EMG changes in human thenar motor units with force potentiation and fatigue.

Few studies have analyzed activity-induced changes in EMG activity in individual human motor units. We studied the changes in human thenar motor unit EMG that accompany the potentiation of twitch force and fatigue of tetanic force. Single motor unit EMG and force were recorded in healthy subjects in response to selective stimulation of their motor axons within the median nerve just above the elbow.

Incidence of F waves in single human thenar motor units.

F-wave generation, axon conduction velocities, and contractile properties were compared in 44 healthy individual human thenar motor units. Force and muscle action potentials were recorded when single motor axons were stimulated intraneurally about 10 cm proximal to the elbow. Each stimulus usually evoked only one electromyographic (EMG) potential.

Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor.

1. While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean +/- s.

Muscle fatigue induced by stimulation with and without doublets.

Muscles are usually stimulated by shocks delivered at some constant rate. However, human thenar motor units generate optimum force per pulse when excited by impulse trains that begin with one or two short interpulse intervals ("doublets"), followed by longer intervals. Our aim was to determine whether the rate of force and force-time integral reduction during fatigue of thenar muscles is influenced by an initial doublet, and/or the number of pulses per train.

Pattern of pulses that maximize force output from single human thenar motor units.

We assessed the sequence of nerve impulses that maximize force output from individual human thenar motor units. When these motor units were stimulated intraneurally by a variable sequence of seven pulses, the pattern of pulses that elicited maximum force always started with a short (5-15 ms) interpulse interval termed a "doublet. " The twitch force summation caused by this "doublet" elicited, on average, 48 +/- 13% (SD) of the maximum tetanic force.