Physiological characterisation of the "warm up" effect of activity in patients with myotonic dystrophy.

Contractile properties of adductor pollicis muscle were examined over a range of stimulation frequencies in patients with myotonic dystrophy and normal subjects. In patients, fresh muscle demonstrated impaired relaxation, weakness at all frequencies and selective loss of force and excitation at high frequencies. During stimulated "fatiguing" activity, patients showed improvements in force and relaxation which appeared to result from normalisation of membrane excitation. Normal twitch potentiation also occurred during activity suggesting intact excitation-contraction coupling. These electrophysiological findings help to characterise and explain the "warm up" effect described by patients.