Interval training is an essential component of most endurance training programs, because of its ability to improve cardiovascular fitness in relatively short periods of time—ie. maximize performance gains while minimizing injury risk. In fact, interval training is so essential to most endurance and triathlon programs that many athletes hardly even think about it. Recently, colleagues and I conducted a meta-analysis and systemic review of the research done on interval training when used to optimize time trial performance. What we found can provide insight into the programming, structure, and benefits of intervals. Below I will discuss five new findings from the meta-analysis that may change how you structure your own interval programs.
Before discussing the findings, it is important to lay out the precise criteria used to define the two forms of interval training frequently studied—since the literature tends to be somewhat unclear in this area. High-intensity interval training (HIIT) consists of repeated bouts of exercise at a power output or velocity within the severe-intensity domain. The lower limit of the severe domain is commonly demarcated by a measure known as critical power or critical speed. The upper limit of the severe domain, defined as maximal aerobic power or maximal aerobic velocity, can be described as the highest power or velocity that can still allow for the attainment of maximal oxygen consumption (VO2 max). On the other hand, sprint interval training (SIT) is performed in the extreme intensity domain which occurs at a power output or velocity above those associated with maximal aerobic power.
In other words, SIT is all-out, anaerobic, above VO2 max, unsustainable efforts—which, by definition, can only be done for very short intervals. HIIT training is hard intervals but at or just below VO2 max—and can then be sustained for repeats of a few minutes, in general.
The Five New Interval Training Findings
In our comprehensive review and analysis of the studies done on both HIIT and SIT training, we found a few key takeaways:
1. There are no sex-based differences in the change in time-trial performance following HIIT or SIT. This is an important finding because there are differences in the acute physiological responses that occur in men as compared to women. For example, men tend to utilize a greater amount of carbohydrates compared to women. Furthermore, it is understood that a woman’s menstrual cycle phase can influence performance. In fact, there is evidence that testing results in women may differ if the assessments are not performed in the same phase of the menstrual cycle. Surprisingly, the results of the meta-analysis showed there to be no difference in the adaptive responses between men and women even though training periods for the included studies ranged from two to ten weeks in duration.
2. Exercise intensity with HIIT does not influence change in time-trial performance. Certainly, exercise intensity is important, since HIIT is performed in the severe intensity domain, which is a relatively high intensity. However, it is common for coaches and exercise physiologists to program HIIT exercise at an intensity the coincides with the power or velocity at VO2max, the upper limit of the severe domain. This is because there is a relationship between exercise intensity and changes in VO2max. However, the results of the meta-analysis showed there to be no relationship between changes in exercise intensity and changes in time-trial performance following HIIT. Therefore, training at 90-100% of VO2max does not appear to be necessary.
3. Longer HIIT work bouts will lead to greater performance gains. The meta-analysis included studies with work bouts that ranged from one minute to 5 minutes in duration. The results showed there to be approximately a 0.5% improvement in time-trial performance for every additional minute per work bout. Since it takes approximately two minutes to reach peak oxygen levels, longer duration intervals may allow for exercise at a high percentage of VO2max for a longer continuous period of time. Although the exact mechanistic adaptations have yet to be determined, it is believed that a greater time spent at high levels of oxygen consumption will produce greater aerobic adaptations. An alternative hypothesis is that longer intervals may improve an individual’s anaerobic metabolism by improving the ability to clear metabolites.
4. Maximal gains in performance with SIT only require two weeks of training, a similar timeframe to that of an explosive strength training program. Both SIT and explosive strength training have been shown to improve endurance sport performance. Interestingly, the adaptive responses and the time frame for the responses are very similar between the two modes of exercise. Heavy and explosive resistance exercise has been shown to improve VO2 max, and increase measures of mitochondrial protein content, exercise economy, and the rate of force development. A direct comparison of SIT with resistance exercise showed no difference in the improvements in VO2 max or submaximal exercise performance. Furthermore, changes in force development with strength training can occur in as little as two weeks. This is similar to the minimal timeframe observed for changes in time trial performance with SIT. The primary difference between resistance training and SIT is that SIT can be performed in the same exercise mode as a TT test, making it a sport-specific form of exercise.
5. Performing continuous training during an interval training program (HITT or SIT) may be detrimental to performance in trained individuals. The results of the meta-analysis showed that an interval training only program (HIIT or SIT) led to a 2% greater improvement in time-trial performance when compared to programs that included continuous training. These results were unexpected, to say the least. As an exercise physiologist, triathlete coach, and recreational athlete, I have always incorporated long duration bike rides or runs in a training program. An important limitation to these results is that the exercise intensity and the total number of continuous training sessions were not clearly defined. A previous meta-analysis of mine demonstrated that training programs that incorporated a polarized model led to greater improvements in time-trial performance than a threshold model. The key difference between the two models is that a polarized model includes continuous training performed at very low intensities (<lactate threshold), whereas a threshold model includes continuous training performed near threshold levels (between the lactate threshold and critical power).
What Does This Mean for Your Interval Training
The results of the meta-analysis have far-reaching implications for the development and implementation of evidence-based guidelines and best practices for training programming designed to optimize endurance sport performance. The current findings can be used to formulate uniquely designed programs that take into account both discrete individual characteristics and the variables associated with an interval training protocol.
Michael Rosenblat, PT, PhD, CEP, runs Evidence Based Coaching in Canada to prepare athletes for national and international competition.