutes in duration are the most used in training programmes targeting aerobic capacity improvement (NSCA,2017).

The purpose of aerobic interval training is to improve speed and speed endurance. That is, to be able to hold a faster speed for longer. For an endurance athlete, the idea is to complete intervals at a speed that is faster than their typical race pace. This higher intensity of work can’t be maintained as long, but with rest intervals employed (and repeat sets) the volume of running at this higher intensity can be progressively increased raising both VO2 Max and lifting the lactate threshold.

So, what does the research say about aerobic interval training?

While interval training appears to be made for sporting athletes, several studies have shown that an increased volume of aerobic high-intensity endurance training is effective for improving aerobic characteristics and endurance performance in highly trained endurance athletes also (Sandbakk et al, 2012). It appears that longer aerobic intervals have more relevance to endurance athletes than shorter duration higher intensity intervals.

Sandbakk et al (2012) compared the effects of longer aerobic intervals (5-10 minutes) and shorter intervals (2-4 minutes) and found that while both interventions lead to improvements, longer duration intervals were most effective for improving endurance performance and oxygen uptake at the ventilatory threshold. In the study, participants were asked to perform the intervals at the highest intensity they could maintain for the duration of the interval. The longer interval group total session time included 40-45 minutes of work, compared to 20-25 minutes work for the higher intensity shorter interval group. A control group that performed sessions of 60-100 minutes of steady-state cardio at 60-74% of heart rate maximum was also included in the study as a comparative control. The authors concluded that it was advantageous for endurance performance to use slightly reduce intensity and use longer duration of intervals for maximal aerobic enhancement.

It appears there may also be a “sweet-spot” for aerobic interval duration. Seiler et al (2013) compared the effects of three different interval training approaches on recreational cyclists (4 x 16-minute intervals, 4 x 8-minute intervals and 4 x 4-minute intervals). The subjects were instructed to complete intervals at the maximum sustainable intensity. While all interventions improved markers of aerobic fitness over 7 weeks, the 4 x 8-minute interval group exhibited the largest improvements. This group maintained completed 32 minutes of work during sessions at around 85-90% of heart rate maximum compared to 64 minutes of work at 75% of heart rate maximum in the 4 x 16-minute interval group and 16 minutes at 94% of heart rate maximum in the 4 x 4-minute interval group.

It was surmised by the authors that the reason the 4 x 8-minute interval approach resulted in better conditioning outcomes was because these intervals generated lower lactate levels than the 4 x 4-minute approach as well as a lower perceived rate of exertion resulting in the most “gain for the pain”. The 8-minute intervals also allowed a higher work rate than the 16-minute interval group, which placed more stress on the CV system, allowing for greater adaptation in half the time. Improvements resulting from the 8-minute intervention were a right shift in the lactate threshold, increased VO2 Max and a large improvement in time to exhaustion at 80% of peak aerobic power. The 4 x 8 minute interval group also reported less symptoms of fatigue in the days after training (Seiler et al, 2013) than both the other intervention groups.

Billat (2001) explored a range of work intensities and durations for interval training and reported the following:

• One- or two-minute intervals run at a maximum sustained intensity induces high blood lactate levels. A one-to-one work:rest ratio does not completely clear these levels, and low-level active recovery (less than 40% of VO2Max) might be more effective at doing this than total rest.
• The benefit of aerobic interval training is that it allows athletes to work at a higher percentage of VO2 Max across the session than longer continuous forms of training with comparisons of up to 2.5 times the work time spent at running velocities attributed to VO2 Max with intervals vs continuous running sessions.
• Intervals of between 5 and 8 minutes duration, run at an intensity between the onset of blood lactate accumulation (OBLA) and the velocity that achieves VO2 max (somewhere around 90-95% of VO2 Max running velocity) are thought to elicit optimal cardiovascular fitness improvements because an athlete is able to double their time spent training at VO2 max velocities throughout the session. These interval durations appear to be best used with a 2:1 work:rest ratio (I.e. 5 minutes work:2.5 minutes recovery) and low-intensity active recovery is preferred over static recovery.
• Longer intervals (up to 8 minutes duration) have been able to show rapid increases in VO2 Max in just 4 weeks (training twice a week).
• Shorter intervals (up to 2 minutes duration) also show impressive improvements in VO2 Max (+7%) and blood lactate threshold running velocity (25%) when used with a 1:1 work:rest ratio over 7 weeks (4 trainings a week).

Consider the following example. The effectiveness of interval training for targeting speed and speed endurance can be seen in this simple table showing 2 different aerobic approaches:

Workout A: Steady state	Total Distance covered during workout	Average speed maintained throughout workout
A 4k steady aerobic effot running around the park	4000m	12kph
Work out B: Intervals	Total Distance covered during workout	Average speed maintained throughout workout
An interval training session of 10 x 400m w/90sec rest	4000m	16kph

The obvious difference between the workouts is the difference in speed maintained for the same distance covered. Due to the shorter interval distance and the rest periods, the pace maintained in the intervals is 25% faster than in the steady-state workout. This leads to a greater stress response and improved fitness outcomes.

Applying the FITT principles to aerobic interval training

Type

Interval training can be used with any form of cardiovascular training. All you need is a measure of intensity (from % of heart rate max, to perceived rate of exertion, or velocity of VO2 Max) and correct work:rest allocation and you can perform any mode of activity you like!

Frequency

The ACSM suggests that interval training sessions are more exhaustive than continuous training methods so should be introduced gradually into training programmes. Their suggestion is to start with one interval training session a week, then progressing to two interval workouts a week with at least 48 hours separating them. Elite runners may complete up to 3 interval sessions a week, but most sources suggest 2 sessions a week is ideal when performing other forms of training concurrently.

Time (Duration)

Most literature indicates aerobic training intervals should be between 1 and 8 minutes in duration with intensity adjusted to suit. Aerobic intervals longer than 8 minutes could be considered split-tempo runs.

Intensity

The intensity that an aerobic interval can be performed at depends of conditioning level of the athlete and the duration of the interval. Much of the research on interval training has been performed on well-trained subjects who we know are able to sustain higher work intensities for prolonged periods. In fact, much of the information discussed above promotes intervals of 1-8 minutes completed at around 90% of VO2 Max training pace (vVO2 MAx). The velocity of VO2 Max running pace (vVO2Max) and maximum heart rate are not the same thing. Essentially, 100% of velocity of VO2 Max (vVO2 Max) is the minimum pace you can move at that will result in reaching VO2 Max in the next couple of minutes, i.e. the point at which you can maximised oxygen uptake in muscle via aerobic means before becoming reliant on the anaerobic energy system.

The issue with percentage of heart rate maximum to programme for interval training

Unlike other cardio training programmes, there is an issue with using a percentage of heart rate maximum to set training intensities for interval training. That is, the heart rate during intervals is not constant. For example, during an 8-minute interval, the heart rate climbs gradually across the majority of the interval (rather than sitting in one steady state). This is because you are working above the lactate threshold which places increasing stress on the system across the interval. In fact, the highest heart rate you will achieve is often in the first 20-30 seconds of the recovery period. For this reason, a movement pace is considered the best way to programme for intensity for interval training. This is where the percentage of vVO2 Max comes in.

The image below shows the relationship between running pace and vVO2 Max. Things to remember when looking at this image:

• VO2 is simply your body’s ability to consume oxygen.
• VO2 Max is the body’s maximal ability to consume oxygen
• vVO2 Max is the minimum pace that (if maintained) will achieve VO2 Max within the next few minutes, i.e. the pace that elicits the highest point of oxygen uptake, with the lowest total anaerobic system contribution (usually only 10-20% anaerobic contribution in most trained athletes). The reason vVO2 is used to describe working pace is because it takes into account both aerobic capacity (VO2 Max) and movement economy.

In the following example, you can see how oxygen consumption rises linearly and then plateaus. For this runner we can see that oxygen uptake is very close to maximum uptake at 20km/h and then plateaus when running speed increases to 21.5km/h.

Training at 90% of this pace, means you sit just below that threshold. 90% of vVO2 pace would be somewhere in the vicinity of OBLA (onset of blood lactate accumulation). Less than 75-90% and you are working somewhere around the lactate threshold pace (depending on fitness level). So, in short, longer aerobic intervals are completed at a pace that is higher than what you could maintain for longer tempo or LSD runs, but slower than the pace that would push you into a reliance on the anaerobic energy system. This pace ensures that the maximum aerobic stress is placed on the system. Obviously, this pace can’t be maintained for extended periods, thus rest periods are used to ensure volume of training is sufficient for adaptation.

The percentage of vVO2 Max pace you can maintain is also reliant on the duration of the working interval. Intervals of longer than 5-8 minutes will require a slight reduction in the percentage of vVO2 Max pace used (usually 90-95%). Shorter intervals of up to 2 minutes often use vVO2 Max pace (or even supra-maximal vVO2 Max pace in the case of 60 second efforts). Complete even shorter (sprint) intervals and you are now using anaerobic interval training methodologies.

The information to follow discusses suggested variables for programming aerobic intervals. This information represents generic approaches only. The exact intensities, durations and work:rest ratio will be dependent on the athlete's training age, level of competition and the training goals they have.

It is actually very difficult to find consistent information on ideal rest/recovery periods to apply to different interval durations. Research online will result in a number of different suggestions. This is because the exact amount of rest between intervals depends on a number of factors including:

• The length and intensity of the work interval
• The level of condition the athlete has (fitter athletes can sustain higher intensities and recover faster)
• The goal of the training approach (i.e. does the sport the athlete play allow for full recovery in games?)
• The nature of rest in the athlete's sport, e.g. is rest static or active?

For these reasons, it is very difficult to arrive at exact rest period recommendations for interval programmes. Most sources suggest that monitoring heart rate during rest periods might be the best way to measure ideal work:rest ratios for most athletes. Ideally, recovery periods should allow heart rates to drop between 100-120bpm before the next interval begins. This means that recovery period lengths may need to be extended as the interval repetitions continue.

Recovery

Most research supports the use of active recovery between working intervals. However, both passive (static) and active (low-intensity movement) may have their place. The general approach for rest in interval training is that passive (static) rest periods are best used during HIIT training (short duration, supra-maximal intervals) as they seem to allow for greater work output during repeat maximal efforts. Active recoveries appear best to use between aerobic intervals (over 60 seconds) as they appear to assist with the clearance of lactate and its conversion into energy (Recovery Intervals: A Key Part of VO2 Max Training, n.d.).

If active using active recovery, movement should be no more than 40% of max effort (pace) with some sources suggesting as low as 10% effort. If using a return of heart rate to 100-120bpm as a guide to the duration of rest period, then the active rest movement effort should allow for a steady reduction in heart rate to these levels.

Common approaches to aerobic interval training

The following information is compiled from a range of sources and should be used as a general guide (starting point) only with further consideration given to an individual athlete's sport or training goals.

Longer intervals (5-8 minutes)

Likely to be used primarily by endurance athletes (during specific preparation and competition phases) and sporting athletes (in their preparation phases). These intervals need to be completed at approximately 90% of an athlete’s vVO2 Max pace. Endurance athletes will likely complete these longer intervals as part of a longer continuous effort, where rest periods are completed as active rest (around 40% effort of movement). Sports athletes will likely take static or walking rest between sets due to the more intermittent nature of their sports.

The work:rest ratio should be 2:1 (meaning an athlete doing an 8-minute interval would have 4 minutes of rest before the next). Longer rest periods can be used when starting out with progressive overload applied to reduce the rest interval to the desired ratio. The guide to the appropriate length of rest interval should be the return of their heart rate to the 100-120bpm zone.

With these longer intervals, the aerobic stress on the body will build up over the first few minutes of the interval, with the maximum benefit obtained in the last part of the interval as athletes' approach VO2 max levels of work. Generally, 4-5 repetitions will be performed, with endurance athletes potentially extending the repetitions as their training progresses. Sporting athletes will likely start to move towards shorter, more intense intervals.

Mid-length intervals (2-5 mins)

These can be completed at a slightly higher intensity (faster pace). Again, this will be the maximum pace that an athlete can maintain for the duration of the interval. This is likely to be around 95-100% of vVO2 Max pace. In endurance athletes, this approach will start to resemble Fartlek training with shorter faster efforts in amongst lower intensity efforts of the same duration. In sporting athletes, this approach will help them transition into shorter interval training. For field sport athletes like football and hockey, these mid-length intervals represent the longer phases of play that will occur in games before a whistle or break occurs.

The work:rest ratio should be 1:1 to begin with and as progressive overload is applied over the training period the usual approach would be to reduce the rest interval to a 2:1 ratio. Generally, 4-6 repetitions will be performed depending on the length of intervals and the total time available for the session.

Shorter intervals (1-2 minutes)

Many experts believe that intervals of this duration primarily focus on the lactic acid (glycolytic) energy system, but they can still significantly impact the aerobic system. The reason for this is that the aerobic benefit of such intervals is accumulated across the series of repetitions, rather than during individual repetitions. This is because intervals completed over 1-2 minutes elicit a larger contribution from the anaerobic energy systems than the intervals discussed above. That said, the aerobic system is always involved in repeated effort training. A study by Spencer and Gastin that looked at energy system contribution during 200m-1500m running in highly trained athletes showed that the aerobic energy system contribution during each of these running distances was:

• 29% for 200m
• 43% for 400m
• 66% for 800m
• 84% for 1500m

Given that most people will run over 400m in a 1–2-minute period, the likelihood is that the aerobic system will provide at least 50% of energy creation during these sets (with this contribution increasing at repeated repetitions are completed).

These shorter duration intervals are also performed at the maximal pace that can be maintained for the duration of the interval. This is likely to be at vVO2 maximum pace for intervals of 90 seconds to 2 minutes, and slightly above vVO2Max (105%) pace for 60-second intervals. This is the first of the interval approaches that utilises work:rest ratios that will apply a rest period longer than the work interval with a 1:2 work:rest ratio usually applied (when starting out with these shorter intervals a work:rest ratio of 1:3 or even 1:4 can be used until athlete tolerance is improved). Again, monitoring heart rate to ensure it drops into the 100-120bpm range can be used to determine rest requirements.

For sports where full recovery is not always possible, trainers may implement a shorter rest interval to mimic the demands of the sport. In this instance it would be usual practice to perform these intervals in sets with an extended break allowing full recovery after each block of 3-4 repetitions. This approach would be suitable for sports where continuous play periods are often between 60-90 seconds duration (e.g. rugby, where multiple phases are strung together before the ball is put out of play, or a mistake is made resulting in a penalty or scrum).

The table below summarises the details discussed above:

Interval length	Intensity/pace	Work:Rest Ratio	Reps/sets
6-8 minutes	90% of vVO2 Max.	Start with a 1:1 ratio, then move towards a 1:0.5 (i.e. rest is half the duration of work efforts)	4-5 reps
3-5 minutes	95-100% of vVO2 Max	Most literature supports a 1:1 ratio, however, if working with athletes that have less than full recovery between efforts in games a 1:05 can be used	5-6 reps
1-2 minutes (allowing for full recovery)	At or slightly above vVO2 Max (100-105% of vVO2 Max)	Most common 1:2 ratio (rest twice as long as the work effort). May have to start with a higher work:rest ratio of 1:3 or 1:4	10-20 reps
1-2 minutes (not allowing for full recovery)	At or slightly above vVO2 Max (100-105% of vVO2 Max)	1:1 work:rest ratio or less	3-4 sets of 3 reps. Each set followed by a return of HR to between 100-120bpm

Shorter aerobic intervals run at vVO2max are also used by many trainers in a team training environment. Gamble (2013) suggests the most common work:rest formats featured in the literature involve shorter intervals are either 30 seconds of work with 15 seconds of rest or 15 seconds of work with 15 seconds of rest.

Applying progressive overload to aerobic interval training

When starting out with aerobic interval training it is best to work at a slower than suggested percentage of vVO2 pace to reduce your chances of injury. You can then slowly apply the progressive overload principle to add either distance or duration to intervals, intensity/velocity to the work efforts, or reduce the rest intervals to better match the requirements of your athlete’s sport.

For those athletes competing over shorter distances or looking to use intervals to improve aerobic condition for a sport, the typical approach would be to start with longer intervals (at slightly lower intensities), then decrease the interval length and raise the intensity and manipulate the rest periods gradually as the weeks go on (with the ultimate aim being to match the work:rest ratios involved in the sport). For a sports athlete, this approach would likely start towards the end of the general preparation phase (longer intervals) and continue to be used through the specific preparation and competition phases (shorter more intense intervals). For endurance athletes, aerobic intervals would likely be used in the specific preparation phase of training (build phase) and throughout the competitive season.

Working out vVO2 training pace

So, we have been discussing vVO2 Max for a while now. But how do you estimate or calculate this? The benefit of using vVO2 Max as a measure of intensity over a percentage of heart rate maximum is that it allows you to programme working intensities based on pace. vVO2 Max is described by multiple sources (e.g. Billat et all, 1999; Hill et al, 1997) as the maximum speed an athlete can sustain for between 4-8 minutes. This has led to a number of field-based tests that can help estimate your vVO2 Max.

The most common of these is the ½ Cooper Test or 6-minute time trial. The objective is simple. Find a flat surface where you can run unimpeded for 6-minutes. Complete a meaningful warm-up, then run as fast as you can for 6 minutes and record the distance covered. Next, you need to calculate your average running velocity during the 6 minutes to estimate your vVO2 Max. Here is an example of this estimation process:

John runs 1800m in 6 minutes. There are 360 seconds in 6 minutes, so by dividing John’s distance covered (1800m) by the time he took to run it (360 seconds), we can work out his average running velocity over the test.

1800m / 360 sec = 5m/sec

This means John ran at an average pace of 5m/sec during the 6 minutes. As long as this was a true representation of his best effort, this pace is his vVO2 Max. Most fitness trackers, apps and cardio machines use km/hr to show movement speeds. You can convert this m/sec value to km/hr by multiplying the answer by 3.6. For example:

5m/sec x 3.6 = 18km/hr

This means that John’s vVO2 Max running pace is 18km/hr. From there we can calculate percentages of vVO2 Max to apply to the different interval training approaches discussed above.

For example, if John wished to perform some 5-minute intervals at 95% of his vVO2 Max he would apply the following equation:

18km/hr x 0.95 (95%) = 17.1 km/hr

John should therefore run his 5-minute intervals at approximately 17km per hour (note: John is a pretty good runner!). He can track this using his fitness watch or can work out the distance he should cover in each 5-minute interval using the following calculation. There are 12 x five-minute segments in 1 hour. Therefore, if John runs at a pace that will cover 17km in one hour (16km/hr), if he divides this distance by 12, he will find out how far he should run in each 5-minute interval. 17km / 12 = 1.42km or 1420m. This means John should try and run 1.42km in each of his 5-minute intervals to be on 95% vVO2 pace. The reason you might calculate distances for each interval is so you can track running volume. By multiplying the distance of each interval by the number of repetitions completed, you have the total volume of high-intensity running completed in the session. This can be used to programme additional volume as you apply the progressive overload principle.

Important Note: This 6-minute baseline test should be repeated every 3-4 weeks to re-establish a new vVO2Max pace.

The key difference between anaerobic and aerobic interval training is that working intervals are performed for shorter durations with an “all out” (maximal) effort approach. Anaerobic sessions are usually shorter in nature due to the higher intensity of effort. HIIT training is usually used by general fitness athletes and sporting athletes, and less used by endurance athletes because it has little relevance to the demands of their discipline.

HIIT is a popular training method. HIIT has claimed a sport in the top 10 fitness trends since 2014 as surveyed by the American College of Sports Medicine (ACSM). HIIT training was first introduced in the 1950s as “sprint training”. It was mainly used by shorter and middle-distance Olympic level track athletes who ran at maximum effort for short distances and and broke this up with extended rest intervals (Harvard School of Health, n.d.).

By definition, anaerobic interval training involves working interval durations of less than 1 minute, performed at near maximal or maximal intensity. While efforts of between 1 and 2 minutes would technically be classed as anaerobic, the repeated effort approach of interval training sees an increasing reliance on the aerobic system as repetitions are completed.

Another key difference between anaerobic and aerobic interval training is the application of comparatively longer rest intervals. This is because the purpose of anaerobic intervals is to maximise the production of repeated high intensity efforts. The longer rest periods allow for more complete recovery, clearance of lactic acid and a return to equilibrium, permitting another very high intensity work interval.

There are two key approaches to anaerobic interval training. Intervals of between 30- and 60-seconds targets anaerobic endurance, while intervals of between 10-30 seconds target speed, anaerobic power and capacity. Intensities used are typically maximal or supra-maximal so require significant recovery between efforts. Shorter rest periods preclude recovery and necessitate a shift in energy production to the aerobic system, therefore reducing intensity and power output.

The requirements of the sport an athlete plays may require a change in approach. Pure anaerobic power is best developed using the variables discussed above, however, sport is intermittent in nature. While there are some sports that are primarily anaerobic in nature (e.g. volleyball, badminton etc), most sports don’t give athletes that opportunity to fully recover. Field sports are largely aerobic, but punctuated by short bursts of anaerobic maximal, or near maximal, efforts. For these athletes, a period of pure anaerobic training would be followed by period of training using high intensity efforts with incomplete recovery periods to mimic the requirements of the sport.

Gamble (2013) suggests that anaerobic interval training can be split into two divisions. The first involves more ATP-PC orientated supra-maximal activities lasting less than 10 seconds using longer rest to work ratios (e.g. 1:5), and the second involves efforts of up to 30-60 seconds, that utilise comparatively shorter rest to work ratios (1:1 or 1:0.5) to better simulate match conditions. The approach used by trainers should ultimately be tailored to the demand of the sport. For example, the average duration of a rally in a game of elite tennis is 4.2 shots, with 30% of all rallies only 1-shot (Kuzdub, 2020). This means most rallies are all over within 10 seconds. The official rules of tennis state the next point should be started in 20 seconds (so a 1:2 work to rest ratio). A rest of 30-60 seconds is common between games, and a 2-minute break is allocated between sets (United States Tennis Association). This means interval training could be tailored to match the demands of the sport by adapting work:rest ratios to mimic the requirements of a game. In this case it would seem prudent to plan for sets of intervals of around 10-15 seconds, with a 1:2 work to rest ratio and 2 minutes between sets.

Sports like American Football have an average work to rest ratio of 1:10 (4 seconds per play, 40 seconds between plays), whereas others like basketball have average ball in play times like 40-50 seconds with short rest intervals applied. Each sport has specific requirements that interval training can be adapted for to apply the principle of specificity.

The general approach to anaerobic interval training for sport would be to start by using pure anaerobic training principles of longer work to rest intervals and apply the principle of specificity as the competitive period draws closer. This would see most sports athletes using more structured anaerobic training in their specific preparation phase of training, the continuing to use repeated anaerobic efforts in the form of conditioning games at trainings that utilise the specific sporting movements associated with gameplay.

Suitable participants for HIIT

HIIT training is for anyone who wants to improve their ability to produce repeated high-intensity efforts over time (just like in sport). While endurance athletes may use hit training sparingly in favour of longer aerobic intervals, some endurance athletes (particularly shorter distance athletes) use HIIT training to maintain the intensity of training while reducing total training volume. This would be most commonly used in the lead-up to race events and used in conjunction with longer, less intense forms of training. While this intensity is less specific for endurance athletes, research has shown it to improve exercise performance. In fact, HIIT Workouts for Endurance Athletes (n.d.) suggests that it is important to include some training at these intensities within any endurance training schedule as they may limit their endurance potential if they don’t train at these intensities.

HIIT training definitely has more alignment with sports athletes who require repeated high-intensity efforts throughout their game performances.

Additionally, HIIT training has been used successfully by people seeking general fitness improvements, weight loss, and even to treat a variety of health and chronic conditions. Before we get into athletic HIIT training models, let’s have a look at how HIIT training has been used with those suffering from chronic conditions.

Although the higher intensities reached with HIIT training may appear too risky to use with people who have chronic conditions, research has shown that the intermittent rest intervals and overall reduced volume of exercise associated with HIIT make it a potentially feasible (and safe) option for even more serious chronic conditions like lung disease, heart disease and chronic kidney disease when completed under close supervision and with a few minor modifications (Harvard School of Public Health, n.d.). Common modifications applied include using specific modes of exercise, longer and more gradual warm-ups and cool-downs, limiting the percentage of maximum intensity and adjusting work:rest ratios (Harvard School of Public Health, n.d.).

While more research is needed before the blanket use of HIIT is recommended for these at-risk groups, the Harvard School of Public Health has compiled the following research findings related to HIIT training in chronic disease:

• Most research on HIIT and chronic disease has been conducted on those with cardiovascular disease (CVD). Studies show that HIIT can achieve a greater reduction in CVD risk factors than moderate-intensity continuous training (MICT). The rest intervals and shorter durations of higher intensity intervals of HIIT workouts may help cardiac patients complete the workout and achieve greater stimulation of the heart. The use of HIIT programmes has shown improvements in cardiovascular health in patients who have had a heart attack, coronary artery bypass surgery, or congestive heart failure. HIIT workouts can be modified depending on an individual’s fitness level and have been successfully prescribed for cardiac rehabilitation programs.
• A meta-analysis looking at the effect of HIIT and MICT on cardiovascular risk factors in adults who were overweight or obese found that both groups significantly decreased weight, body fat percentage and total cholesterol, and improved the body’s use of oxygen despite being 10 minutes shorter than the MICT sessions.
• Meta-analyses of randomised trials found that HIIT was as effective as MICT in lowering resting blood pressure and systolic blood pressure in people with borderline or established high blood pressure. Some of these studies found that HIIT showed a greater reduction in diastolic blood pressure than MICT, and HIIT more effectively improved total oxygen efficiency (VO2max) than MICT.
• HIIT has been found as equally effective as MICT in improving aerobic endurance and reducing shortness of breath in people with chronic obstructive pulmonary disease (COPD). The HIIT rest periods allow participants temporary relief from the high breathing demands required with higher-intensity exercise, which then promotes better engagement in the bouts of high-intensity movements that are needed to improve lung functioning.
• A meta-analysis of controlled trials following overweight or obese subjects for an average of 10 weeks who exercised three times a week found that MICT elicited the same body composition improvements (i.e., decreased fat mass and waist circumference) despite taking 40% less exercise time than MICT. The authors found that HIIT programs incorporating running were more likely to show fat mass losses than with cycling. Studies also suggest that HIIT may be more effective at reducing abdominal (visceral) fat than MICT. This may be due to the release of hormones like epinephrine and growth hormone that promote the breakdown of fat, in response to the high exercise intensities achieved.
• HIIT should only be performed in people with diabetes who have well-controlled blood glucose levels and should be avoided if one has diabetic retinopathy, which increases the risk of detachment of the retina. In short to medium-term interventions (up to 16 weeks), HIIT has been found to be at least equally effective as MICT (and in some cases more effective) at reducing fasting blood glucose, fat mass, and insulin resistance in people with type 2 diabetes.

As you can see, HIIT training has more applications than just athletic performance. The best thing to come out of the research above was that closely monitored HIIT was effective, safe, took less time than traditional continuous approaches and rated higher in terms of client adherence and enjoyability.

Applying the FITT principles to anaerobic interval training (HIIT)

Type

Anaerobic intervals can be performed using any form of cardio. Because this form of training is used in later preparatory phases and in competition, the mode of cardio should ideally be the predominant form of movement used in the sport/discipline the athlete competes in. Interval approaches can also be adapted to incorporate movement forms used in gameplay because most sports are not comprised of linear movements. For those with more general fitness goals, a variety of cardio modes can be employed. While sporting athletes should perform the bulk of their interval training on the surface on which they compete, it can be difficult to monitor movement speed (particularly without the presence of a coach/trainer. This is why many athletes find using a cardio machine to perform interval training on is the easiest way to ensure the correct pace is maintained in intervals.

Frequency

Anaerobic interval training is high-intensity and therefore places significant stress on the systems. For this reason, particularly if you are using higher-impact modes of training like running, you should limit the frequency of HIIT to a couple of times a week to allow for sufficient recovery and the completion of other forms of training (Sayer, 2022). The number of HIIT workouts you perform a week can be increased if the mode of exercise you use has lower levels of impact (e.g. cycling, swimming and rowing) as the body recovers faster from these types of exercise. For example, a runner may perform one of their interval sessions on a Watt bike to gain additional cardiovascular benefits without adding additional stress in the form of impact to bones, joints and muscles.

There is clear evidence that overdoing HIIT can lead to negative responses. Flockhart et al (2021) examined the effects of increasing frequency of high-intensity cardio exercise modes and found that this can lead to compromised mitochondrial function and blood sugar regulation and may also increase circulating levels of the stress hormone cortisol, leading to impaired recovery. In this study, the authors increased the frequency of HIIT training from 2 sessions in week one to three sessions in week 2 and five sessions in week three. Flockhart et al (2021) found that performance improvement increased over the first two weeks, then stagnated in week 3 (despite an increase in training load). They also found that mitochondrial function was reduced by up to 40% during this “excessive” HIIT week. This reduction in mitochondrial function also appeared to reduce the glucose tolerance in the subjects (as mitochondria is where glucose is converted to ATP in the muscle cell).

The following image shows the effects of increasing HIIT loading across the 4-week study. The fourth week of the study was a de-load week. While the super-compensation effect of this de-load week still resulted in a net increase in performance, the authors hypothesised that this effect would have been more pronounced had the “over-training” effect not occurred in week 3.

This negative effect was only apparent when HITT frequency was increased from 3 to 5 sessions in a week, supporting the limitation of HIIT training to around 3 sessions a week.

Time (Duration), Intensity and Rest

Intensity in this context is simple. Anaerobic intervals should be completed at maximal intensity (i.e. the fastest pace you can maintain for the duration of the interval). Given that the ATP-PC system lasts for about 15 secs of all-out effort, anaerobic endurance intervals targeting the glycolytic system (30-60 seconds duration) would have to be completed at just under all-out pace in order to maintain the pace for the duration of the set. Traditional approaches to anaerobic endurance intervals use a 1:3-1:5 work:rest ratio, with work intervals kept under 45-60 seconds to ensure the glycolytic system doesn’t “time out” (Interval Training Overview, n.d.).

Shorter anaerobic power intervals utilise a mix of the glycolytic (10-30 second efforts) and ATP-PC energy (6-15 second efforts) systems and will be completed at maximal and supra-maximal paces. These require a 1:10-1:20 work:recovery ratio (HIIT Workouts for Endurance Athletes, n.d.).

HIIT sessions should be kept relatively short. The literature is inconclusive regarding exact timelines with sources suggesting somewhere between 30-60 minutes per session. This is due to the wide range of rest:work ratios that can be applied for different training outcomes. Most sources indicate a session of at least 20 -30 minutes appears necessary. The danger of long HIIT sessions is that intensity is unlikely to be maintained throughout the session at the required level to derive maximum benefit. Less than 20 minutes and the working volume achieved may be too low to elicit optimal effects (given that the majority of a HITT session is rest intervals).

Applying progressive overload to anaerobic interval training (HIIT)

As with aerobic intervals, progressive overload can be applied to anaerobic interval training in a number of ways. Given that anaerobic intervals are performed at maximal intensity and that increasing session frequency can lead to negative training effects, progressive overload is primarily achieved by increasing the number of repetitions of high-intensity intervals performed, the movement types used and by reducing the duration of rest intervals to better mimic the demands of gameplay. Athletes would typically begin an anaerobic interval training approach with a limited number of repetitions and sets, performed with more linear movement approaches used, then gradually increase repetitions over time. Once sufficient volume has been established, changes to movement patterns used can be implemented to better align with gameplay movement requirements (e.g. shuttles, forward and backward running and change of direction – e.g. agility drills). Rest periods can also be manipulated to better match gameplay requirements. As most sports require a mix of running distances and high-intensity movement durations, athletes can also begin to use mixed distance/duration approaches within the same working set to simulate gameplay.

Another way of applying progressive overload to interval training is by changing the surface/environment where the intervals are completed to make the efforts more challenging. Many athletes do this by including hill sprints or treadmill incline sprints, beach sprints or stair running intervals to name a few.

Common approaches to anaerobic interval training (HIIT)

When prescribing HIIT sessions, it can be useful to categorise workout approaches to better understand how to apply workout variables. The main categories of anaerobic interval training have been described as follows:

• Anaerobic conditioning workouts
• Anaerobic speed endurance workouts
• Repeated short sprits
• Mixed approaches
• Tabata training.

Let’s look at each of these in more detail.

Anaerobic conditioning workouts

Short anaerobic intervals coupled with short active recovery periods. These often use a 1:1 work:rest ratio and are completed as sets of continuous work. An example of this would be the 30:30 approach which involves running very intensely for 30 seconds, then easy for 30 seconds. A lot like fartlek training, but the “hards” are at very high intensity. This means rather than completing a longer Fartlek run, this workout approach needs to be broken up into shorter sets of work as the lactate accumulation during this approach is high. This workout is sometimes called a “lactate stacker” session as the blood and muscle acidity levels rise with each work interval.

The purpose of this approach is to improve anaerobic capacity and the ability to tolerate increased muscle/blood acidity during repeated high-intensity efforts.

An example of this approach is 4 sets of 5 x 30 second maximum effort intervals separated by 30-second easy active recovery periods. After each set the athlete is given 3-5 minutes of easy active recovery (returning heart rate to between 100 and 120bpm). This approach is known as Smit Intervals. The workout is depicted in the following image.

Anaerobic speed endurance workouts

Speed endurance intervals focus on pushing the upper limits of anaerobic energy production. Typically, this involves completing 30-60 second intervals at close to maximal “sustainable” intensity. We then combine this with a long recovery period using 1:5–1:10 work:rest ratios.

While the intensity used must be slightly lower than maximal sprints over 10-15 seconds, these repeat intervals are more challenging and result in a significant rise in lactate levels and muscle/blood acidity. This is why longer rest protocols are necessitated.

The purpose of this approach is to target the maintenance of high-end speed over repeated efforts.

An example of this approach would be completing 5 x 200m intervals at close to maximum intensity, with each interval separated by 3-4 minutes of rest, walking or very light jogging. This rest should be sufficient to allow the heart rate to return between 100-120 bpm and may have to be extended in later reps.

The key to this form of training is that repeat repetitions of the distance should be completed with a high degree of similarity in running time. The set should end before running times over the 200m start to increase by more than a second. A longer recovery period of 5 minutes should be used between sets to allow more complete recovery (athletes should be encouraged to keep moving to encourage more efficient lactate removal). The number of reps/sets will be dependent on the condition level of the athlete. Most sources appear to use between 8-12 total repetitions (broken up into working sets) in their studies. Progressive overload should be applied by a gradual increase in session running distance (i.e. the inclusion of more repetitions).

Repeated short sprints

The focus of this approach is simple. Move at the fastest possible speed, highest intensity, or power that you can sustain over a very short duration. Since we can only sustain maximum power and speed for a matter of seconds; these intervals need to be very short—normally limited to around 10 seconds. In order to maintain intensity in repeated efforts, these intervals require a long recovery (generally using a 1:20 work:rest ratio).

While these intervals are very intense, they do not lead to big increases in blood lactate as the ATP-PC system does not produce this (as it relies on stored ATP and O2 for energy). Despite the high intensity of the intervals, this approach is actually one of the least demanding (from a metabolic standpoint) forms of interval training, so is great for in-season training.

An example or this approach could be a runner doing 10-second hill sprints broken up by 2-3 minutes of very easy jogging. Another example could be field sport athletes doing maximal 60m-70m sprints followed by a 2-minute walking (or static) recovery.

Most research indicates that between 10 and 16 maximal sprints can be performed using this method before sprint times start to increase. The exact number of repetitions and sets used will depend on the athlete’s condition and their ability to maintain sprint times. Repetitions could likely be increased by inserting a longer period of rest after half the prescribed repetitions are completed. This can later be removed as athlete's condition improves.

A summary of the key training variables for each HIIT approach is detailed in the following table:

HIIT Approach	Average Duration of Session	Duration of Intervals	Reps and Sets	Work:Rest intervals
Anaerobic Conditioning Workouts	35-50 mins	30-45 seconds	4 sets of 4-5 reps (depending on condition)	1:1 (3-5 mins between sets)
Anaerobic Speed Endurance Workouts	30-70 mins	30-60 seconds	8-12 total repetitions	1:5 – 1:10 (3-5 mins between sets)
Repeated Short Sprint Workouts	40-50 mins	6-15 seconds	10-16 reps (broken into 2-3 sets initially)	1:20 (2-3 minutes between sets)

Mixed approaches

Many sports are made up of intermittent high-intensity sprints of various speeds. Therefore, it makes sense that sporting athletes should train over a range of different distances, at a range of different speeds within the same session to better simulate the requirements of the sport. This means many trainers will employ mixed interval training approaches within the same conditioning session. Depending on the training phase or sport, this may involve mixing aerobic and anaerobic approaches within the same session. Some common mixed approaches include:

• Interval ladders
  These interval sessions use intervals that progress from shorter to longer repetitions or vice versa in each set. Recovery intervals can either remain constant (meaning some intervals get better work:rest ratios than others). Distances involved in ladder approaches would likely be longer in preparatory phases and more reflective of average game high-intensity running distances in-season. An example for field sport athletes during a preparation phase of training might use a mix of aerobic and anaerobic efforts like this 3km interval set:
  • 200m, 400m, 600m, 800m, 1000m (or from 1000m back to 200m). Rest intervals could be kept the same for each rest interval, e.g. 2 minutes or be adjusted for each interval. Monitoring of heart rate is likely the best way to determine rest intervals (i.e. a return to between 100-120bpm before starting each interval).
    In-season it could look like multiple sets of the following approach (600m set):
  • 30m, 50m, 70m, 100m, 150m, 200m (or vice versa). Again, rest periods can be kept at a constant duration (e.g 1 minute), or adjusted for each interval. This depends on the purpose of the session (i.e. maximising the speed of each effort, or repeated efforts without complete recovery).
• Pyramid Intervals
  Essentially the same principle as above, but this is a longer interval approach in which the athlete climbs the ladder and then comes back down. With this approach you can use the same distances and work:rest ratios as above, or intervals could be based on running duration. For example, in a general conditioning block of training an athlete might maintain the fastest pace possible for each of the following time frames (adjusting their speed accordingly):
  • 15 seconds intense effort, 15-seconds rest.
  • 30 seconds intense effort, 30-seconds rest.
  • 60 seconds hard effort, 60-seconds rest.
  • 90 seconds hard effort, 90-seconds rest.
  • 90 seconds hard effort, 90-seconds rest.
  • 60 seconds hard effort, 60-seconds rest.
  • 30 seconds hard effort, 30-seconds rest.
  • 15 secs hard effort, 15-seconds rest.

Tabata training

Another popular form of interval training is Tabata training. This is a relatively new approach to interval training that was the brainchild of Dr Izumi Tabata. Dr Tabata developed this form of training in the 1990s while working with the Japanese Olympic speed skating team. This form of interval training is used in both resistance training and cardiovascular training modalities. It works on the basis of short, intense periods of work, with very short recovery intervals. The idea is that due to incomplete recovery, each interval of work accumulates the training effect across a set.

The most common approach is a 4-minute interval set consisting of 8 x 20-second work intervals, broken up by 10-second rest intervals. Due to the short recovery periods, clients would have to have a good level of base conditioning prior to taking on a Tabata workout. It is best suited to general fitness clients looking for high-calorie burn and variety and could also be used for sporting athletes looking to improve repeated effort with incomplete recovery.

Tabata has become a very popular training method and is easy to implement using a raft of Tabata-inspired songs that programme the work and rest periods for you. Search “Tabata songs” on Spotify (or Google) for a list of these songs. You can also add work:rest ratios and find songs that correspond with the work:rest ratios you wish to apply.

The physiological benefits of interval training have already been covered earlier in this topic including increased VO2 Max, mitochondrial density, improvements in blood sugar control and enhancement of endothelial cell function. This section will instead focus on what the research has to say about the effectiveness of interval training, how much improvement can be made, and the timeframes associated with such improvements.

Rosenblat et al (2022) performed a systematic review and meta-analysis on the effect of interval training of factors effecting VO2 Max. The studies included in the review focused on the effects of two types of interval training, HIIT (which for the purpose of this review encompassed repeated efforts at and around lactate threshold, OBLA or vVO2 Max (longer intervals)) and sprint interval training (SIT) which encompassed shorter repeated bouts of exercise completed at above VO2 Max intensities (shorter intervals). Rosenblat et al (2022) reported the following findings:

• There was no significant difference between the effect on VO2 Max between HIIT and SIT.
• There was no significant difference in VO2 Max improvements between males and females.
• There were significant differences in improvement in VO2 Max between highly trained and less trained subjects.
• Left ventricular hypertrophy – Both HIIT and SIT were shown to produce increases in left ventricular mass over 3 months of training. HIIT returned average increases across the studies of 7.4% while SIT average increase was 5.3%. HITT was also compared to moderate intensity continuous training (MICT) and was found to produce significantly greater improvements. SIT improvements were questionable due to a lack of quality studies. This indicates that longer intervals may return the greatest left ventricle mass increases leading to greater stroke volume and cardiac output. When improvements to cardiac output were compared between HIIT longer intervals and sprint intervals (SIT), HIIT training resulted in 8% more improvement in cardiac output than SIT.
• Increase in capillary density in skeletal muscle – Both HIIT and SIT promoted increases in capillary density in skeletal muscle. Research comparing the effects of the two exercise approaches on capillary density is a little conflicting, but it would appear HIIT approaches more consistently return improvements at around the same level as MICT. SIT training improvements in this area were less conclusive.
• Mitochondrial density and function – Mitochondria are responsible for the conversion of fuel (using O2) into ATP during exercise. The stress induced by interval training appears to lead to increased expression of enzymes that stimulate mitochondria production in muscle. This is thought to be because interval training calls on more type 2 muscle fibres than continuous aerobic training methods. The nature of interval training forces the type 2 fibres to produce more mitochondria to keep up with the muscle demand for ATP. Most studies showed that both HIIT and SIT protocols lead to a similar increase in mitochondrial density and function, however when time spent exercising is factored in, SIT appears to be the superior form of exercise to enhance mitochondrial function. This is great to know for athletes who have limited time to train using longer aerobic exercise approaches.
• Performance - While HIIT protocols are thought to produce greater VO2 Max improvements than SIT approaches and resulted in an average of 2% better performance measures overall (e.g. in time-trial tests), the reduced training times associated with SIT approaches still make this form of training a useful tool to improve VO2 max markers whiles simultaneously improving anaerobic qualities and speed. It was also suggested that SIT training time-trial improvements were significant after a training duration of just 2 weeks, whereas HIIT approaches of 4-weeks or longer were required to achieve their optimal performance increases. This indicates that SIT approaches may be best to use when only short training windows are available (e.g. in a small window f time between races for endurance athletes).

Interval training is used by both endurance and sports athletes to improve VO2 MAX and lactate threshold. The effectiveness of an interval training programme can therefore be tested using a variety of appropriate fitness tests. However, in sports athletes, interval training is also used to improve their ability to perform repeated high-intensity efforts over time (this probably holds more relevance to athletes than VO2 Max).

Types of fitness tests

There are a number of fitness tests that measure an athlete’s ability to perform repeated intermittent activity. While some of these tests are essentially VO2 Max estimation tests, they are very much aligned with the requirements of intermittent sports and will give a clear indication of how interval training has improved an athlete’s ability to continue to produce intermittent efforts over time (as they will be required to do in their sport). Let’s take a look at some common fitness tests that can be used.

The 30-15 intermittent fitness test

In this test, an athlete runs for 30 seconds, then walks for 15 seconds paced by a pre-recorded series of beeps. A 40m distance is marked out with cones at either end and at the mid-point (20m). Cones are then placed 3m either side of the zero, 20m, and 40m cones. These areas are known as “tolerance zones”.

The athlete starts behind one of the end cones (zero or 40m cones) and then begins running at the first beep towards the 40m cone. The pace of the run is timed to coincide with the 2nd beep at the 20m cone and then a double beep at the 30-second mark at which point they stop running (this may be at the 40m mark, or they may have already reached that mark and turned to head back to the start. The athlete then walks slowly to the next line (which could be at the end or middle) and waits for the start of the next level (15 seconds after the previous one ends). The initial running velocity is 8km/hr, with increments increasing by 0.5km/hr at every level. The test ends when an athlete does not cover 40m (or at least enter the tolerance zone) three times. The athlete’s score is the speed of the final level they complete in full - Velocity In Final Stage (VIFS).

This test can be used to estimate an athlete’s VO2 Max using the following scoring system:

VO2 Max Estimation Key: The speed of the final stage completed in full is VIFS. G stands for gender (female = 2; male = 1), A for age, and W for weight.

VO2max (ml.kg-1.min-1) = 28.3 – (2.15 x G) – (0.741 x A) – (0.0357 x W) + (0.0586 x A x VIFS) + (1.03 x VIFS)

However, if you are more interested in the ability of the athlete to produce repeated efforts, than their VO2 Max, you can also start this test at a higher running velocity and simply see how far an athlete moves through the stages. A good starting speed for this adapted test would be 12km per hour for females and 14km per hour for males.

The interval shuttle test

Very similar to the 30:15 test with the same work:rest intervals but using different running speeds and only a 20m coned distance. Tolerance zones are once again placed 3m before the end of each of the 20m cones. Participants run for 30 seconds and walk for 15 seconds (prompted by a series of pre-recorded beeps). They start behind one of the cones and begin to run on the first beep trying to time their run so that they hit the 20m cone by the 2nd beep. They then turn and run back to the first beep and continue running until they hear the double beep at 30 seconds (this may be beyond the first cone – I.e. keep running until the double beep). After the double beep, they have 15 seconds to walk back to the start cone ready to begin the next level. The starting speeds and increment rises are slightly different for this test. The initial speed is 10km/hr and the levels rise by 1km/hr after every 90 secs (I.e. 2 shuttles at each level). Once the running speed reaches 13km/hr, the increase in speed is 0.5km/hr every 90 seconds. The test ends when the athlete fails to make it into the tolerance zone twice. The test score is the last fully complete level (or the number of completed 20m shuttles achieved).

The FIFA (Fédération Internationale de Football Association) High Intensity Interval Test

This test was designed by FIFA for football referees but can be used in a variety of sporting contexts. The test involves 6 x maximal 40m sprints, followed by a maximum of 1-minute recovery. The purpose of the test is to assess the ability to recover between sprints and repeatedly produce high-intensity sprints. It has a high degree of relevance to many field-based sports.

The test can use either timing gates (preferred) or cones placed 40m apart. A start line distance 1.5m before the first cone is also marked. The athlete starts with their foot behind the start line and when they are ready, start their first sprint. The sprint time is either recorded by the timing gates, or by a trainer who starts a stopwatch when they break the first cone line and stops it when they cross the 40m mark (may need two timers). The athlete has 1 minute until they must start the next sprint. A total of 6 maximal sprints are completed using this process.

For the official referees’ test, they must complete each sprint in under 6.2 seconds for males and 6.6 seconds for males. For more generic assessment of repeated sprint ability, simply record the 6 sprint times and divide them by 6 for an average sprint time. Improvements in athlete recovery can also be assessed by wearing a heart rate monitor and noting the athlete’s heart rate at the end of each 1-minute rest period.

FIFA interval test (longer distance)

FIFA also developed a longer distance interval test that can also be used to assess an athlete’s ability to repeatedly perform running intervals over a prolonged period of time (as athletes are required to do in most team sports). The test involves repeated 75m hard running efforts. A flat running area of at least 100m is required. A start cone is placed on the first line, a second cone is placed at the 72m mark (tolerance zone), a third cone is placed at the 75m mark, and a final cone is placed at the 100m mark.

On the first whistle, the athlete must complete a 75m run in under 15 seconds (males) or 17 seconds (females). They then have to walk 25m to the 100m cone, turn and wait ready to start the next interval. Males have 18 seconds for the recovery walk before a whistle starts the next interval sprint (females have 20 seconds). If the athlete fails to get one foot inside the tolerance zone or is not ready to start the next interval sprint within the recovery period, they receive a warning. The 2nd time this happens the test is ended.

The total distance covered is recorded (i.e. each interval is 100m). As an example, international referees must achieve a 4000m total distance (40 intervals) to pass the test. This test is best suited to athletes involved in intermittent sports with larger running distances covered during a game (e.g. football, AFL etc).

Repeat sprint ability test

This test assesses anaerobic capacity and the ability to recover between sprints while producing the same level of power repeatedly. It involves repeated 20m maximal sprints. Athletes should record a maximal sprint time before the official test begins (to use as a baseline). A 3-minute rest is administered after this test (before the official test begins).

To complete the test, the athlete begins from a standing start and sprints 20m without slowing down before the 20m line. Timing is best performed with timing gates but can be done using a stopwatch. Once the first sprint is performed, the athlete will turn and walk back to the 20m cone ready to sprint in the other direction. The 2nd sprint starts 20 seconds after the first. The athlete performs 10 x 20m sprints in this fashion with each effort’s time recorded.

Scoring: The total time of all 10 sprints is calculated. The percentage decrement is calculated using the following formula:

Total Time - (best time x 10) / Total time x 100

Other repeated sprint tests

A number of other repeated sprint tests targeting different sprint lengths and recovery times can be found at the following links:

• Sprint Fatigue Test – 10 x 30m sprints with 30m rest intervals
  Topendsports- Sprint-fatigue
• AFL Sprint Recovery Test – 6 x 30m sprints with 20 second rest intervals - AFL Sprint Recovery Test (topendsports.com)
• Phosphate Recovery Test – 7 x 7 second sprints with 23 seconds of active recovery between each - Phosphate Recovery Test (topendsports.com)

Interest Topic: Over-use of HIIT

While HIIT training is a fantastic training tool with exceptional benefits for performance and health, it seems there can be too much of a good thing. Over-training, in general, comes with a raft of negative outcomes including impaired sleep, injury and mood disturbance (to name a few) and it appears when you over-train using HIIT, these effects can be amplified even further (Weiner, 2021).

As discussed earlier in the topic, Flockhart et al 2021 investigated the dose response of HIIT training and reported that when subjects performed HIIT training 5 days a week they saw a decline in mitochondrial function and performance output. Subjects in this study also showed less stable blood sugar control and clear evidence of increased oxidative stress (a type of cell damage linked to chronic illness and premature aging) when exposed to heavy HIIT training loads. Subjects in the 5-x weekly HIIT session protocol also reported higher fatigue and inflammation scores. While these participants were able to recover somewhat after a week of de-load training their mitochondria were still impaired following this easier training week.

Caprito (2021) also reports that high volumes of HIIT (with insufficient recovery) can increase in cortisol levels (the primary stress hormone in the body). Chronic increased levels of cortisol can cause unwanted side effects like digestive issues, sleep impairment, bloating and weight gain and have even been linked to increased anxiety responses.

The American Council on Exercise (2011) recommends a maximum of three sessions of HIIT training a week (no more than 90 minutes of total volume) ensuring at least 24 hours of recovery between sessions to reduce these potential effects. They also recommend programming HIIT training periodically over 6-week periods (mesocycles) and then taking a break from HIIT to allow for super-compensation and to avoid burnout and injury.

Of course, athletic populations will use HIIT training alongside other training modalities, so it is important that the balance and placement of HIIT is well thought out to ensure appropriate recovery is possible.

Right, time to apply what you have learned. Head to your assessment for an assessment guide video and instructions on submitting your assessments.

The assessment guide video explains your assessment task, which requires you to use the information you have learned on this topic to help a case study client.

This assessment will require you to apply the knowledge you have learned and practised by completing the following tasks:

• Design an Interval Training mode programme for an athlete
• Justify your selection of programme variables (e.g. modality of training, exercise selection, sets, reps, rest etc.)
• Design a progressed programme for the end of the mesocycle
• Justify the changes you implement to the progressed programme
• Select a relevant fitness test to test this fitness component.