This single fibre EMG study compares the standard method of neuromuscular jitter measurement in voluntarily activated muscle to that by intramuscular electrical stimulation of motor axons in a group of normal subjects. The latter method avoids the interdischarge interval-dependent jitter, as well as a possible failure to recognise split muscle fibres. The mean MCD on axonal stimulation was only 5.2 microseconds less than in the voluntary activation study and was thus 8% more than theoretically expected for single motor end plates. The difference could be due to an axonal jitter and some other factors. Axonal stimulation has proved to be a relatively easy and reliable method for routine estimation of neuromuscular jitter, provided that the resolution of time measurement is better than 2 microseconds, so that low jitter due to occasional direct muscle fibre stimulation is not mistaken for a normal reading. The upper normal limits for the extensor digitorum communis muscle suggested by the present study are 40 microseconds (individual muscle fibres) and 25 microseconds (mean of 30 muscle fibres).
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