SUMMARY1. Glycogen depletion pattern in human skeletal muscle fibres was studied after bicycle exercise of varying intensity performed at different pedalling rates. Work intensities studied were equivalent to 30-150 % of 70o, max. with pedalling rates of 30-120 rev/mi.2. Glycogen depletion increased dramatically with increasing exercise intensity; depletion was 2-7 and 7-4 times greater respectively at workloads demanding 64 and 84 % ro2 max. than at workloads calling for 31 % 1702 max. Even greater rates of glycogen utilization occurred at supramaximal loads.3. Slow twitch, high oxidative (ST) fibres were the first to lose glycogen (reduced PAS staining) at all workloads below T°2 max. Progressive glycogen depletion occurred in fast twitch (FT) fibres as work continued. Large quantities of glycogen remained in the muscle after 3 hr of exercise at low exercise intensity. This was almost exclusively found in FT fibres. At workloads exceeding maximal aerobic power, there was an initial depletion of glycogen in both fibre types. Varying the pedalling rate and, thus, the total force exerted in each pedal thrust had no effect on the pattern of glycogen depletion in the fibres. 4. Results point to primary reliance upon ST fibres during submaximal endurance exercise, FT fibres being recruited after ST fibres are depleted of glycogen. During exertion requiring energy expenditure greater than the maximal aerobic power, both fibre types appeared to be continuously involved in carrying out the exercise.

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SUMMARY1. Six healthy males performed sustained contractions with different tensions related to their maximal voluntary contraction (MVC). The isometric exercise consisted of efforts to extend the knee when flexed at an angle of 900.2. Biopsy samples were taken from the lateral portion of M. quadriceps femoris before and after different periods (6--45 min) during a series of sustained contractions. Total glycogen content was determined on each muscle sample. In order to evaluate whether the glycogen depletion occurred preferentially in slow twitch (ST) or fast twitch (FT) fibres, serial sections of the muscle samples were stained for myofibrillar ATPase and glycogen (PAS reaction).3. In all experiments a selective glycogen depletion was observed. At low tensions, the ST fibres and at higher tensions the FT fibres became glycogen depleted. The critical tension at which this conversion in glycogen depletion from ST to FT fibres took place was 20 % MVC.4. It is concluded that at sustained contractions of less than 20 % MVC there is a major reliance upon ST fibres and above that level a primary dependence upon FT fibres. It is further suggested that restriction of blood flow and thus availability of oxygen at forces higher than 20 % MVC may be the explanation for the present findings.

PIEHL, K. T i m e course for refilling of glycogen stores in h u m a n muscle fibres following exercise-induced glycogen depletion. Acta physiol. scand. 1974. 90. 297- 302.Muscle biopsy samples were obtained from the thigh muscle of 4 subjects before and after 2 h of work and a t selected intervals during the following 46 h when a carbohydrate enriched diet was consumed. Mean glycogen content declined 103 (from 125 to 22) mmol glucose units x kg-l following exercise. 5 and 10 h after consuming the carbohydrate enriched diet muscle glycogen increased to 64 and 86 mmol glucose units X kg-l, respectively. During the first 5 h there was a marked storage of glycogen in the muscle which was related to the carbohydrate intake, but pre-exercise concentrations of muscle glycogen were observed first after 46 h. T h e increase in glycogen occurred in both fibre types, but the fast twitch fibres replinished their glycogen somewhat faster than did the slow twitch fibres suggesting a higher glycogen synthetase activity. At glycogen concentrations above 80-90 mmol no differences in the glycogen content of the two fibre types could be discerned with histochemical methods.

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