1. The effect of up to 2.5 h circulatory occlusion on muscle pH and energy metabolism of the quadriceps muscle was studied in 13 patients operated on for an inveterate knee injury. Muscle samples were taken by the percutaneous needle biopsy technique at different times during the arterial occlusion and after restoration of circulation.

2. Energy was provided by a complete utilization of phosphocreatine and by a low rate of glycolysis during 2 h of occlusion. The ATP level remained unchanged throughout the occlusion period.

3. Muscle pH decreased from 7.0-7.1 to about 6.8 after 2 h occlusion. Muscle pH was essentially normalized within 20 min after restoration of the blood flow.

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