Changes in muscular excitability and contractility caused by fatigue in Steinert's disease.

To study effects of fatigue on muscle excitability and contractility in myotonic dystrophy (MyD), we evaluated, by ulnar nerve supramaximal stimulation, both single shock and 40 Hz tetanus, M wave and force parameters from adductor pollicis. In 8 MyD patients and in 6 controls amplitude of M wave, electromechanical delay, single twitch and tetanus tension, contraction and half-relaxation times were recorded in basal condition and at different times after 75 sec. of maximal voluntary contraction. Reduction of force related to fatigue was per cent lesser in MyD compared to controls. Electro-mechanical delay, basally longer in MyD, showed after fatigue 15% increment compared to 47% in controls. Half-relaxation time increased in both groups, but in MyD recovery was faster. Peculiar alterations of excitation-contraction coupling and contractility occurring in MyD can explain the observed modifications of fatigue phenomena in this disease.