High-intensity interval training (HIIT) is a type of interval training that alternates intense efforts (typically 90-100% VO₂max) with less intense recovery periods (typically 50% VO₂max). The length of both the work interval and recovery interval can vary greatly but is largely dependent on the intensity of the work interval – submaximal (below VO₂max) intervals tend to have longer work intervals and a larger work to rest ratio (e.g. 5:1 work to rest ratio), whereas supra-maximal (intensities above the VO₂max) intervals tend to have shorter work intervals and smaller work to rest ratio (e.g. 1:5 work to rest ratio).
HIIT type training sessions have become increasingly popular amongst serious fitness enthusiasts and athletes alike and are believed to be one of the most effective ways to enhance cardiovascular fitness. HIIT is known to be particularly effective at improving the maximal aerobic capacity (VO₂max), Lactate threshold, the economy of motion and endurance exercise performance.
The Benefits of High Intensity Interval Training:
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 performance whilst decreasing carbohydrate oxidation at submaximal intensities (80%Wpeak equivalent to around 85-88% VO₂max) and increasing fat oxidation (Westgarth-Taylor et al., 1997;).
Types of High Intensity Interval Training:
High intensity interval training can typically be broken down into three intensity levels:
1) Submaximal Aerobic Intensities – Training at intensities slightly below VO₂max (typically around 90-95% VO₂max);
2) Maximal Aerobic Intensities – Training around the minimum speed or power output at the VO₂max;
3) Supra-maximal Intensities – Training at speeds or power outputs above the VO₂max intensity.
1) High Intensity Interval Training at Submaximal Intensities (e.g. 90-95% VO₂max)
Training at these submaximal intensities (90-95% VO₂max) places a lower level of stress on the muscular and physiological systems than “maximal” or “Supra maximal” intensities. However, the intensity is above the lactate threshold and therefore results in a significant rise in blood lactate concentrations. Because the intensity is below maximal, or supra-maximal intensities, the interval length and the total work load can be increased above that of the higher intensity training. 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 intensities.
One advantage of this interval intensity is that recovery periods can be reduced meaning that the average intensity of the workout – a combination of work interval and the rest interval – increases e.g. we can estimate that the average intensity of 4x5mins@95% VO₂max, with 90-second recoveries @60% VO₂max is around 89% VO₂max. Therefore even when you take into account the recoveries the average intensity is above the lactate threshold of most athletes.
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 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).
In addition to improvements in VO₂max, training at these intensities appears to be particularly important for enhancing exercise performance and can reduce blood lactate concentration, at a given submaximal intensity, 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.
Research in runners has found that these training intensities (90-<100% VO₂max ~10k-5kpace) appears to be particularly important for improving running economy/exercise economy (Franch et al., 1998; Daniels, 1985; Fallowfield and Wilkinson, 1999). Running economy appears to be improved at the intensities used in training and therefore sufficient training time should be devoted to training at around race intensities. Therefore training at around 90-95% VO₂max (5k-10k pace) would be more beneficial to 5k-10k middle/long-distance runners than it would to a 1500m runner or 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).
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 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). Further research using an electromyogram to study the effects of a similar HIIT sessions (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 that the workload is shared amongst a greater number of muscle fibres and effectively reduces the workload of individual muscle fibres.
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.
In order to maximise the benefits of training at these intensities the recovery periods should be kept relatively short with a work to rest ratio of around 3:1 – 5:1 (Gosselin et al., 2012; Stepto et al., 1999; Babineau and Leger, 1997). It also appears that active recoveries may be more effective than passive recoveries (Menzies et al., 2010;).
Examples of submaximal HIIT sessions include:
6x1000m @ 10km pace with 60-75 seconds jog recoveries
6x1000m @ 5km pace 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
2) High Intensity Interval Training at VO₂max Intensity
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).
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 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). More recently 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 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.
Examples of VO₂max HIIT sessions include:
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
3) High Intensity Interval Training at Supra-maximal Intensities
Supra-maximal HIIT sessions 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 enhancement of performance with improved 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 HIIT 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. 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 it is not clear whether endurance runners will derive the same benefit from this type of training. One variation of supra-maximal interval training that may prove more effective for endurance runners is the use of “lactate stacker” sessions in which athletes complete short work intervals (above VO₂max) separated by short active recovery intervals. Due to the intense work intervals, and short recovery periods, the production of lactate becomes greater than lactate metabolism and therefore blood lactate levels increase with each work interval – hence the name “lactate stacker” session. Research has demonstrated that this type of session (e.g. 15 x 40-secs at 110%VO₂max, with 20-sec recoveries) can lead to significant increases in blood lactate (~15x greater than at rest) and may lead to 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 (e.g. 3-4 x (5 x 40-secs at 110% VO₂max, with 20-sec recoveries) – separate each set of 5 work intervals with 2-3mins easy jogging).
It appears that training at intensities that are greater than the VO₂max, may prove a useful tool as final preparation for an 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.
Examples of Supra-maximal HIIT sessions:
8-12 x 30-secs @ 175%PPO, with 4-5mins active recoveries (e.g. 30-50% PPO).
8-12 x 200m sprint, with 4-5min slow jog recovery
8-10 x 45-seconds @ 125%PPO, with 2.5mins active recoveries (e.g. 30-50% PPO).
8-10 x 300m @ 800m pace, with 2.5-3min slow jog recovery
2 x (10x40secs @ 110%VO₂max, with 20 second recoveries), 5 min between sets of 10
High Intensity Interval Training Workouts and The Risk of Overtraining
The amount of high intensity interval training sessions that can be undertaken is limited due to an increased risk of overtraining. This is due to increases in stress hormones following excessive use of high intensity training. When researchers looked at the effects of increasing the number of weekly HIIT sessions 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).
It is generally recommended 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). However, the amount of HIIT sessions that you can complete without increasing the risk of overtraining will be largely determined by the intensity of the session e.g. completing more than 5% of your training as supra-maximal HIIT sessions would be more likely to place you at risk of overtraining than 10% of Submaximal HIIT sessions (e.g. 90% VO₂max). It is generally recommended that two HIIT sessions 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).
**Please note this is in addition to a Tempo session.
High Intensity Interval Training (HIIT) Summary:
- HIIT 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 sessions enhance fatigue resistance and exercise efficiency is believed to be 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 Interval Training – below the point of maximum oxygen uptake (e.g. 90% VO₂max); 2) Maximal Interval Training – Training at an intensity that corresponds with the VO₂max; 3) Supra-maximal 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 is generally recommended that HIIT sessions 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 fairly fast with improvements occurring after just 4-6 weeks of training and may be a particularly effective training method in the build-up to a competitive phase.