The compound muscle action potential (MAP) and the isometric twitch force of the adductor pollicis of 25 normal subjects were registered after local cooling. Intramuscular temperature ranged from 36.6 to 18.0 degrees C. The amplitude of the MAP increased while the twitch force decreased at lower temperatures. The increase of the MAP could be caused by the effect of cooling directly at the muscle cell membrane. The decreased twitch force probably is due to an affect on the contractile apparatus.

The effect of local cooling has been studied in 27 normal subjects, 8 cases of myotonia congenita, 5 of myotonic dystrophy and one of paramyotonia. Using the adductor pollicis we registered the compound muscle action potential, the isometric twitch force and the time to half relaxation, the maximum tetanic force and to time 3/4 relaxation. 1. In normal subjects the twitch force and maximum tetanic force decreased after cooling (Fig. 2). The amplitude of the action potential increased. 2. Myotonia congenita and myotonic dystrophy were not aggravated by cooling. Muscle force was reduced only in the same proportion as in normal subjects (Fig. 2). The myotonic after-contraction was made normal by cooling (Figs. 5 and 6). 3. In paramyotonia initial tonic stiffness with a pronouncedly prolonged twitch relaxation occured directly after cooling (Fig. 1 B). Paradoxical myotonia occured only after exercise and was accompained by increasing paresis (Figs. 3 and 8). The results indicate that exposure to cold has a specific effect on muscle function only in paramyotonia.