High Intensity Interval Training (HIIT) Workouts for Endurance Athletes

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

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

Not only is it an extremely time efficient way to train—giving you the biggest training benefit in the shortest amount of time—it boosts maximal aerobic capacity (VO2max), the lactate threshold, and exercise efficiency.

In this article we’ll take an in-depth look at: 

• The science of high-intensity interval training

• How to adapt HIIT Workouts

• The 3 key types of HIIT workouts (submaximal, VO₂max and sprint), including examples

• How to reduce the overtraining risk

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WHAT IS HIGH INTENSITY INTERVAL TRAINING (HIIT)?

HIIT is an acronym for High-Intensity Interval Training—a form of cardiovascular training, involving repeated high-intensity intervals, broken up 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. We achieve this by breaking up the workout into several small high-intensity intervals. 

By using this approach you can train harder, and spend more time at higher intensities, compared with continuous training. As a result, HIIT workouts provide a much better training stimulus and generate greater training adaptations.

The Benefits of HIIT

Research is unequivocal regarding 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 adaptability. It’s also extremely time effective.

Another advantage is we see very rapid results. In fact, just a few sessions of HIIT training can yield substantial improvements in several factors related to health, fitness, endurance and exercise performance.

Health Benefits

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

Some health benefits of HIIT include:

• Faster metabolic rate
• Improved cardiovascular fitness
• Lower resting heart rate and blood pressure
• Greater fat metabolism and weight loss
• Lower blood sugar levels and improves insulin resistance

Endurance Training Benefits

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

Importantly, it can produce additional improvements—in aerobic and anaerobic metabolism—beyond those gained through basic aerobic fitness training.

In fact, research indicates that among well-trained endurance athletes, it may be the best approach to bring about further improvements in aerobic fitness (Acevado and Goldfarb, 1989; Billat et al., 1999; Stepto et al., 1999).

HIIT sessions improve fitness in several ways, including:

• Greater fat oxidation
• Increased stroke volume of the heart
• Improved lactate uptake and clearance
• Higher VO₂max
• Increased aerobic and anaerobic energy production
• Improved neuromuscular co-ordination and efficiency
• Greater speed, or power output, at lactate threshold and VO₂max
• Improved endurance exercise performance

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

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5 COMPONENTS OF HIIT WORKOUTS

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 intervals

#1 Interval Intensity

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

So what intensity should you use?… To be “high intensity”, intervals should 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.

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

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, emphasize 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 intervals (3-5minutes).

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

• 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 consideration is: when interval duration increases, work intensity decreases.

For this reason, 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 largely depends on the intensity and duration of the work interval.

As you might expect, the more intense the interval, 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 affects the overall training effect. For instance, during some workouts we keep the recovery shorter, to improve 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?…

First, the recovery intensity affects the intensity of work intervals. In this way, recoveries must be easy enough for you to recover so you can maintain the intensity of the intervals.

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

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

When you increase the intensity of the recovery, you increase the average intensity of the workout. In contrast, having a complete rest between intervals reduces the average intensity of the workout.

Third, an active recovery can be beneficial during HIIT workouts. And may enhance 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 adjust the recovery intensity, this affects the average intensity of the workout and the interval intensity.

This is 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 maintain interval intensity.

#5 Volume of Intervals

Clearly the training volume is a key factor for 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 training volume than sprint intervals, because of 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).

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TYPES OF HIIT WORKOUTS

As we’ve seen, there are almost unlimited variations of HIIT workouts. However, from an endurance training perspective, we can typically divide these into three intensity levels:

1. Submaximal HIIT Workouts: where intensity is 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.

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SUBMAXIMAL HIIT TRAINING

These are completed at just below the intensity of the VO2max.

This type of training is essential 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.

Why use Submaximal Intervals?

1. Allows a greater volume of intervals 

Because the intensity is just below VO2max intensity, there’s 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 amount of HIIT compared with VO2max, or sprint intervals.

Research (Zuniga et al., 2011) indicates that athletes looking to maximize the volume of high intensity exercise, should focus 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. Higher average workout intensity

Another advantage is an increased work to rest ratio. This raises 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 comprising 4x5mins@95% VO2max, with 90-second recoveries @60% VO2max, would be around 89% VO2max. So, even with the recoveries, the average intensity of this session would be above lactate threshold intensity for many athletes.

So, not only are these effective for developing the upper limits of aerobic capacity, they’re also great for lactate threshold training.

3. Increased aerobic fitness, cardiac stroke volume and efficiency

Training at these intensities is more effective at increasing VO2 max than lower intensities, including the lactate threshold intensity (Gormley et al., 2008; Helgerud et al., 2007). It also turns out to be effective at enhancing the muscle’s ability to oxidize fats and carbohydrates (Perry et al., 2008).

Research indicates the improvements in VO2max relate to increased 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 blood lactate concentrations

Besides improvements in VO2max, these intensities appear to be important for improved endurance exercise performance. Importantly, it can also lower blood lactate concentrations at submaximal intensities.

In fact, research has demonstrated 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, an increased ability to sustain higher work intensities.

Training in this way places a powerful stimulus on both maximal aerobic power and lactate threshold.

5. Improved exercise efficiency

Research in runners has identified that these intensities (~10k-5kpace) are important for improving running efficiency/exercise economy (Franch et al., 1998; Daniels, 1985; Fallowfield and Wilkinson, 1999).

We know from research that running efficiency improves at the intensities used during training. Therefore, it’s essential to exercise at race intensities.

In this sense, 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 backed by research which found continuous distance running training, and long interval training, improved running economy at speeds used during training. In comparison, 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 discover later in this article: new research has found sprint intervals are effective at improving running efficiency.

6. Increased recruitment of slow twitch muscle fibers

Research, using an electromyogram, to study the effects of submaximal HIIT workouts (8x5mins at 82%PPO, with 60 second active recoveries), observed that improved exercise performance, and fatigue resistance, was possibly due to increased recruitment of slow twitch muscle fibers (Jemma et al., 2005).

So why is this important?… By increasing the recruitment of slow twitch fibers, we distribute the workload across a greater number of muscle fibers. This reduces the workload of individual muscle fibers and improves fatigue resistance.

7. Increased 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 agrees 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 discovered 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).

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

Optimum recovery periods

In order to maximise the benefits of submaximal HIIT training, recovery periods should 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.

Here, the workout comprises:

• Warm-up, consisting of some short efforts, gradually building to VO2max intensity.
• Main session comprising 8 x submaximal intervals, separated by a short active recovery. Here, the work to recovery ratio is 3:1, but we can extend it up to 5:1.
• Cool down.

As you can see, this allows a significant amount of training time to at near VO2max intensity. At the same time we increase the average intensity, because of the short active recoveries.

• 6-8x1000m @ 10km pace, with 60-75 seconds jog recoveries
• 5-6x1000m @ 5km pace, with 75-90 seconds jog recoveries
• 6x4mins @ 85%PPO, with 90-120 seconds @ 30-40%PPO recoveries
• 6x5mins @ 80%PPO, with 60-90 seconds @ 30-40%PPO recoveries

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VO2 MAX INTERVALS

What are VO2max workouts?… These are intervals completed at the speed, or power output, corresponding with the maximum oxygen uptake (VO2max).

Compared with Submaximal intervals, the work rate is higher. However, the recovery duration is longer, which means that the average intensity (Work interval + recovery) is lower.

Research has established that training at this intensity produces a potent 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 adaptation to this intensity is quick—often athletes achieve improvements after just 4-6 weeks. So, it’s a useful training stimulus in the buildup to important races.

VO2 Max Intervals and Running

Research in runners has found VO2max intervals (referred to as vVO2max intervals) can improve running performance, VO2max, the velocity at the lactate threshold (vLT), running economy, vVO2max and neuromuscular co-ordination (Esfarjani and Laursen, 2007; Dendai et al., 2006; Smith et al., 2003; Billat et al., 1999).

In one study, one weekly vVO2max interval session led to a 3% improvement in vVO2max and a 6% improvement in running economy (Billat et al., 1999). Here, the interval duration was 50% of the time that the athlete could sustain vVO2max (something referred to as tlimvVO2max or Tmax), which averaged around 3 minutes.

In another study, vVO2max training improved 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).

VO2 Max Intervals and Cycling

Similar results have been seen in cyclists. With cycling, we set training intensity to the power output that corresponds with VO2max, rather than the velocity at VO2 max.

In one study the use of 8 intervals at Pmax—the power output at VO2max—led to notable improvements in 40km time trial performance (~5%) and peak power output (~3-6%), in a group of highly trained cyclists (Laursen et al., 2002). They based the interval duration on the percentage of the time the cyclists could sustain P(max). Here, researchers set interval duration to 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

So, what’s the best interval duration for VO2max intervals?… Interestingly, researchers have observed positive results using 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 athletes can sustain VO2max (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 yield a better training stimulus.

That said, shorter duration intervals also produce 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 could sustain an additional 5 minutes at VO2max, compared with a continuous run to exhaustion.

In another study, researchers noted 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, compared with 3minute intervals (Zuniga et al., 2011). They concluded that shorter intervals may allow athletes to complete longer duration interval sessions, while 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 intervals. That said, my preference is to complete longer VO2max intervals, while completing shorter intervals at intensities above VO2max intensity.

In fact, restricting shorter intervals to VO2max intensity may limit the training benefits.

Example VO2max Workouts

The image below shows a popular VO2max interval training workout. In this example, the warm up includes short accelerations, building to just above VO2max intensity. The main workout then comprises 5 x 3minute intervals at VO2max intensity, separated by 3minute recoveries at half the work rate of the intervals.

• 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.

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SPRINT INTERVALS

Supramaximal, or sprint interval workouts, use intensities that are significantly above the speed or power output at the VO2max.

Training at these intensities involves a strong anaerobic component. And often leads to an accumulation of lactate in the muscles and blood. There’s also a corresponding rise in blood and muscle acidity.

While this intensity is less specific for endurance athletes, research has shown it to improve exercise performance. In fact, it’s important to include these intensities within any endurance training schedule. And you may 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 Short Sprints

Here, the focus is purely on exercising at the fastest possible speed, highest intensity, or power that you can sustain. Since we can only sustain maximum power and speed for a matter of seconds; these intervals need to be very short—normally limited to ~10seconds. And, in order to maintain intensity, these require a long recovery—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 broken up by 2-3minutes of very easy jogging.

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

Despite the high intensity, they are one of the least demanding—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. We then combine this 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 more challenging. And, there is a greater risk of over-training. We also see a significant rise in lactate levels and muscle and blood acidity.

#3 Anaerobic Conditioning Workouts

One further variation of sprint interval training involves completing short anaerobic intervals, coupled 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 useful for developing aerobic and anaerobic capacity. They also improve your capacity to tolerate significant increases in blood and muscle acidity.

Because of the intense work intervals, and short recoveries, the production of lactate is greater than lactate metabolism. Consequently, blood and muscle acidity levels, increase with each work interval, leading to a steady 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 increased lactate clearance. However, they are also very challenging.

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

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

Sprint Workouts and Endurance Cycling

There is growing research backing the benefits of intense anaerobic intervals. One sport where these are really beneficial is cycling.

Stepto et al. (1999) found that including 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 suggested that this improvement may have been because of improved buffering capacity, which allowed a greater amount of work before fatigue limited performance.

The benefits of sprint training for cyclists, were confirmed in a study where the same sprint interval session (12 x 30 seconds at 175% PPO, with 4.5 minute recoveries) led to a big 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) compared the effects of sprint intervals (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 discovered that sprint training was equally effective as tempo training. This was despite the tempo session being 17x longer (3.5mins vs. 60mins). They suggested that sprint training is an effective training method for athletes looking for a time-efficient training strategy.

Sprint Workouts and Endurance Running

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

10km running

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 research, runners completed 10 sessions of 5-10 x 30sec speed endurance intervals. The intervals were run at maximal running intensity, and separated with a 3:30min walk recovery.

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

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

In another study (Lum et al., 2019), researchers discovered that even shorter sprint intervals improved 10km running performance and peak power. Here, the interval training comprised 12 sessions of 12-16 x 30-50m sprints. So, even extremely short sprint intervals can improve endurance running performance.

3km running

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

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 Summary

Training at these intensities is an extremely time efficient method for increasing neuromuscular co-ordination, power and endurance exercise performance. Part of the training benefits come via specific adaptations within slow twitch muscle fibers.

One important factor is the time course of these adaptations: improvements can happen after as little as 2 weeks of training. Making these an ideal training method in the buildup to key events.

Clearly, this is an important training intensity—even for endurance athletes. And it’s wise to incorporate these intensities within your training.

One important point: because of the increased levels of physiological stress and CNS fatigue, it’s important not to use these too close to key events—leaving around 10 days before competition is a good approach.

Example Sprint Interval Workouts

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).

Anaerobic conditioning cycling example:

• 4 x (5x30secs @ 110-120%VO2max Power, with 30 second recoveries at 50-60% VO2max Power), 3-5 min recovery between sets

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 90-95% effort, with 4-5min slow jog recovery
• 8-10 x 300m @ 90% effort, with 4-5min slow jog recovery

Read more about sprint training for distance runners. 

Anaerobic conditioning running example:

• 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

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HIIT AND OVERTRAINING RISK

The frequency and volume of HITT workouts need to be carefully designed to reduce the risk of over-training. This is because of 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 over-training.

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

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

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

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

The general recommendations are that high-intensity training sessions should make up approximately 10-15% 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 over 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.

We recommend 1-2 HIIT sessions and one tempo/threshold session to be the weekly upper limit, if you are trying to reduce the risk of over-training. 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).

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SUMMARY

• HIIT combines intense efforts with easier recovery periods.
• Some of the benefits include improved VO2max, enhanced aerobic and anaerobic energy systems, increased cardiac output, efficiency and oxygen transport, increased lactate threshold, improved speed/power, greater 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 fibers. By increasing the recruitment of type I muscle fibers, we reduce the workload of individual muscle fibers.
• We can divide HIIT sessions into three intensity levels: 1) Submaximal—intensities just below the point of peak oxygen uptake (e.g. 90-95% VO2max); 2) VO2max—training at VO2max intensity; 3) Supramaximal or Sprint—intensities that are significantly above VO2max and involve a strong anaerobic component.
• The rest interval varies depending upon the intensity of the work intervals—the higher the intensity, the longer the recovery period.
• HIIT sessions are a very powerful and a time-efficient method for enhancing fatigue resistance, lactate threshold, race performance, and exercise efficiency.
• It’s recommended that HIIT should make up around 15% of an endurance athletes total training volume, with a maximum of 2 weekly sessions.
• The time course of the training adaptation to HIIT training is rapid, with improvements occurring after as little as 2 weeks of training. For this reason, HIIT is effective in the build-up to competition and races.

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