Electrical and mechanical responses in the platysma and in the adductor pollicis muscle: in normal subjects.

In the platysma of 34 normal subjects the amplitude of the action potential and twitch tension and tetanic tension were lower, the contraction time of the isometric twitch was 1.4 times shorter, and the potentiation of twitch tension in a staircase and after tetanus was two to four times greater than in m. adductor pollicis. Differences in twitch kinetics and potentiation were related to the four times higher incidence of fast fibres in the platysma than in the adductor pollicis muscle, as determined by histochemistry. Ninety-five per cent confidence limits were established for comparison with patients with myasthenia gravis.