A study of repeated lateral pinch grip in myotonic dystrophy

Subjects with myotonic dystrophy present with progressive muscle weakness, myotonia and fatigue. The aim of this study was to determine whether there was a difference in response to fatiguing exercise in myotonic dystrophy individuals compared to normal subjects. If no difference was found, a similar response to a physiotherapy exercise programme as seen in normal subjects might be expected.

Ten individuals with myotonic dystrophy were compared to eight normal subjects in their response to ten repetitions of maximal lateral pinch grip efforts each five seconds in duration and separated by a ten-second rest. The root mean square (RMS) values, initial median frequency (Fmed) and slope of the median frequencies were recorded by electromyography (EMG) for the first dorsal interosseus, flexor pollicis brevis, flexor digitorum profundus and extensor digitorum communis muscles in the forearm and hand. Simultaneously, the rate of grip development, rate of grip release and work done during each grip effort were recorded by dynamometer. The RMS values for all muscles from subjects with myotonic dystrophy increased over the ten repetitions.

The initial Fmed for all myotonic dystrophy muscles was lower than for the normal subjects. The Fmed slopes for the first and last repetition showed no significant difference to the normal subjects. Rate of grip development was no different between groups over ten repetitions. Rate of grip release was slower and work done less for the individuals with myotonic dystrophy.

Results suggest the main difference between muscles affected by myotonic dystrophy and normal ones was the smaller size of muscle fibres. The increase in rate of grip release that was found is supportive of the 'warm-up' phenomenon. This appears to indicate that muscles affected by myotonic dystrophy could benefit from standard physiotherapeutic exercise methods.