High Intensity Interval Training: A guide to HIIT workouts for endurance

HIIT Workouts - High Intensity Interval Training

HIIT (High intensity interval training) refers to a specific type of workout, involving repeated high intensity exercise, separated by short periods of rest or low intensity exercise. 

Whilst HIIT training has recently gained massive popularity in the fitness industry, it’s been used by elite and club level endurance athletes for many years. In fact, HIIT training has been used by runners for over 100 years. 

Not only is it an extremely time effective way to train – giving you the greatest training benefit in the shortest amount of time – it’s particularly effective at developing the upper limits of maximal aerobic capacity (VO2max), increasing the lactate threshold, and improving exercise efficiency and endurance exercise performance.

In this article we’ll take an in-depth look at the science behind HIIT – from an endurance training perspective – covering everything from the different components of high intensity training, the three key types of HIIT workouts (including examples), and look at how to incorporate these into your training and avoid overtraining.

Table of Contents

What is HIIT training?

HIIT is an acronym for High Intensity Interval Training – a form of cardiovascular training, involving repeated high intensity intervals, separated by periods of rest, or low intensity exercise.

So why use HIIT training? The main purpose of HIIT, is to increase the volume of training completed at high intensities. This is achieved by dividing the workout into smaller high intensity intervals. 

By separating these intervals with short recovery periods, you can train harder, and complete more training at higher intensities, than during continuous training. As a consequence, HIIT workouts significantly increase the training stimulus and the level of adaptation when compared with continuous training.

The benefits of HIIT 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.

One advantage of HIIT training is the level of versatility. It’s also extremely time effective. 

Another advantage is that we see very quick results. In fact, just a few weeks of HIIT training can yield significant improvements in a number of factors related to health, fitness, endurance and exercise performance.

Health benefits of HIIT workouts:

HIIT training is highly beneficial to fitness enthusiasts, where it allows individuals to maximize cardiovascular fitness and calorie expenditure in a minimal time period.

Some of the health benefits of HIIT include:

  • Increases metabolic rate
  • Improved cardiovascular fitness
  • Reduces heart rate and blood pressure
  • Increased fat metabolism and weight loss
  • Lower blood sugar levels and improves insulin resistance

Endurance training benefits of HIIT workouts:

HIIT training is a key training method for endurance athletes. Put simply, if you want to achieve your full potential, then high intensity interval training should feature in your training schedule. 

Importantly, it can bring about additional improvements in aerobic and anaerobic metabolism, beyond those achieved through basic aerobic fitness training.

In fact, research suggests that amongst well trained endurance athletes, it 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).

How HIIT improves endurance

HIIT sessions are believed to improve performance in a number of ways including:

  • Improved fat oxidation
  • Increased stroke volume of the heart
  • Improved lactate uptake and clearance
  • Can lead to improvements in the VO₂max
  • Improved aerobic and anaerobic energy production
  • Enhanced neuromuscular co-ordination and efficiency
  • Increased speed, or power output, at lactate threshold and VO₂max
  • Can lead to significant improvements in endurance exercise performance

Of particular importance for endurance athletes, is the way HIIT training improves exercise performance, decreases carbohydrate oxidation at sub-maximal intensities (below VO2max), and at the same time increasing fat oxidation (Westgarth-Taylor et al., 1997).

The 5 components of HIIT Training

One advantage of HIIT training is the level of adaptability. In fact, there are five components that we can adapt to alter both the training effect and purpose of the workout.

  1. Interval intensity
  2. Interval duration
  3. Recovery duration
  4. Recovery intensity
  5. Volume of HIIT intervals

#1Interval Intensity

As you might expect, HIIT workouts are all about intensity.

So what intensity should you use? To be considered “high intensity” the interval intensity really needs to be at least of 90% of VO2 max (~95% of maximum heart rate). As an example, this would be somewhere between 10km and 5km running pace for many runners.

Having said that, there is actually a very wide range of intensities that we can use: ranging from just below VO2 max intensity, right up to maximal sprint intensity.

Not surprisingly, the training benefit from these sessions varies depending on the intensity used during these intervals:

  • Sub-maximal Intervals (just below VO2max), develop aerobic conditioning and muscular endurance.
  • VO2max intervals, place the emphasis on increasing aerobic capacity, and the velocity or power at VO2max.
  • Supra-maximal/Sprint Intervals increase the emphasis on strength, power, anaerobic capacity and anaerobic endurance.

#2 Interval duration

The interval duration can vary from short sprints (10-15seconds), to longer aerobic intervals (3-5minutes).

So, what interval duration works best? Again, it all depends on the purpose of the session:

  • Developing aerobic conditioning and muscular endurance: 3-5minutes.
  • VO2max: normally from 30secs-3minutes, but sometimes longer.
  • Anaerobic endurance: normally 30-60secconds
  • Speed, power and anaerobic capacity: 10-30seconds.

One factor to consider, is that when interval duration increases, work intensity decreases. As such interval length is an important factor when structuring workouts.

Interval intensity is also affected by the recovery between intervals.

#3 Recovery duration

The length of the recovery interval is largely dependent on the intensity and duration of the work interval. As you might expect, the more intense the effort, the longer the recovery period will need to be.

In the same way that interval intensity and duration vary significantly, there can be great variability in the length of recovery:

  • Sprint intervals/anaerobic capacity: the recovery period can be 10-20x longer than the work interval (1:10-1:20 work to recovery ratio)
  • Anaerobic endurance/conditioning: the recovery interval is normally 3-5x longer than the work interval (1:3-1:5 work to recovery ratio). However, sometimes this may be as low as 1:1.
  • VO2max Intervals: the recovery period is normally equal to the work interval (1:1 work to recovery ratio).
  • Aerobic conditioning/muscular endurance: the work interval is much longer than the recovery period (typically 3:1-5:1 work to recovery ratio)

The key factor here is: the recovery must be long enough to allow you to maintain the target interval intensity.

The length of the recovery also influences the overall training effect. For instance, during some workouts the recovery is delivery kept shorter, to develop muscular endurance, aerobic capacity, or anaerobic endurance.

Another factor to consider with recoveries is the intensity of recoveries.

#4 Recovery Intensity

The recovery intensity can vary from:

  • Complete rest,
  • Very light activity,
  • Moderate intensity.

Why does the recovery intensity matter?

Firstly, the recovery intensity affects the intensity of work intervals. In this way, recoveries must be easy enough to allow you to maintain the goal intensity of the intervals.

Secondly, the recovery intensity affects the average intensity of the workout:

Average workout intensity =  (interval intensity + recovery intensity) ÷ workout duration

As such, when you increase the intensity of the recovery, you also increase the average intensity of the workout. In contrast, taking a complete rest between intervals reduces the average intensity of the workout.

Thirdly, an active recovery can be beneficial during HIIT workouts. And can improve the rate of recovery between intervals. I’ve covered this in greater detail in the following article: active vs passive recoveries during interval training.

The key point here is: when we alter the recovery intensity, this affects the average intensity of the workout and the interval intensity.

This can be particularly important when the focus is on developing aerobic conditioning and muscular endurance. However, as I mentioned: it’s important that the recovery is sufficient to allow you to main the goal interval intensity.

#5 Volume of intervals

Clearly the volume of training is a key factor when it comes to training adaptations. And HIIT training is no different. However, this depends on the intensity of the intervals and purpose of the training session.

For instance, sub-maximal HIIT intervals allow a much larger total training volume than sprint intervals, due to the lower intensity.

As an example, a well conditioned endurance runner might complete 30minutes of submaximal intervals (6 x 5minute 10k pace intervals). In contrast, the same athlete might only complete 2minutes of maximal hill sprints (10 x 12 second hill sprints).

Types of HIIT workouts

Clearly, there are almost limitless variations of HIIT workouts. However, from an endurance training perspective we can typically divide these into three intensity levels:

  1. Submaximal HIIT Workouts: training at intensities slightly below VO2max (typically around 90-95% VO2max);
  2. VO2max HIIT Workouts: training at the speed or power output of the VO2max;
  3. Supramaximal/Sprint Interval Training Workouts: training at intensities that are significantly above VO2max intensity.

Submaximal HIIT training

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 or 40K time trials, cross country skiers, triathletes and duathletes.

There are a number of advantages to using submaximal HIIT workouts:

1. Allows a greater volume of HIIT training 

Because the intensity is slightly below VO2max intensity, this places less stress on your muscular and physiological systems, compared with “maximal” or “Supra maximal” HIIT workouts.

So why is this important? Simply put, this allows you to increase the interval duration, and workout volume. By doing this, you can complete a greater volume of HIIT compared with VO2max, or sprint intervals.

This is supported by research (Zuniga et al., 2011), which suggests that athletes looking to maximize the volume 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.

In fact, training volume is a key factor in developing both muscular endurance, aerobic conditioning and lactate threshold.

2. A higher average workout intensity.

Another advantage of submaximal HIIT workouts is: the ratio of work to rest can be increased. This increases the average intensity of the workout – a combination of the 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. So, even when including the recoveries, the average intensity of this session would be above lactate threshold intensity for most athletes.

This means that not only are these intervals effective for developing the upper limits of aerobic capacity, they are also great for lactate threshold training.

3. Increased aerobic conditioning, VO2max, cardiac stroke volume and efficiency

Not surprisingly, training at these intensities is more effective at increasing VO2max than lower intensity training, including training at the lactate threshold intensity (Gormley et al., 2008; Helgerud et al., 2007). It also appears to be particularly effective at enhancing the muscles ability to oxidize fats and carbohydrates (Perry et al., 2008).

Research suggest that the improvements in VO2max appear to be related to improved stroke volume of the heart (Helgerud et al., 2007). In addition, submaximal 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 endurance exercise performance. Importantly, it can also reduce blood lactate concentrations, at submaximal intensities.

In fact, research has shown that training at these intensities, allows athletes to race at faster speeds, or power outputs, without increasing blood lactate concentrations. Interestingly, these improvements occur independently of any changes in VO2max (Acevado and Goldfarb 1989; Londere 1997). Suggesting, that it increases your ability to sustain higher work intensities.

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 (~10k-5kpace) may be particularly important for improving running efficiency/exercise economy (Franch et al., 1998; Daniels, 1985; Fallowfield and Wilkinson, 1999).

We know from research, that running efficiency is often improved at the intensities used during training. Therefore, sufficient training time should always be devoted to training at race intensities.

In this way, submaximal HIIT workouts completed at 90-95% VO2max (5k-10k pace for runners), are more beneficial for 5km running and 10km running training than for a 1500m runner, or, an ultra-distance runner.

This is supported by research that found continuous distance running training, and long interval training, led to improved running economy at speeds close to the training intensity. In contrast, shorter intervals, run at faster speeds did not appear to have a significant effect on running efficiency at slower speeds (Franch et al., 1998). However, as you will see later in this article: recent research suggests that short sprint intervals can be highly effective at improving running efficiency.

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 was possibly due to increased recruitment of slow twitch muscle fibres (Jemma et al., 2005).

So why is this important? Increasing the recruitment of slow twitch muscle fibres – essentially recruiting a greater percentage of the slow twitch muscle fibres – shares the workload across a greater number of muscle fibres. Consequently, this reduces the workload of individual muscle fibres.

7. Increased endurance exercise performance

Research in cyclists (Stepto et al., 1999), looked at the effects of five training intensities (175%, 100%, 90%, 85% and 80% peak power) and interval duration (30secs, 1mins, 2mins, 4mins, 8mins). From this, researchers predicted a maximum training benefit – in 40km time trial performance – when interval duration was ~3-6minutes, and intensity was ~85% peak power (~95% VO2max).

Interestingly, 85%PP is just slightly above 40km TT intensity. This is in agreement with research looking at endurance runners, which found that optimum interval training intensity should be around, or slightly above, target race intensity.

Further to 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).

Similar performance benefits have been seen with rowers (Driller et al., 2009). In this study, researchers found that using 8 x 2.5 minute intervals at 90% vVO2max was more effective at improving 2000m rowing performance and relative VO2max, than traditional training.

Optimum recovery periods for submaximal HIIT workouts

In order to maximise the benefits of submaximal HIIT training, recovery periods should be kept relatively short – ideally 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 are more effective than passive recoveries (Menzies et al., 2010;).

Example submaximal HIIT workouts

The image below, shows a typical submaximal HIIT workout, used by endurance athletes, including myself.

In this case, the workout consists of:

  • Warm-up including some short efforts, gradually building to VO2max intensity.
  • Main session consisting 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.
  • Cool down.

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.

Submaximal HIIT Training example
  • 6-8x1000m @ 10km pace, with 60-75 seconds jog recoveries
  • 5-6x1000m @ 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

VO2 max HIIT Training

What are VO2max HIIT workouts? Simply put, these are intervals completed at the speed or power output that corresponds with maximum oxygen uptake (VO2max).

Compared with Submaximal HIIT intervals, the work rate of VO2max HIIT intervals is higher. However, the increased emphasis on recovery, means that the average intensity (Work interval + recovery) is lower.

Research has shown that training at this intensity 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). In fact, the training response to this intensity is quick – often athletes may see improvements after just 4-6 weeks of training. As such, it can prove a particularly useful training stimulus, in the build up to target races.

VO2max HIIT Intervals: Endurance Running performance

Research in runners has found that VO2max intervals (often referred to as vVO2max training) can improve running performance, VO2max, the velocity at the lactate threshold (vLT), 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). In this case, the interval duration was 50% of the individual time that the athlete could sustain vVO2max (referred to as 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).

VO2max HIIT Intervals: Endurance Cycling performance

Similar benefits have been observed with the use of VO2max training in cyclists. In the case of cycling, 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 Pmax – the power output at VO2max – 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). As with vVO2max running training, the interval duration was based on a percentage of the time that cyclists could sustain P(max). In this case, interval duration was kept at 60% of Pmax.

In another study, shorter duration Pmax intervals (8-12 x 60s at Pmax, 75 second recoveries) also proved to be a potent stimulus for improving exercise performance. The researchers also observed increased mitochondrial function within the muscle (Little et al., 2010).

Optimum Interval Length for VO2max Intervals

So, what’s the best interval duration for VO2max intervals? Interestingly, positive results have been demonstrated across a range of different interval durations: from 30seconds right 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.

However, shorter duration intervals, also appear to also provide a powerful training stimulus. Billat et al., (2000) demonstrated that by using short intervals (30 seconds at vVO2max, 30 seconds at 50% of vVO2max) runners were able to sustain an additional 5 minutes at VO2max, compared with a continuous run to exhaustion.

In another study, researchers demonstrated that shorter intervals (30s intervals, with 30s recoveries) allowed athletes to train at a higher total and average VO2, HR and with lower blood lactate, when compared with 3minute intervals (Zuniga et al., 2011). The researchers concluded that compared with longer intervals, shorter 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.

Short vs Long VO2max intervals

Both short and longer VO2max intervals can be very effective for improving endurance exercise performance. However, the training benefits from each are slightly different.

With this in mind, it’s wise to use both short and longer VO2max intervals. Having said that, my preference is to complete longer VO2max intervals, whilst completing shorter intervals at intensities above VO2max intensity.

In fact, restricting shorter intervals to VO2max intensity can be limiting in terms of training benefits.

Example VO2max HIIT workouts

Below is a widely used VO2max HIIT interval training workout. In this example, the warm up includes short accelerations, building up 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.

VO2max Interval example
  • 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. The recoveries should be at approximately 50% of the speed, or power, of the work interval.
  • 10-12 x 60-seconds @ a pace/intensity 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 Sprint Intervals

Supramaximal, or sprint interval training workouts, involve training at intensities that are significantly above the speed or power output at which the VO2max occurs.

Not surprisingly, training at these intensities involves a strong anaerobic component. Consequently, this often leads to an accumulation of lactate in the muscles and blood. There’s also a corresponding increase in blood and muscle acidity.

Whilst these intervals are less specific (in terms of intensity) for endurance athletes, they have been shown to improve endurance exercise performance. In fact, it’s important to include these intensities within any endurance training schedule. And, I would argue that you will limit your endurance potential if you don’t train at these intensities.

While there are many variations of sprint intervals, we can loosely categorise these into 3 distinct types:

#1 Maximal Sprint Intervals

With maximal sprint intervals, the focus is purely on exercising at the fastest possible speed, highest intensity, or power that you can sustain. Since, maximum power and speed can only be sustained for a matter of seconds; these intervals need to be kept very short.

In fact, the work interval would normally be limited to 10seconds. And in order to maintain intensity, this must be combined with a long recovery period – typically, using a 1:20 work to rest ratio.

As an example, a runner might complete a series of very short maximal hill sprints, with each lasting just 10seconds and separated by 2-3minutes of very easy jogging.

While these intervals are very intense, they do not lead to significant increases in blood lactate. In fact, the main energy source comes from alactic energy stores within your muscles.

Despite the high intensity, they are one of the least stressful (from a metabolic standpoint) forms of interval training.

#2 Anaerobic Speed Endurance Intervals

Speed endurance intervals focus on pushing the upper limits of anaerobic energy production – both alactic and lactate energy production. Typically, this involves completing 30-60second intervals at close to maximal “sustainable” intensity. This is then combined with a long recovery period – typically using a 1:5 – 1:10 work to rest ratio.

As an example a runner might complete 200m intervals at close to maximum intensity, with each interval separated by 3-4minutes of rest, walking or very light jogging.

While the Intensity is lower than maximal sprints, these are far more challenging. Consequently, there is a greater risk of overtraining. We also see a significant rise in lactate levels and increased muscle and blood acidity.

#3 Anaerobic Conditioning Intervals

One further variation of sprint interval training, involves completing short anaerobic intervals, combined with short active recoveries. An example session might involve, completing 30second intense efforts, separated by 30 seconds of very light exercise, or rest.

These sessions – sometimes referred to as “lactate stacker” sessions – can be effective for developing aerobic and anaerobic capacity. They also improve your ability to tolerate significant increases in blood and 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 and muscle acidity levels, increase with each work interval, leading to a gradual decrease in muscle ph.

These sessions can be a useful way to develop anaerobic conditioning, helping to increase buffering capacity – the ability to tolerate increased muscle and blood acidity – and improved lactate clearance. However, they are also extremely challenging.

Below is an example anaerobic conditioning workout. In this case, the workout consists of 4 sets of (5 x 30-secs intervals, 30sec recoveries), with each set separated by 3-5mins of active recovery.  

Sprint Interval example

Let’s take a look at how sprint intervals benefit endurance athletes.

Sprint intervals and endurance cycling performance

There is growing research supporting the benefits of intense anaerobic intervals on endurance exercise performance. One sport where these are particularly beneficial is cycling.

Stepto et al. (1999) found that the inclusion of 12 x 30 second intervals at 175% PPO (the peak power sustained in a max test) led to a substantial improvement in 40-km cycling time trial performance. The researchers speculated that this improvement may have been due to improved buffering capacity, which allowed a greater amount of work to be completed before fatigue limited performance.

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 cycling time trial performance, in a group of highly trained cyclists (Laursen et al., 2002;).

In another study, 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)).

Interestingly, they found that sprint training was equally as effective as tempo training. This was despite the tempo session being 17x longer (3.5mins vs. 60mins). The researchers suggested that sprint training, may be an effective training method for athletes looking for a time-efficient training strategy that maximises training time.

Sprint Intervals and endurance running performance

Similar results have been demonstrated when looking at the benefits of sprint interval training in endurance runners (Skovgaard et al., 2018).

Sprint intervals and 10km running performance

In one study, researchers looked at the benefits of speed endurance training in a group of trained endurance runners (Skovgaard et al., 2018). During the study, the runners completed 10 sessions of 5-10 x 30sec speed endurance intervals. The intervals were completed at maximal running intensity, and separated with a 3:30min walk recovery.

So, what did the researchers find? Firstly, speed endurance training improved 10k performance by 3.2%. Secondly, running efficiency improved and the oxygen cost of running decreased.

Interestingly, the researchers noted that the improved running efficiency appeared to be related to an adaptation within slow twitch muscle fibres. So, developing sprint speed endurance, actually benefited slow twitch muscle fibres. From this, the researchers speculated that the improved 10km running performance may be related to both improved running efficiency and increased anaerobic capacity.

In another study (Lum et al., 2019), researchers found that even shorter sprint intervals improved 10km running performance and peak power. In this case, the interval training consisted of 12 sessions of 12-16 x 30-50m sprints. So, even very short sprint intervals can benefit endurance running performance.

Sprint intervals and 3km running

Similar results were observed when researchers looked at the effects of 30second sprint interval training on 3000m running performance (Koral et al., 2018). In this case, just 6 sessions (5-7 x 30second sprint intervals) led to a 6% improvement in 3km time trial performance.

In addition, there were significant improvements in peak power, mean power and the time to exhaustion. The researchers noted that sprint interval training is an extremely time efficient training method, that’s useful for improving endurance and power in trained runners.

Clearly, sprint interval sessions are at the opposite end of the training spectrum to what most endurance athletes are used to. This highlights, the importance of using a range of training intensities, within any effective training program.

Sprint interval training summary

Training at these intensities is an extremely time efficient method for increasing neuromuscular co-ordination, power and endurance exercise performance. Perhaps surprisingly, it seems that at least part of the training benefits come via specific adaptations within slow twitch muscle fibres. 

One important factor, is the time course of these adaptations. In fact, improvements can happen after as little as 2 weeks of training. Making these an ideal endurance training method in the build up to key events. 

Having said that, this is an important training intensity – even for endurance athletes. And as such, it’s wise to incorporate these intensities routinely within your training. 

One important point: due to the increased levels of physiological stress, as well as CNS fatigue, it’s important not to use these too close to key events.  

Example Sprint Interval Training workouts

Sprint Interval Training Cycling Workouts

  • 10-16 x 10second maximal sprints, with 2-3mins easy recoveries between sprints.
  • 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).
  • 4 x (5x30secs @ 110-120%VO2max Power, with 30 second recoveries at 50-60% VO2max Power), 3-5 min recovery between sets

Sprint Interval Training Endurance Running Workouts

  • 6-12 x 10second maximal hill sprints, separated by 2-3mins of easy jogging.
  • 5-8 x 30second near maximal short hill repeats, 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

HIIT Training and the risk of overtraining

The frequency and volume of HITT workouts needs to be carefully controlled, to reduce the risk of overtraining. This is due to a combination of increased levels of stress hormones, and the high levels of CNS stimulation during these sessions. If used excessively, this can increase the risk of overtraining.

So, how much HIIT training is too much? There tends to be wide variation in the individual tolerance of high intensity training. Some athletes are able to complete 3 high intensity sessions per week, whereas others may struggle with just one or 2 sessions.

Some research (Billat et al., 1999) suggests that completing 3x weekly HIIT workouts may increase the risk of overtraining as indicated by increased levels of norepinephrine (a stress hormone). However, one important point: this depends largely on the type of HIIT workout.

Risk of overtraining from HIIT workouts – from lowest (1) to highest (4):

  1. Very short sprint intervals (sprints lasting around 10 seconds)
  2. Submaximal HIIT
  3. VO2max HIIT workouts
  4. Anaerobic capacity sprints

The general recommendations are that high intensity 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, this needs to consider the type of HIIT workout. For example, completing more than 5% of your training as anaerobic capacity sprints, would be more stressful than completing 10% as submaximal HIIT workouts. Of all the HIIT workouts, very short sprints – like 10second hill sprints – are the least stressful. 

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
  • There are 5 components of HIIT intervals: interval intensity, interval length, recovery duration, recovery intensity and workout volume.
  • One way in which HIIT workouts enhance fatigue resistance and exercise efficiency is through increased recruitment of type I (slow twitch) muscle fibres. And 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 – training below the point of maximum oxygen uptake (e.g. 90-95% VO2max); 2) VO2max HIIT Interval Training – training at VO2max intensity; 3) Supramaximal Sprint Interval Training – training at intensities that are significantly above VO2max, and involving a strong anaerobic component.
  • The rest interval for HIIT sessions, varies depending upon the intensity of the work intervals. In general, the higher the intensity the longer the recovery period.
  • HIIT sessions are a very effective and a time-efficient method for 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 as little as 2 weeks of training. As such HIIT can be a particularly effective training method, in the build-up to a competitive phase.
High Intensity Interval Training References:

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