Interval recoveries: take a rest or keep on moving?

Sports Performance Bulletin takes a look at some new research on how to potentially maximize interval training results by manipulating the nature of the rest periods in between each effort

In recent years, a growing body of evidence has accumulated showing that performing regular bouts of high-intensity interval training is an extremely effective training tool for athletes seeking to maximize performance. Put simply, short sessions of high-intensity intervals are a great way of producing gains in aerobic power, enabling an athlete to sustain a higher intensity/pace/workload for longer before fatigue sets in. What’s less clear however is the precise recipe of any interval program for maximizing gains.

An interval recipe

In simple terms, there are three key questions that need to be considered when designing an interval program:

  1. The intensity of the interval efforts
  2. The number of efforts in an interval session
  3. The length of the rest periods in between each interval effort

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.

Rest length

For athletes performing medium-length intervals (around 4 minutes per interval) some fairly recent research on runners has demonstrated that the physiological loading  on the body (basically how much a session takes out of the body) was similar whether the rest duration was 1, 2 or 3 minutes(1). However, running velocities were higher when participants took a 3-minute rest in between efforts compared to taking a one or two-minute rest (see this article for a more in-depth discussion). 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 with less risk of putting the body under too much physical stress.

What kind of rest?

Some scientists have argued that the nature of the recovery periods in between efforts can and should be manipulated too. For example, instead of a simple passive recovery – where the athlete just rests in between efforts or at the end of a session – a better option could be to remain active at a reduced intensity – ie to do some low-moderate intensity jogging.

This kind of rest period is known as a period of ‘active recovery’, and should in theory aid the clearance of lactate from muscles – especially after hard efforts. Clearing lactate from fatigued muscles rapidly is desirable because 1) it hastens the time when muscles are ready to work hard again, and 2) it can reduce the subjective feelings of fatigue at the end of an interval session, and may even reduce the severity and duration of any delayed onset muscle soreness (DOMS).

But is there any scientific evidence for the benefits of an active recovery over a passive one? In a previous PP article, which you can read here, we looked at evidence that an active recovery (as opposed to a passive one):

  • Accelerates the clearance of lactate from exercising muscles during recovery after an incremental rowing test in rowers(2).
  • Improved subsequent performance in a second 20-minute bout of exercise in professional climbers(3).

However, in both of these studies, the bouts of exercise (ie effort lengths) were quite long, which begs the question: does the inclusion of active rest intervals between efforts improve recovery when the effort lengths are shorter – ie in the 30-second to 3-minute range? To try and answer this, we can turn to some brand new research published just a couple of weeks ago by Thai scientists in the International Journal of Sports Physiology and Performance(4).

Answers from swimming

In this study, researchers compared the effectiveness of three different recovery protocols on muscle oxygenation, blood lactate, and subsequent performance during maximal-effort 200-metre repeated swimming (front crawl) efforts. Twelve college swimmers completed three separate sessions consisting of two consecutive 200-m front-crawl efforts with each session using one of three recovery protocols:

  • A 15-minute active recovery
  • A 15-minute passive recovery
  • A combination recovery consisting of five minutes of active recovery (very gentle swimming) and ten minutes of passive recovery.

To test the effectiveness of each recovery protocol, muscle oxygenation levels were recorded at biceps femoris (hamstring muscles), along with blood lactate concentrations, and heart rates; the measurements were taken at rest, immediately after the trial, and at 5, 10, and 15 minutes of recovery. In addition of course, the swimmer’s times were recorded in order to see whether any of the recovery protocols resulted in improved times during the second effort (ie after each type of recovery).

The findings

The first main finding was that the active recovery resulted in a slightly longer 200m swim time in the second trial (156.79 vs. 157.79 seconds), the passive recovery resulted in a very similar swim time (156.54 vs. 156.30 seconds) while the combination recovery resulted in a slightly quicker second swim time (156.50 vs. 155.55 seconds). However, the differences between the three protocols were not large enough to be considered statistically significant.

More interestingly however, while muscle oxygen saturation index rapidly declined immediately after the first 200-metre swim and then gradually returned to baseline (ie re-oxygenation of the muscles) during the rest period in all three protocols, the re-oxygenation was faster and more complete after the combined ‘active plus passive’ recovery. Moreover, the combined recovery protocol also resulted in significant reductions in blood lactate and heart rate during the recovery period – suggesting the swimmers were physiologically ready to repeat the trial earlier with a combined recovery (or to put it another way, for the same rest period, they were better rested and prepared for the second swim).

Implications for athletes

The researchers in this study concluded that compared with either an active or passive recovery, the combination recovery was more effective in enhancing muscle re-oxygenation after a 200-metre swim – but that its beneficial effect didn’t affect performance in a statistically meaningful way. Why was this? Well firstly, the study size (12 subjects) was quite small. A larger-scale study may well have observed statistically meaningful effects. Perhaps more importantly however is that only two efforts separated by one rest period were examined. Given the differences in muscle oxygenation and lactate after just one rest period, it’s quite likely that a much more significant difference would have been observed with repeated efforts – for example six intervals and five rest periods.

Is a combined active/passive recovery protocol worth trying in your own training? It’s too early to say (more research along the lines mentioned above is needed) but there would be no harm in giving it a try. Athletes who monitor their interval sessions can check for themselves if a combined rest approach helps; this would be evident by lowers heart rates at the end of the rest period and less perceived effort during the following interval(s). The key here is to maintain a one third active/two thirds passive ratio. For example, a two-minute rest period would consist of 80 seconds of passive rest and 40 seconds of gentle active rest!

References

1. J Sci Med Sport. 2018 Sep 28. pii: S1440-2440(18)30942-3. doi: 10.1016/j.jsams.2018.09.230

2. Phys Fitness. 2015 Oct;55(10):1058-63J Sports Med Phys Fitness. 2014 Jun;54(3):271-8

3. Med Sci Sports Exerc. 2009 Jun;41(6):1303-1

4. Int J Sports Physiol Perform. 2020 Sep 17;1-8. doi: 10.1123/ijspp.2019-0537. Online ahead of print

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