Energy metabolism and contraction force of human skeletal muscle in situ during electrical stimulation.

The quadriceps femoris muscles of nine volunteers were stimulated with intramuscular electrodes for 50 s. The stimulation frequency was 20 Hz and the voltage adjusted to produce an initial tension of 50-75% of the maximum voluntary contraction force. The force decreased after 25-30 s of contraction to reach a mean of 78% of the initial value after 50 s. During the stimulation up to four muscle biopsy samples were taken from each leg. The ATP turnover rate was initially 5.6 mmol kg-1 dry wt. s-1 and decreased during the later stages of contraction to 4.0 mmol kg-1 dry wt. s-1. The force declined at approximately the same rate as the ATP turnover during the contraction. The phosphocreatine (PCr) store decreased exponentially during contraction and was practically depleted at 50 s. Glycolysis began within 5 s after initiation of contraction. The rate of glycolysis measured as lactate accumulation increased with time throughout the entire contraction period.