Neurophysiology of fastest voluntary muscle contraction in hereditary neuropathy

Abstract In patients with hereditary motor and sensory neuropathy types I (demyelinative) and II (neuronal) and in normal subjects isomatic force and electromyographic activity of the first dorsal interosseous muscle were recorded during fastest voluntary contractions and during twitches evoked by nerve stimulation. The maximum voluntary force of the first dorsal interosseous muscle was also measured. In patients, fastest voluntary contraction time (i.e., time from ooset of contraction to peak force) was prolonged and inversely proportional to maximum voluntary force. Maximum rate of rise of tension (i.e., slope of rise in force) was reduced and directly proportional to maximum voluntary force. In patients with hereditary motor and sensory neuropathy type I, contraction time was longer and the maximum rate of rise of tension was lower than in those with hereditary motor and sensory neuropathy type II. In patients and normal subjects, voluntary contraction time was closely correlated with the duration of electromyographic bursts. In patients, the twitch contraction time was prolonged and inversely proportional to maximum voluntary force. Twitch contraction amplitude was diminished and directly proportional to maximum voluntary force. Neither twitch contraction time nor amplitude were dependent on the type of hereditary motor and sensory neuropathy. Twitch contraction time evoked by proximal nerve stimulation was minimally longer than that evoked by distal stimulation. In fastest voluntary muscle contraction performed by patients with hereditary motor and sensory neuropathy, it is argued that: (1) Decrease in maximum rate of rise of tension is due to a reduced number of motor units, an increase in motor unit contraction time, and possibly lower motor unit firing rates in type I compared to type II neuropathy patients. (2) The central nervous system appears to adapt to a decreased rate of rise of tension by prolonging duration of the agonist electromyographic burst, thus lengthening contraction time. This may result in the higher peak forces than if burst duration were normal.