RELATIVE EFFECTS OF GLYCOGEN DEPLETION AND PREVIOUS EXERCISE ON MUSCLE FORCE AND ENDURANCE CAPACITY

Endurance capacity of human vastus lateralis muscles was observed 24 h after hard exercise followed by either a carbohydrate-restricted or a carbohydrate-loaded diet (depletion and repletion conditions). In a control condition the subjects did no previous exercise and ate their normal diet. Each of these conditions was followed by an experimental protocol in which the five male subjects made a series of alternating 25-s static contractions of each leg at 50% maximal voluntary contraction until one leg failed to achieve the required force (T(lim)). Glycogen concentration before the experimental protocol in both legs was significantly lower in the depletion than in the repletion condition. Muscle lactate and creatine phosphate concentrations were within normal limits before the static contractions. The number of contractions the repleted (12.7 ± 2.2) and depleted (10.3 ± 1.5) legs could sustain before T(lim) were not different from each other, but both were 35% (P < 0.05) fewer than the control (17.6 ± 3.0). Surface electromyogram (EMG) amplitude was higher in depleted than in repleted or control muscles. At T(lim), EMG amplitude was maximal, creatine phosphate was 50-70% depleted, and lactate increased fourfold. Average glycogen utilization per contraction in both the repletion and depletion conditions was 5.8 mmol/kg dry wt, but postexercise lactate concentrations were lower in depleted (14.4 ± 3.6 mmol/kg dry wt) than in repleted (43.2 ± 7.4) muscles. The EMG frequency distribution shifted downward in all conditions during the experimental protocol and was independent of muscle lactate concentration. We have drawn the following conclusions about this form of exercise from these data: 1) previous exercise is a more potent determinant of muscle endurance capacity than glycogen availability, and 2) glycogen depletion, previous exercise, and lactate accumulation do not influence the shift in the EMG frequency distribution.