HIIT - High Intensity Interval Training
High-intensity interval training (HIIT) is one of the most effective types of endurance training you can use. HIIT workouts are extremely time effective giving you the greatest training benefit in the shortest amount of time.
High intensity interval training has been used by elite and club level endurance athletes for many years. Recently HIIT training has also gained popularity amongst serious fitness enthusiasts, due to the many health benefits that can be gained from these highly effective workouts.
HIIT workouts are particularly effective at working the upper limits of maximal aerobic capacity (VO₂max), increasing the lactate threshold, and improving exercise efficiency and endurance exercise performance.
One real benefit of HIIT training is that we often see very quick results. In fact, just a few weeks of HIIT training can yield significant improvements in a number of factors related to fitness and endurance performance.
What is HIIT training?
HIIT or High intensity interval training refers to a specific type of training involving repeated high intensity efforts, separated by short periods of rest, or low intensity exercise.
The high intensity used during HIIT training workouts can vary from just below VO2max intensity, to significantly above VO2max. As such, the training benefit from these sessions can vary depending on the intensity used during the intervals.
The main purpose of HIIT workouts is to allow you to spend significantly more training time at high intensities. This is achieved by separating the work intervals with short recovery periods.
The length of the recovery interval is largely dependent on the intensity and duration of the work interval. However, the primary focus with the recovery period is to allow sufficient recovery for you to maintain a high work rate during the intervals.
By using this approach you can complete several high intensity intervals during a training session. And, as a consequence HIIT workouts significantly increases both the training stimulus and level of adaptation when compared with continuous training.
Research is clear with regards to the benefits of HIIT training – it’s one of the most effective types of training you can use. It’s also extremely time effective and we often see significant fitness improvements after just a few HIIT intervals.
What are the benefits of HIIT Training?
High Intensity Interval Training (HIIT) workouts have a number of benefits, these include:
HIIT training is highly beneficial to fitness enthusiasts, where it allows individuals to maximize cardiovascular fitness and calorie expenditure in a minimal time period.
HIIT training is also a key training method for endurance athletes. Importantly, it can bring about additional improvements in aerobic and anaerobic metabolism, that are beyond those achieved through basic aerobic fitness training.
In fact, research suggests that amongst well trained endurance athletes the use of HIIT sessions at 90-100% VO₂max may be the best way to bring about further improvements in aerobic fitness (Acevado and Goldfarb, 1989; Billat et al., 1999; Stepto et al., 1999).
Amongst well trained endurance athletes HIIT sessions are believed to improve performance in a number of ways including: increasing the stroke volume of the heart, enhancing fatigue resistance, improving neuromuscular co-ordination, reducing blood lactate levels and increasing the recruitment of slow twitch muscle fibres.
Of particular importance for endurance athletes is the way that HIIT training improves exercise performance, whilst also decreasing carbohydrate oxidation at submaximal intensities (80%Wpeak equivalent to around 85-88% VO₂max), and at the same time increasing fat oxidation (Westgarth-Taylor et al., 1997;).
Different types of HIIT training
High intensity interval training can typically be broken down into three intensity levels:
- Submaximal HIIT Workouts – Training at intensities slightly below VO₂max (typically around 90-95% VO₂max);
- VO₂max HIIT Workouts – Training around the minimum speed or power output at the VO₂max;
- Supramaximal Interval Training Workouts – Training at speeds or power outputs above the VO₂max intensity.
Submaximal HIIT training (90-95% VO₂max)
Submaximal HIIT training involves completing intervals at just below the intensity of the VO₂max.
This type of training is particularly important for athletes competing in endurance events. So, runners competing in events of 5k or longer, cyclists competing in events like 10mile time trials – or longer, cross country skiers, triathletes and duathletes, will all benefit from these sessions.
There are a number of advantages to using submaximal HIIT workouts:
1. Training at submaximal HIIT intensities places less stress on the muscular and physiological systems, compared with “maximal” or “Supra maximal” HIIT workouts.
So, why is this important? Because the intensity is below maximal, or supra-maximal intensities, it allows us to increase both the interval length, and the the total duration of high intensity training within a workout. This allows more total work to be completed compared with both maximal, or supra-maxima intervals. This is supported by research (Zuniga et al., 2011) which suggests that athletes looking to maximize the total time of high intensity exercise and total VO₂ of an exercise session should concentrate on submaximal intensities (i.e. 90-95% VO₂max) rather than maximal, or supra-maximal intensities.
2. Submaximal HIIT training allows you to maintain a higher average intensity across the workout
Another advantage of submaximal HIIT intensity workouts is that the ratio of work to rest can be increased as we do not need long to recoveries between intervals. This increases the average intensity of the workout – a combination of work interval and the rest interval.
As an example we can estimate that the average intensity of a session consisting of 4x5mins@95% VO₂max, with 90-second recoveries @60% VO₂max would be around 89% VO₂max. Therefore even when you take into account the recoveries the average intensity of this session is above the lactate threshold of most athletes.
This means that not only are these intervals great for pushing the upper limits of aerobic capacity, but the relatively short recovery periods mean that the average intensity is very close to the lactate threshold.
Below is a typical submaximal HIIT workout used by endurance athletes including myself. In this case, a warm-up including some short efforts gradually building to VO2max intensity. The main session then consists of 8 x submaximal intervals, separated by a short active recovery – here the work to recovery ratio is 3:1, but it can be extended up to 5:1.
As you can see this allows a significant amount of training time to be spent near VO2max intensity. At the same time the average intensity is increased due to the short active recoveries.
3. Training at these intensities can increase aerobic metabolism, VO2max, and the stroke volume and efficiency of the heart
Training at these intensities has been shown to be more effective at increasing VO₂max than lower intensity training including the lactate threshold intensity (Gormley et al., 2008; Helgerud et al., 2007), and appears to be particularly effective at enhancing the muscles ability to oxidize both fats and carbohydrates (Perry et al., 2008).
Research suggest that the improvements in VO₂max with this type of training intensity appear to be related to improved stroke volume of the heart (Helgerud et al., 2007). In addition HIIT appears to increase cardiac efficiency and the maximal mitochondrial capacity of the heart (Hafstad et al., 2011).
4. Reduced submaximal blood lactate concentrations
In addition to improvements in VO₂max, training at these intensities appears to be particularly important for improving exercise performance and can reduce blood lactate concentrations, at submaximal intensities, independently of any changes in VO₂max (Acevado and Goldfarb 1989; Londere 1997). Research has shown that following interval sessions at these intensities athletes are able to race at greater speeds, or power outputs, without increased blood lactate concentrations.
Training in this way places a significant training stimulus on both maximal aerobic capacity and lactate threshold.
5. Improved exercise efficiency
Research in runners has found that these training intensities (90-95% VO₂max ~10k-5kpace) appears to be particularly important for improving running economy/exercise economy (Franch et al., 1998; Daniels, 1985; Fallowfield and Wilkinson, 1999).
We know from research that running economy appears to be improved at the intensities used during training. Therefore sufficient training time should be devoted to training at around race intensities.
In this way submaximal HIIT workouts completed at 90-95% VO₂max (5k-10k pace), are more beneficial to 5k-10k middle/long-distance runners than for a 1500m runner, or, an ultra-distance runner. This is supported by research that found continuous distance training and long interval training led to improved running economy at speeds close to the training intensity, but shorter intervals run at faster speeds did not have a significant effect on running economy at slower speeds (Franch et al., 1998).
6. Increased recruitment of slow twitch muscle fibres
Some of the training benefits from submaximal HIIT workouts appear to be related to increased slow twitch fibre recruitment. Research, using an electromyogram to study the effects of submaximal HIIT workouts (8x5mins at 82%PPO, with 60 second active recoveries), observed that the improved exercise performance and fatigue resistance following the HIIT sessions were possibly due to increased recruitment of slow twitch muscle fibres (Jemma et al., 2005).
Recruitment of additional slow twitch muscle fibres is important, since it means the workload is shared amongst a greater number of muscle fibres – effectively reducing the workload of individual muscle fibres.
7. Increased endurance exercise performance
Research in cyclists (Stepto et al., 1999), looking at the effects of five training intensities (175%, 100%, 90%, 85% and 80% peak power) and interval lengths (30secs, 1mins, 2mins, 4mins, 8mins), predicted a maximum enhancement in 40km time trial performance when interval training length is approximately 3-6minutes and intensity is around 85% peak power (~95% VO₂max).
Interestingly, 85%PP is just slightly above 40km TT intensity and is in agreement with research in runners that found that training intensity that is around, or slightly above, race intensity is optimum for enhancing race performance.
In line with this researchers found that when highly trained cyclists replaced approximately 15% of their total training volume with HIIT sessions (6-8 x 5mins @ 80%PPO, with 60secs recoveries) they significantly improved PPO, fatigue resistance and 40km time trial performance (Lindsay et al., 1996).
Research in rowers has also demonstrated the benefits of sub-maximal HIIT sessions on rowing performance (Driller et al., 2009). In this study the use of 8 x 2.5 minute intervals at 90% vVO₂max improved 2000m rowing performance and relative VO₂max more than traditional training.
Optimum recovery periods for submaximal HIIT workouts
Example submaximal HIIT workouts
- 8x1000m @ 10km pace intervals with 60-75 seconds jog recoveries
- 6x1000m @ 5km pace intervals with 75-90 seconds jog recoveries
- 6x4mins @ 85%PPO with 90 seconds @ 30-40%PPO recoveries
- 6x5mins @ 80%PPO with 60 seconds @ 30-40%PPO recoveries
VO₂max HIIT Workouts
Training at the maximal aerobic capacity involves training at the minimum speed or power output that elicits the maximum oxygen uptake (VO₂max). Research has shown that training at these intensities provides a powerful training stimulus (Little et al., 2010; Esfarjani and Laursen, 2007; Smith et al., 2003; Dendai et al., 2006; Laursen et al., 2002; Billat et al., 1999; Smith et al., 1999).
Below is a popular VO2max HIIT interval training workout. Here the warm up includes short accelerations to just above VO2max intensity. The main session then consists of 5 x 3minute intervals at VO2max intensity, separated by 3minutes at half the workrate of the the VO2max intervals.
Compared with Submaximal Hiit intervals, the work rate of VO2max Hiit intervals is higher. However, the longer recovery periods mean that the average intensity (Work interval + recovery) is lower.
Research in runners has found that the use of high intensity interval training sessions at the velocity at which VO₂max occurs (vVO2max) can lead to improvements in running performance, VO₂max, the velocity at the lactate threshold, running economy, vVO₂max and appears to be a powerful stimulus to neuromuscular co-ordination (Esfarjani and Laursen, 2007; Dendai et al., 2006; Smith et al., 2003; Billat et al., 1999;).
In one study the use of one weekly interval session at vVO₂max led to a 3% improvement in vVO₂max and a 6% improvement in running economy (Billat et al., 1999). The interval duration was set to be 50% of the individual time that the athlete could sustain vVO₂max (the time that VO₂max can be sustained is called tlimvVO₂max or Tmax) which averaged around 3 minutes.
In another study vVO₂max training was found to improve 1500m running performance, 5000m performance, the velocity at OBLA (onset of blood lactate accumulation), running economy, and vVO₂max in a group of well-trained runners (Dendai et al., 2006).
Similar benefits have been observed with the use of VO₂max training in cyclists, where training intensity is set to the power output that corresponds with VO₂max rather than the velocity at which VO₂max occurs.
In one study the use of 8 intervals at P(max) – the power output at VO₂max – for 60% of the time that this power output could be sustained led to significant improvements in 40km time trial performance(~5%) and peak power output (~3-6%) in a group of highly trained cyclists (Laursen et al., 2002).
In another study 8-12 x 60 second intervals at P(max), with 75 second recoveries, was found to be a potent stimulus for improving exercise performance and increasing mitochondrial function within the muscle (Little et al., 2010).
The optimum length of VO₂max intervals is not clear although positive results have been found with a range of different interval lengths from 30 seconds up to 3-4 minutes (Little et al., 2010; Esfarjani and Laursen, 2007; Smith et al., 2003; Laursen et al., 2002; Billat et al., 2000; Billat et al., 1999; Smith et al., 1999;).
Researchers have often used a percentage of the time that VO₂max can be sustained (Tmax) to determine the interval duration – in most cases researchers have used either 50% Tmax (Billat et al., 1999) or 60-75% of Tmax (Esfarjani and Laursen, 2007; Smith et al., 2003; Laursen et al., 2002; Smith et al., 1999), with 60-75% of Tmax believed to provide a greater training stimulus.
The use of shorter duration intervals, also appear to also provide a powerful training stimulus when the work to rest ratio is kept around 1:1. Billat et al., (2000) demonstrated that the use of 30 second intervals at vVO₂max alternated with 30 seconds at 50%vVO₂max allowed the subjects to sustain an additional 5 minutes at VO₂max, than during a continuous vVO₂max run to exhaustion.
The effect of exercise to rest ratio may also have a significant effect on the VO₂max attained and blood lactate response during short intermittent runs at close to or above the v VO₂max (Ballor and Volovsek, 1992). The use of a work to rest ratio of 2:1 at 90% VO₂max caused a significantly greater oxygen uptake than a work to rest ratio of 1:1 at 110% VO₂max with both having similar lactate responses (Ballor and Volovsek, 1992).
Researchers have demonstrated that the use of 30 second intervals, with 30 second recovery periods, allowed athletes to train at a higher total and average VO₂, HR and with lower blood lactate than 3 minute interval durations (Zuniga et al., 2011). The researchers concluded that shorter duration intervals may allow athletes to complete a longer duration interval sessions, whilst placing a greater metabolic stress (as measured by total and average VO₂) with reduced blood lactate levels, when compared with longer duration intervals, with the same work to rest ratio.
Because of the short time response (~4 weeks) to training at the VO₂max intensity, it may prove particularly useful if utilised 4-6 weeks prior to a major competition as a final training stimulus.
Example VO₂max HIIT workouts
- 5-8 x 3-4mins @ a pace/intensity that you could sustain for around 6-8mins in an all-out effort with 3-minute recoveries – recoveries should be at 50% vVO₂max (Running) or 30-40% of Pmax (cycling)
- 10-12 x 60-seconds @ a pace/intensity that you could sustain for around 6-8mins in an all-out effort with 60-second recoveries as above
- 15-20 x 30-seconds @ a pace/intensity that you could sustain for around 6-8mins in an all-out effort with 30-second recoveries as above
Supramaximal Interval Training (SMIT) workouts
Supramaximal Interval Training SMIT workouts involves training at intensities above the speed or power output at which the VO₂max occurs. Training at these intensities involves a strong anaerobic component, and therefore causes high levels of lactate to accumulate in the muscles and blood stream.
Because of the very high intensity of the supra-maximal intervals, a greater emphasis must be placed on recovery periods in order to maintain the quality of the efforts. The duration of these intervals are normally kept short, typically around 30-60 seconds, but may be as long as 90 seconds.
There is growing research supporting the benefits of intense anaerobic intervals on endurance exercise performance. Stepto et al. (1999) found that the inclusion of 12 x 30 second work bouts at 175% PPO (the peak power sustained in a max test) led to a substantial improvement in 40-km time trial cycling performance. The researchers speculated that this improvement may have been due to improved buffering capacity that would allow a greater amount of work to be performed.
The benefits of supra-maximal sprint training on cycling performance were confirmed in a later study where the same sprint interval session (12 x 30 seconds at 175% PPO with 4.5 minute recoveries) led to a significant improvement in PPO and 40km time trial performance in a group of highly trained cyclists (Laursen et al., 2002;).
When researchers (Psilander et al., 2010) looked at the effects of supra-maximal sprint training (7 x 30 second all out efforts), and tempo training (3 x 20mins at 87% VO₂max), on genetic markers of mitochondrial biogenesis in a group of elite national level cyclists (VO₂max 68 ± 1 mL kg(-1) min(-1)), they found that sprint training was equally as effective as tempo training, despite the tempo session being 17 times longer (3.5mins vs. 60mins). The researchers suggest that supra-maximal sprint training may be an effective training method for athletes looking for a time-efficient training strategy that maximises training time.
Due to the very high training intensity the recovery time must be increased in order to allow effective recovery and maintenance of the intensity of the work interval. Typically, the work to rest ratio is normally kept in the region of 1:8 – 1:10 so a 30 second work effort would typically require around 4 – 5 minutes of active recovery. If the exercise intensity is reduced (e.g. from 175%PPO to 125-150%PPO then the work to rest ratio can be reduced to the region of 1:3 – 1:5.
Although this type of supra-maximal sprint training has been found to be an effective training technique for cyclists. There is less research looking at the effect of supra maximal sprint training on running performance. However, it is always wise to use a range of different training intensities. This will improve you as a runner and allow you to gain greater benefits from other training sessions.
One further variation of supra-maximal interval training involves completing short work intervals (above VO₂max) separated by short active recoveries. These sessions – sometimes referred to as “lactate stacker” sessions – can be effective for developing aerobic and anaerobic capacity, as well as the ability to tolerate significant increases in muscle acidity.
Due to the intense work intervals, and short recovery periods, the production of lactate is greater than lactate metabolism. As a consequence blood lactate levels increase with each work interval, leading to a gradual decrease in muscle ph. Research has demonstrated that this type of session can lead to significant increases in blood lactate. This can lead to an increased buffering capacity, improved lactate metabolism and clearance.
This type of session can be particularly tough and athletes who are new to this type of training may find it easier to break the session into smaller segments. An example session would be: 4 x (5 x 30-secs at 110-120% VO₂max, with 30-sec active recoveries – separate each set of 5 work intervals with 3-5mins of active recovery).
Training at intensities that are greater than the VO₂max, can prove a useful as a final preparation for a key endurance event, by increasing the muscle buffer capacity and enhancing neuromuscular co-ordination.
It has not been established what the time course of the adaptation is, however, improvements in blood buffering capacity are likely to occur fairly rapidly with this type of training and could be used over the last 2-3 weeks prior to a major competition. Due to the high levels of physiological stress as well as CNS fatigue, it’s important not to do these too close to key events.
Example Supramaximal Interval Training workouts
Supramaximal Interval Training Cycling
- 8-12 x 30-secs @ 150-175%VO₂max Power, with 4-5mins active recoveries (e.g. 40-60% VO2max Power).
- 8-10 x 45-60seconds @ 125%VO₂max Power, with 2:30-3:00mins active recoveries (e.g. 40-60% VO2max Power).
- 2-3 x (10x30secs @ 110-120%VO₂max Power, with 30 second recoveries at 50-60% VO2max Power), 3-5 min recovery between sets
Supramaximal Interval Training Endurance Running
- 5-8 x 30second near maximal short hill intervals, 4-5min slow jog recovery
- 8-12 x 200m at 400/800m pace, with 4-5min slow jog recovery
- 8-10 x 300m @ 800m pace, with 2.5-3min slow jog recovery
- 3 x (4x200m @ 800/1500m pace, with 100m jog recovery – aiming to run the recovery in same time as the 200m interval), 3-5 min easy recovery between sets
High Intensity Interval Training and the risk of overtraining
The frequency of high intensity interval training/HITT workouts should be limited due to an increased risk of overtraining. This is due to a combination of increased levels of stress hormones following excessive use of high intensity training, and the high levels of CNS stimulation during these sessions. This combination can potentially lead to overtraining and burnout if overused.
When researchers looked at the effects of increasing the number of weekly HIIT workouts on exercise performance and stress hormone levels, they found that increasing HIIT sessions to 3x per week did not improve performance and led to increased levels of norepinephrine (a stress hormone), indicating an increased risk of overtraining (Billat et al., 1999).
The general recommendation is that high intensity interval training sessions should make up approximately 5-15% of total training volume (Jemma et al., 2005; Neumann et al., 2000; Bompa, 1999; Daniels, 1998; Lindsay et al., 1996).
There is wide variation in the individual tolerance levels to these types of sessions. In addition, the amount of HIIT sessions that you can tolerate without increasing the risk of overtraining will be largely determined by the intensity of the session.
Completing more than 5% of your training as supra-maximal HIIT sessions, would be more likely to place you at risk of overtraining than completing 10% of Submaximal HIIT workouts.
It is generally recommended that two HIIT sessions and one tempo/threshold session should be the weekly upper limit, if you are trying to reduce the risk of overtraining. In addition you may wish to ensure that at least one of your weekly HIIT sessions is a submaximal session (e.g. 90-95% VO₂max).
High Intensity Interval Training (HIIT) summary:
- HIIT training is a type of interval training in which intense efforts are alternated with easier recovery periods.
- Some of the benefits of HIIT sessions include improved VO₂max, enhanced aerobic and anaerobic energy systems, improved cardiac output, efficiency and oxygen transport, enhanced lactate threshold, improved speed/power, enhanced fatigue resistance and enhanced exercise efficiency
- One way in which HIIT workouts enhance fatigue resistance and exercise efficiency is through increased recruitment of type I (slow twitch) muscle fibres – by increasing the recruitment of type I muscle fibres the workload of each individual muscle fibre is reduced.
- HIIT sessions can be divided into three intensity levels: 1) Submaximal HIIT Training – below the point of maximum oxygen uptake (e.g. 90% VO₂max); 2) Maximal HIIT Interval Training – Training at an intensity that corresponds with the VO₂max; 3) Supramaximal Interval Training – Training at intensities above the VO₂max.
- The rest intervals for HIIT sessions varies depending upon the intensity of the work intervals – typically submaximal, maximal and supra-maximal sessions have work to rest ratio’s of 5:1, 1:1 and 1:5-1:10 respectively.
- HIIT sessions appear to be a particularly effective and a time-efficient means of enhancing fatigue resistance, lactate threshold intensity, race performance, and exercise efficiency.
- It’s generally recommended that HIIT workouts should make up no more than 15% of the total training volume with a maximum of 2 weekly sessions.
- The time course of the training adaptation to HIIT training is quick, with improvements occurring after just 4-6 weeks of training. As such these may be a particularly effective training method in the build-up to a competitive phase.