Isometric and isotonic contractions

When muscles contract they exert a force on whatever they are attached to (this force is equal to the tension in the muscle) and they shorten if they are permitted to do so. Hence we can measure two different variables during the contraction of a muscle: its length and its tension. Most often one of these two is maintained constant during the contraction. In isometric contractions the muscle is not allowed to shorten (its length is held constant) and the tension it produces is measured. In an isotonic contraction the load on the muscle (which is equal to the tension in the muscle) is maintained constant and its shortening is measured.

Strain gauge

Strain gauge

Fig. 9.2. An isometric lever system for measuring the force exerted by a muscle without allowing it to shorten. Semiconductor strain gauges are bonded to a steel bar (a), and form two arms of a resistant bridge connected to a battery (b).

An isometric recording device has to be stiff, so that it does not in fact allow the muscle to shorten appreciably while the force is being measured. A simple method is to use a lever which is attached to a stiff spring and writes on a smoked drum. A more sophisticated device consists of a small steel bar to which semiconductor strain gauges are bonded. The electrical resistance of the strain gauges then varies with muscle tension and so can be used to give an electrical measure of the tension, and this can then be displayed on an oscilloscope or a chart recorder (Fig. 9.2). The force exerted by the muscle is usually measured in newtons or grams weight.

Isotonic recording devices usually consist of a moveable lever whose motion can be recorded either directly on a smoked drum or indirectly via an electrical signal. The lever can be loaded to different extents, perhaps by hanging weights on it. Fig. 9.3 shows a typical arrangement.

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