Interval training is widely used in the training of endurance athletes. However, it is also frequently used in fitness settings. Programs like Cross Fit, P90X, Insanity, etc. all use variations of interval training. For endurance athletes, it has several benefits including teaching endurance athletes to be fast, improving oxygen consumption, providing variety, and increasing the athlete’s anaerobic threshold. In a fitness setting, it’s promoted as a “better” way to do cardio (i.e. do cardio without the cardio – that sort of thing).
Jacobs et al, in the September issue of the Journal of Applied Physiology, studied the impact of six interval training sessions on untrained individuals. Over two weeks, the subjects performed six interval training sessions on a cycle ergometer. The sessions were set up so that subjects performed 60 seconds of work at their peak power followed by 75 seconds of low-intensity cycling to recover. Over the first two weeks, the subjects did 8 of these; over the third and fourth week they did 10 of these, and over the fifth and sixth week they performed 12 of these. After two weeks:
- Peak oxygen consumption increased by 8%
- Peak power increased by 7%
- Mitochondrial content increased with training
The authors found that the improvements from training came about without a corresponding improvement in cardiac output, blood volume, or changes in the oxygen-carrying characteristics of the blood. In other words, the changes to performance seem to be the result in an increase in the mitochondrial content from training.
I rarely write this, but in this study untrained individuals are a great population to look at. This is because it will be easiest to see immediate adaptations from the training in this population. The adaptations that the authors found in this study are interesting because the performance improvements seem to stem from the changes to the mitochondrial content. It suggests that the mitochondrial changes might be the short-term adaptation to this type of training. Most studies and training programs that use interval training are 4-12 weeks in length or look at the impact of a single episode of interval training.
Now, it needs to be pointed out that the study used a cycle ergometer. This is not the same thing as running, sprinting, kettlebells, heavy ropes, push-ups, etc. As a result, the results only apply with certainty to this exercise mode used in this context – we cannot necessarily extrapolate to other exercise modes, other populations, or other training protocols.
Jacobs, R.A., Fluck, D., Bonne, T.C., Burgi, S., Christensen, P.M., Toigo, M., and Lundby, C. (2013). Improvements in exercise performance with high-intensity interval training coincide with an increase in skeletal muscle mitochondrial content and function. Journal of Applied Physiology, 115: 785-793.
