Interval training: the long and short of rest between efforts

Andrew Hamilton looks at new research on the rest lengths in an interval training session. Could longer be better?

When the topic of interval training comes up, it’s natural to discuss how long and how hard each work interval is, and how many interval repeats are performed. That’s not surprising because we know that it’s the short bursts of relatively intense work that are responsible for the fitness and performance benefits interval training brings. What’s often overlooked, however, is the rest interval, which is an integral part of the interval training process.

Interval length and intensity – what works?

There are three key questions you need to consider when designing an interval programme:

1. The intensity of the intervals.
2. The number of interval.
3. The length of the rest periods in between each interval.

When it comes to points 1) and 2), a LOT of scientific research has been carried out examining the performance benefits of training sessions using a variety of interval lengths and intensities. In one of the earliest studies on this topic (back in 1997), a Japanese professor called Izumi Tabata of the National Institute of Fitness and Sports in Kanoya, Japan found that interval sessions consisting of eight repeats of 20 seconds at very high intensity – 170% of maximum sustainable oxygen uptake – produced excellent gains in aerobic capacity⁽¹⁾. Indeed, Tabata was able to show that his very high-intensity intervals produced superior gains to the same number of sessions of one hour of steady-state, moderate-intensity (70% of maximum oxygen uptake - VO2 max) endurance training on a stationary bicycle!

The (frankly astonishing) results from Tabata’s research sparked a wave of further studies into high-intensity interval training and endurance performance. For example, Swiss researchers compared the following cycling protocols in ten male cyclists competing at national level⁽²⁾:

• High-intensity short intervals consisting of 7 x 30-second ‘all-out’ bouts (sprint intervals).
• Long-duration aerobic intervals consisting of 3 x 20-minute bouts at around 87% of VO2max.

The results showed that the short sprint intervals were just as effective as the longer intervals at raising key markers of fitness, despite the total work performed in the long intervals being 8 times greater and the exercise duration 17 times longer than during the sprint intervals!

Another study by US scientists compared the effects of four different cycling interval-training regimes on recreationally competitive male and female triathletes⁽³⁾:

1. 90% of maximum power output for 30 seconds;
2. 90% of maximum power output for 3 minutes;
3. 100% of maximum power output for 30 seconds;
4. 100% of maximum power output for 3 minutes;

The results showed that although all four protocols results in good fitness gains, the best way to increase the total time spent at high intensity during intervals and maximise oxygen uptake (ie maximise the training effect) is to perform short (30-second) intervals at 90% maximum power rather than to perform longer, harder efforts.

Other research on trained cyclists has found that shorter intervals pedalling at 90% maximum intensity produced better performance gains (in terms of the maximum power that could be sustained for long periods) than cycling at a maximum intensity of 88% using intervals twice as long⁽⁴⁾. Meanwhile, a study comparing cycling intervals of 30 seconds at 89% of peak power to intervals of 10 seconds at 95% of peak power found that although the 30-seconds intervals were superior, the 10 second intervals were still surprisingly effective, despite the 60% reduction in total workload⁽⁵⁾.

Rest – the forgotten ingredient?

While there’s been much research into interval duration and intensity, there’s surprisingly little evidence regarding the optimum length of rest intervals – the third ingredient in any interval training prescription. Traditionally, rest intervals have been set on a ‘needs must basis’. In other words, the rest interval prescribed has often simply been determined by that which allows the set of intervals prescribed to be completed.

For example, consider a recreational athlete seeking to perform 8 x 90-second intervals at 95% maximum oxygen uptake. He/she might find that rest intervals need to be in excess of two minutes to be able to complete the set of eight intervals at 95% max oxygen uptake. Reducing the rest interval length to one minute would result in accumulating fatigue, meaning the last few intervals cannot be completed at that intensity. But does setting the rest interval on such a basis produce the best outcome in terms of fitness gains? Until recently, little research has been carries out to answer this question, but a newly published study on rest interval length provides a valuable insight into this question.

Rest interval length – the evidence

In a study by British scientists, researchers studied the effects of different recovery durations in trained runners who performed high-intensity intervals on a treadmill⁽⁶⁾. In particular, they sought to examine, physiological responses, and ratings of perceived exertion (RPE) for these different recovery durations.

Twelve trained runners participated, and all the subjects began by performing an incremental treadmill exercise test to determine their maximal oxygen uptakes (VO2max) and maximum heart rates (HRmax). In four subsequent visits to the lab, the participants performed a high-intensity interval session on each visit. The four sessions all had the same overall structure, which comprised of six 4-minute work intervals with recovery intervals in between. However, each session varied in terms of the recovery durations in between intervals, which were as follows:

• 1 minute rest
• 2 minutes rest
• 3 minutes rest
• A self-selected rest duration

In all of the four trials, the runners were asked to self pace their interval efforts, ensuring that they were perceived as very hard and that the effort levels were the same for each trial. During the trials, the researchers recorded running velocities and how hard the runners had actually worked in terms of the time spent at different percentages of VO2max.

Longer is better

The first finding was that the runners’ RPE responses were similar across and within the different rest-length protocols, showing a gradual increase with each progressive interval within a set, regardless of the rest length. The second finding was that all of the four trials resulted in similar proportions of time spent at 90% and 95% of VO2max and 90% and 95% of maximum heart rate. The key finding however was that the runners ran their intervals significantly faster when they used the long rest (3-minute) duration. The self-selected rest duration resulted in faster intervals than the 1 and 2-minute rest periods, but not as fast as when 3 minutes was used – ie while the runners may have felt recovered, they were not as well recovered as in the 3-minute rest protocol.

What do these findings means in terms of setting rest intervals? Well, for athletes performing medium-length intervals (around 4 minutes per interval) intervals it turns out that the length of recovery duration seems to have a limited effect on the total physiological strain endured in the training. However, in this study, the actual running velocities were higher when participants received the longest (3-minutes) recovery period. This indicates that longer recovery durations may facilitate a higher external training load (faster running, cycling etc), while maintaining a similar internal training load (physiological stimulus), and may therefore allow for greater training adaptations.

Implications for athletes performing intervals

The overall message is that setting rest intervals on the basis of ‘enough required to complete the session’ may not be optimum. It seems that longer rest intervals may produce additional benefits by allowing athletes to perform at higher velocities, which can only aid race performances where the goal is to maintain the high speed possible.

We need to add a caveat that in this study; the runners were performing 4-minute intervals – ie medium length intervals. For athletes performing much shorter intervals such as 1-minute or 30-second intervals, a 3-minute rest might be excessive. However, it’s still reasonable to assume that somewhat longer rest intervals than ‘just achievable’ could yield additional benefits in terms of allowing a greater training stimulus to be produced. As is so often the case, this is an area where more research is needed.

Having said that, the findings from this study dovetail nicely with some very recent research on a phenomenon known as cardiovascular drift, and which we reported on in a recently published article (see here). This research found that inserting 3 to 5-minute rest intervals into a period of steady state training at medium to high intensity levels improved the time accumulated at maximum stroke volume, which is known to be a powerful training adaptation stimulus for endurance performance. Together, these findings suggest that we may need to rethink the commonly held notion that rest is nothing more than ‘dead’ time in between ‘productive’ efforts when performing intervals. Instead, it seems that adding in adequate amounts of rest in between hard efforts can further increase the training stimulus, while avoiding an increase in physiological loading – a win-win situation for any endurance athlete!

Practical recommendations

These are preliminary findings and more research is still needed. In the meantime however, here are some useful tips for athletes and coaches:

• If you set your rest interval length to be that which is as short as possible while allowing you to complete your interval session, it may be too short. Try increasing it by 50% while keeping the same intensity. The training stimulus is likely to be similar while making the interval session less physiologically demanding.
• Even if you can complete an interval session comfortably, try using longer rest period and upping your speed/intensity. The loading will be no higher but the training stimulus potentially greater.
• Keep detailed records of your interval durations and intensities – AND your rest interval lengths. If you experiment with these, keeping records will help you determine what works best in terms of subsequent performance.
• Research shows that interval lengths varying from 30 seconds to four minutes are all capable of producing good gains in fitness despite targeting different energy systems. To widen the spread of potential benefits from interval training, try to include both short and medium length intervals into your training programme.

References

1. Med Sci Sports Exerc 1997; Volume 29(3), pp 390-395
2. Eur J Appl Physiol. 2010 Oct;110(3):597-606
3. J Strength Cond Res. 2011 May;25(5):1279-84
4. Scand J Med Sci Sports. 2011 Aug 3. doi: 10.1111/j.1600-0838.2011.01351.x
5. Eur J Appl Physiol. 2010 Sep;110(1):153-60
6. J Sci Med Sport. 2018 Sep 28. pii: S1440-2440(18)30942-3. doi: 10.1016/j.jsams.2018.09.230. [Epub ahead of print]