The influence of intramuscular glycogen concentration on performance during maximal intermittent exercise
Date
1990
Authors
Holmyard, David James
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Abstract
It was the purpose of this study to evaluate the effects of muscle glycogen concentration on maximal intermittent exercise performance. Ten male subjects performed an intermittent exercise test (IET) consisting of ten bouts of eight maximal isokinetic leg extensions during each of two conditions. Each bout last:e4 approximately six seconds and there was a thirty second rest interval between each bout. The order in which subjects undertook the treatment conditions was randomised. On the morning of day 1 of the unrepleted (U) condition subjects performed strenous exercise designed to deplete the glycogen concentration of the right leg extensors. After consuming a low carbohydrate diet for approximately 48 hours the subjects performed the IET on the morning of day 3. The repleted (R) condition was identical to the U condition, except that a normal balanced diet, rich in carbohydrates was consumed during days 1 and 2. Biopsies from the vastus lateralis before the IET confirmed a difference in intramuscular glycogen between conditions (67.6±23.2 vs 113.1±27.7 mmol glucosyl units-kg-I w.w. for U and R respectively, p < 0.01). A two-way analysis of variance showed significant (p < 0.001) decreases in performance with increasing bout number for both conditions, but no significant difference was found between the conditions. Nevertheless when the first three exercise bouts were considered alone, a significantly greater amount of work (p < 0.05) was produced during the first three bouts of the IET in the R condition. This was largely attributable to the eccentric component of contraction. Since a reduced eccentric performance in the IET is possibly due to the muscle soreness associated with prior eccentric contraction, it was not possible to attribute the impaired performance in the U condition solely to a reduced availability of glycogen for energy supply. However the significantly lower blood lactates (p < 0.05 - p < 0.01) and lower rates of muscle glycogen utilization (p < 0.001) observed in the U versus the R condition, indicated a reduced rate of glycolysis during the U condition. It was concluded that when muscle glycogen is available (i.e. in the R condition) it makes a substantial contribution to energy supply during high intensity intem1ittent work. Furthermore when glycogen stores are reduced (i.e. in the U condition) then the reduced glycolytic contribution may be compensated for by increased use of PCr and ATP stores.