Less is more: Programming interval training for endurance performance

More isn’t always better. This couldn’t be truer than when it comes to designing an interval training program geared to maximize endurance sport performance. At least that’s the conclusion of our most recent meta-analysis.

This blog discusses findings from our recently published meta-analysis, which describes the effect of manipulating various interval training program characteristics (such as intensity, duration, frequency and interval type).

What is interval training?

Interval training is a form of endurance exercise consisting of repeated work bouts of high-intensity exercise, lasting from seconds to minutes, followed by a recovery period. The work is divided into a set of work-recovery repetitions because exercise at such intensities can only be sustained for short periods of time.

Interval training has been shown to produce greater improvements in maximal oxygen consumption than continuous training, and in a relatively short period of time (Milanović et al., 2015). In other words, interval training can improve an individual’s physiology (maximal aerobic potential) to a greater degree than performing long duration (more than 60 minutes), low or moderate intensity exercise. (For better readability, maximal oxygen consumption is abbreviated in this blog as VO2 max, rather than VO₂ max).

Two forms of interval training are used both in research and in practice. The first is high-intensity interval training (HIIT). HIIT is performed at an intensity above that which can be maintained indefinitely (highest steady state exercise intensity in the severe domain), but below the intensity that coincides with VO2 max. In contrast, sprint interval training (SIT) occurs at an intensity that coincides with a power or velocity above VO2 max (in the extreme domain), typically 20- to 30‑second all-out efforts (Rosenblat et al., 2020).

Measuring change in performance

Due to its high correlation with race performance, VO2 max is one of the most common measures used to assess aerobic fitness in a laboratory environment (Foster, 1983). However, VO2 max doesn’t account for individual differences in other physiological measures (Coyle et al., 1988; Coyle et al., 1991). Therefore, even if 2 athletes have the same VO2 max, they may have different 10 km race times. In addition, VO2 max isn’t a practical outcome measure, because most individuals would need to visit a laboratory to get valid and reliable results. Time-trial (TT) tests can also be used to predict race performance, but they can reliably be performed in both a laboratory and field environment (Smith et al., 2001). For these reasons, our meta-analysis looked at the effect of manipulating interval training program characteristics, including intensity, duration, frequency and interval type, on TT performance.

Recommendations for programming interval training

1. Intensity and duration: 5‑minute repeats in the severe domain

Results of our recent meta-analysis show that there’s no relationship between exercise intensity (within the severe domain) and changes in TT performance (Rosenblat, Lin, et al., 2021). It’s more beneficial to perform longer duration intervals with 5-minute work-bouts at a relatively lower intensity.

2. Frequency: 2 interval sessions per week

Beyond exercise intensity, our meta-analysis shows that performing 3 HIIT sessions per week provides no more benefit than performing 2 sessions per week. It’s also common practice for coaches to prescribe high volumes of low- to moderate-intensity, continuous training during the training week (Rosenblat et al., 2019). However, our results show that adding continuous training to an interval training program is detrimental to improvements in TT performance.

There are similar findings when prescribing exercise volume for sprint interval training (SIT) regarding session frequency and the addition of continuous training. The results of our work on changes in TT performance following SIT are similar to other meta-analyses that investigated changes in VO2 max. Specifically, trained individuals only need to complete 4 repetitions of SIT within an individual exercise session. Furthermore, adaptations in TT performance with SIT peak in as few as 2 weeks.

3. Interval type: HIIT and SIT are both beneficial, but for different reasons

While both HIIT and SIT produce increases in TT performance, our previous work suggests that HIIT can produce greater improvements (Rosenblat et al., 2020). However, it’s important to note that improvements are likely due to different mechanistic adaptations. HIIT has been shown to produce improvements in the heart’s ability to pump blood to exercising muscles (Rosenblat et al., in review). SIT often causes greater changes in how the muscles themselves absorb and use oxygen (Rosenblat, Granata, et al., 2021). Therefore, it would be advantageous to include both forms of interval training in a training program.

Considerations for athlete performance and well-being

High volumes of high-intensity training can lead to an increase in injury risk as well as athlete burnout. Therefore, an optimized interval program is necessary to help athletes safely reach their goals. The findings from our meta-analysis allows endurance athletes to optimize their time to minimize risk of injury and maximize performance gains.

Conclusion

To optimize training by maximizing performance, endurance athletes should use the following prescription to cycle through 2 weeks of SIT followed by 4 weeks of HIIT:

1. SIT programs should consist of 4 repetitions of 30‑second work-bouts performed at maximal effort, with 4 minutes of passive recovery, performed twice a week for 2 weeks.
2. HIIT programs should consist of 5 repetitions of 5‑minute work-bouts at any intensity within the severe domain, with a 2.5‑minute recovery period (active or passive) between work-bouts, performed twice a week for 4 weeks.