In isotonic contraction, the muscle shortens without any change in its tension (or stiffness). To record isotonic muscle contraction, the load on the muscle must be zero. Hence, for isotonic recording, the recording lever should be as light as possible.

The muscle tendon is attached to the lever and the muscle is allowed to shorten freely. The shortening is recorded by the lever. On the other hand, for recording isometric increase in muscle tension, the load on the muscle must be immovable.

In practice, however, the muscle is made to produce a very small displacement of a very heavy load. The displacement, which is measured after magnification, gives an indirect measure of the degree of muscle tension. Isometric recording is made using a tension transducer which serves two purposes:

(i) it acts like an immovable load, allowing only minimal muscle shortening in response to large contractile forces;

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(ii) it produces a large, measurable current in response to small muscle shortening. The recorded current serves as an indirect index of isometric contraction.