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HIT TRAINING
exercise and there is a greater contribution from oxidative metabolism to fuel the last part of the sprint. These changes tie in with the rapid rise in heart rate seen from 15 to 30 seconds in the cycle sprint.
HIT and endurance performance One of the most remarkable findings of recent research into HIT is the dramatic improvement that this type of exercise can have on exercise performance despite the dramatically lower training volume. A group of sub-elite males (who normally trained aerobically) performed six sprint sessions following a progressive HIT protocol similar to the one shown in Table 1, and this resulted in a doubling of the time that could be spent cycling at 80% of pre-training VO2 max (1). However, we know that this type of training does not dramatically change VO2 max in participants who have a high level of aerobic fitness, which suggests the majority of adaptations to this type of training are in the periphery and makes the magnitude of the improvements all the more impressive. Further, in this study a control group, who carried on with their normal training and did not participate in a HIT sprint programme, demonstrated no change in performance. Therefore it is safe to assume that the magnitude of the change is due to a real training effect rather than a learning effect associated with performing the same test twice in two weeks. Subsequent studies have all used tasks that are more similar to normal athletic competition, including cycling time trials and simulated races. All of the subsequent research has demonstrated an improvement in aerobic performance normally in the region of 10–20% after just two weeks of training.
How does HIT improve performance? The factors responsible for the improvement in performance that are seen with HIT are complex and will be determined by many different physiological factors.
Glycogen utilisation A novel aspect of HIT is the relatively large muscle mass that is being recruited during the exercise. This increased muscle recruitment, compared to traditional endurance exercise, in turn would ensure glycogen breakdown due to the fuel demands of the sprint (Figure 1). Therefore there is a greater turnover of glycogen occurring within a greater proportion of fibres than in traditional endurance training. Muscle contraction under conditions of metabolic stress (such as that incurred during HIT) results in a very rapid depletion of glycogen and leads to super-compensation during the recovery phase with skeletal muscle glycogen content increased by approximately 33%. This suggests that there is greater fuel availability as, although the adenosine triphosphate and phosphocreatine stores in the muscle remain the same size as at the start of fixed intensity exercise, a smaller proportion of it is utilised following HIT. Also, power production during exercise is strongly related to skeletal muscle glycogen content and following two weeks of HIT participants’ peak power output is generally increased by 100W.
Lactate utilisation One of the ways that endurance exercise is thought to