Chapter 19:
Physiology of interval training
Interval training refers generally to any activity which alternates periods of high intensity
exercise with periods of lower intensity exercise. While interval training is most commonly
thought of as sprint training, activities such as basketball and football, which are of a stop and
start nature, should be classified as interval training as well. Although weight training is interval
in nature, it is discussed separately in the next chapter.
As was discussed in the previous chapter, the breakdown of glycogen during exercise can
yield either pyruvate or lactate. At low exercise intensities, glycogen breakdown produces
pyruvate which is used to generate more ATP. At higher intensities of exercise, lactate is
produced in greater and greater amounts and begins to accumulate in the muscle and
bloodstream.
Up to a certain point, the body can deal with the increased lactate by reusing it for fuel or
buffering it with bicarbonate to producing water and carbon dioxide. The point at which
production of lactic acid exceeds the body’s ability to cope with increasing amounts is referred to
as the lactate threshold (LT) and, in essence, reflects the body’s switch from primarily aerobic to
primarily anaerobic energy production.
Lactic acid causes fatigue by inhibiting muscular contraction and preventing further
energy production from glycogen breakdown. Thus, the duration of exercise above LT is limited.
Depending on the intensity of activity, the duration of activity will vary from twenty minutes or
more at LT to thirty seconds or less at maximal intensities.
The primary adaptation to interval training is to shift the LT to the right allowing higher
intensities of exercise to be performed before lactic acid buildup causes fatigue. This allows
endurance athletes to perform at higher intensities without fatigue.
Section 1: Adaptations to interval training
At intensities around LT, both Type I and Type IIa muscle fibers are recruited (1). As
exercise intensity approaches maximum, Type IIb fibers are also recruited (2,3,4). Similar to
aerobic exercise, the primary adaptation to interval training is an increase in the oxidative
capacity of the recruited muscle fibers (i.e. Type IIa and IIb fibers) (5). Additionally, with regular
exercise above LT, there is a gradual shift in LT to the right during exercise (5). That is, higher
exercise intensities can be sustained with less lactic acid buildup.
For endurance athletes looking to maximize performance, this is an important adaptation
as the majority of endurance events are performed close to or at the LT. Performance in
endurance events has improved greatly in the past 15 years despite little or no increase in
maximal aerobic capacity. This is explained by the greater amount of training time spent at or
above LT by modern endurance athletes (5).