Influence of contraction strength on single motor unit synchronous activity.

Objective: The influence of contraction strength on motoneurone (MN) synchrony is poorly documented. With stronger contraction, more common and/or synchronized inputs might contribute to greater MN drive and generate more synchronous firings. This effect might be counterbalanced, however, by a negative impact of MN faster firing rates on synaptic effectiveness.

Methods: Pairs of motor units (MUs) were tested at various force levels, in 2-s sequences. MN synchrony was assessed using the index k', the synchronous impulse probability (SIP), and the synchronous impulse frequency (SIF) in cross-correlograms. MU inter-spike interval duration and variability, surface EMG activity and force output were evaluated concurrently.

Results: Both SIP and SIF increased with contraction strength, whereas k' remained unaffected. Faster firing rates and stronger contraction had the greatest effects on SIF.

Conclusions: By testing the same MUs at different force levels, we showed that contraction strength does influence MN synchrony. The enhancement of MU synchrony with stronger contraction suggests an efficient contribution of more common and/or synchronized inputs.

Significance: Force output must be controlled when assessing MN synchrony. Normalizing MU synchronous activity per reference spike is preferable to minimize the influence of firing rate. This is particularly relevant for clinical research, in conditions of poorer neuromuscular control.