MU firing characteristics in human dystrophic muscle.

During isometric contractions of constant force surface EMG as well as intramuscular MU potentials from extensor digitorum communis and biceps brachii muscles were recorded on magnetic tape for further off-line analysis. Surface EMG power spectra were computed and transformed so as to reveal low-frequency peaks which might correspond to MU firing rates. From intramuscular recordings, single MU action potentials were identified with an aid of semi-automatic recognition program. For each single MU action potential train (MUAPT) statistical parameter of interspike intervals (ISIs) were determined and related to the measured muscle force level. Sixty four MUAPTs from 9 patients and 55 MUAPTs from 4 normals were analysed so far. The estimates of MU firing rate from surface and needle EMG corresponded well to each other. The MU firing rates were higher in muscular dystrophy and this difference was more pronounced for higher levels of muscle force. The tendency towards mean MU firing rate increase is stronger for the patients with more advanced disease. The typical dependency of standard deviation of ISIs on their mean value may be approximated by two lines of different slope. There were reported experimental data indicating that the breaking point of this dependency may be an estimate of AHP duration in motoneurones. Our results for dystrophic muscle showed a shift of this point towards shorter ISIs, as compared to normals. This suggests that in muscular dystrophy also motoneurones may be altered, either by the disease itself or as a compensation for changes in muscular part of a MU.