Muscular fatigue

The force exerted during a maximal voluntary isometric contraction in man begins to decline after a few seconds. A similar contraction in which the muscle exerts a tension of 50% of the maximum can be maintained for about a minute, and one of 15% for more than ten minutes. This inability to maintain the tension at a particular level is called fatigue. It is usually accompanied by feelings of discomfort and perhaps pain in the muscle.

Experiments by P. A. Merton and his colleagues on the adductor muscle of the thumb suggest that fatigue is largely a feature of the contractile machinery of the muscle cell. The force in maximal voluntary isometric contractions declined to less than half after 1 to 3 minutes, but there was no reduction in the size of the muscle action potential produced by electrical stimulation of the motor nerve. Maximal force could not be restored by massive direct stimulation of the muscle fibres.

It seems likely that the causes of fatigue are connected with changes in the fuel supply for the contractile machinery of the muscle; the precise details are not yet clear. Fatigue may well act in part as a protective mechanism, avoiding the reduction of ATP concentrations to dangerously low levels: rigor mortis occurs in the absence of ATP.

Fatigue is more rapid for isometric than for isotonic contractions. Strong isometric contractions result in increased pressure in the muscle, which reduces and may completely block the blood flow through it. Consequently the muscle runs out of oxygen and hence its energy supply is soon used up. Rhythmic contractions can be maintained for a much longer time since they produce only intermittent interruptions in the blood supply.

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