The striking result was the blood lactate levels in the subjects exercising for the shortest time did not rise, whereas blood lactate levels in the subjects exercising for 60 seconds each interval rose progressively.
The explanation for this difference would be that the group exercising for only 10 seconds each interval utilized mainly their intracellular ATP and creatine phosphate stores. During recovery, these stores were rapidly replenished by ATP produced in the mitochondria. Because glycolysis was only marginally activated during this exercise, muscle glycogen levels fell only very gradually (see Exercises 3.12a).
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In contrast, the group exercising for 60 seconds per interval activated glycolysis maximally (Exercises 3.11) and produced more lactate and protons during exercise than they could metabolize during the recovery period. Had the researchers also measured muscle glycogen levels, they would likely have found these levels to have fallen precipitously (Exercises 3.12a); thus, both very high muscle proton levels (and therefore low muscle pH) and low muscle glycogen levels would have terminated the interval session.
Subjects in the middle group showed little change in blood lactate levels after the first 3 exercise bouts. This indicates that glycolysis was initially stimulated but that the rate of lactate removal from muscle during the rest period equaled the rate of lactate production during the preceding interval. Thus a steady state was reached in which muscle and blood pH levels did not change further. Glycogen depletion in the muscle, rather than pH changes, would ultimately limit this type of exercise.
The practical implication of this study is that you should alter the length or intensity of the training interval according to which metabolic pathways you wish to train. Exercise to exhaustion in less than 6 to 10 seconds stresses creatine phosphate and glycolytic metabolism maximally and is probably limited by the accumulation of Pi, H+, and Mg++; blood lactate levels remain relatively low.
During intervals lasting 6 to 30 seconds glycolysis is the predominant energy source, but the recovery period allows sufficient time for removal of all the lactate and protons produced during the exercise period. Thus, steady-state blood lactate levels are reached after two or three intervals. The benefit of this form of interval training may be to speed up the rate of lactate removal from muscle during the recovery period. .
Finally, interval sessions of longer than 30 seconds cause a maximum contribution of glycolysis to energy production resulting in a progressive accumulation of lactate and protons, because recovery metabolism is too slow to prevent their accumulation. This type of interval training stresses the buffering systems of muscle maximally and will adapt the muscle for continued performance at low pH levels (Sahlin & Henrikksson, 1984).