Work and power

Mechanical energy is measured as work and the rate of performing work is called power. In an isotonic contraction, therefore, the work done is equal to the force exerted by the muscle multiplied by distance shortened, and the power output at any instant is equal to the force multiplied by the velocity of shortening.

There is no work done by the muscle as a whole during an isometric contraction since there is no shortening, and there is none done during an

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Fig. 9.10. How work varies with load in isotonic twitches.

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Fig. 9.10. How work varies with load in isotonic twitches.

unloaded isotonic contraction since the force exerted is zero. Power will also be zero under both these conditions. Fig. 9.10. shows how the work done in isotonic twitches (measured between the onset of contraction and its peak) varies with the load. During the relaxation process in an isotonic twitch, the load does work on the muscle and so the overall work output during the whole of the isotonic twitch is zero.

The power output during the initial shortening phase of tetanic isotonic contractions is readily calculated from the force—velocity curve, since power is force times velocity. As shown in Fig. 9.11, maximum power is achieved when the load is about 0.3 times the isometric tension, when the muscle will shorten at about 0.3 times its maximum (unloaded) velocity. This has implications for the selection of gears in cycle races: whatever the speed of the cycle, maximum power is obtained from the leg muscles when they are contracting at about 0.3 times their maximum unloaded velocity, which probably corresponds to about two revolutions of the pedals per second.

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