Contractile properties of muscles in myotonic dystrophy.

A study has been made of the contractile properties of plantarflexor and dorsiflexor muscles in 25 patients with myotonic dystrophy and in the same number of closely-matched control subjects. As anticipated, the mean torques developed during maximal voluntary contraction and during the isometric twitch were significantly reduced in the patient population, as were the mean amplitudes of the respective maximum muscle compound action potentials (M-waves). There was considerable variation in weakness between patients, however, and in some there was a striking discrepancy between the results for the plantarflexor and dorsiflexor muscles. It was also found that, in both muscle groups, the mean twitch contraction times were significantly shorter in patients than in controls, but no differences could be demonstrated in relation to fatiguability and post-activation of the twitch. Some patients had great difficulty in obtaining full activation of plantarflexor motor units but there was improvement with repeated effort.