For bodybuilders and other strength/power athletes, the adaptations to interval training
may be detrimental to performance. Several studies have reported a shift of Type IIb muscle
fibers towards Type IIa characteristics with intensive interval training (6,7,8). Therefore,
individuals wishing to maximize muscle size or strength are advised to stick with low intensity
aerobic activity (below LT) under most circumstances (9). Specific guidelines for frequency of
interval training is discussed in chapter 25.
Section 2: Hormonal response to interval training
The hormonal response to interval training is very similar to aerobic exercise. The higher
intensity nature of interval training simply causes a greater hormonal response to be seen as
compared to lower intensity aerobic training.
Adrenaline and noradrenaline both cause an increase in glycogen breakdown in the liver
and muscles and increases in blood glucose and free fatty acids (2). Exercise above the LT
causes a significant increase in both hormones. Blood glucose increases during interval training
due to increased output of liver glycogen. This is further discussed in chapter 21.
Despite the increase in blood glucose during interval training, insulin levels still go down.
However, if blood glucose is elevated following exercise, insulin levels will increase to drive blood
glucose into the muscles. The increase in blood glucose and insulin may de-establish ketosis for a
brief period after exercise.
As mentioned previously, growth hormone (GH) helps to control fuel mobilization by
increasing fat breakdown and decreasing glycogen and protein use. Interval training significantly
increases GH levels most likely by raising lactic acid levels (10,11).
Section 3: Energy Metabolism
By its very nature, interval training will rely on anaerobic energy metabolism, namely
anaerobic glycolysis and the phosphagen energy system. Both are described below.
The Phosphagen System
Recall that ATP is the only fuel that muscles can use directly and all other energy systems
have as their ultimate goal ATP production. When muscle contracts, ATP provides energy by
being broken down to adenosine diphosphate (ADP) in the following reaction with the help of an
enzyme called an ATPase as shown in figure 1.