EMG activation patterns during force production in precision grip. III. Synchronisation of single motor units

Motor unit (MU) synchronisation during isometric force production in the pr ecision grip was analysed in five subjects performing a visually guided ste p-tracking motor task with three different force levels. With this aim mult i-unit electromyographic (EMG) activity of 14 intrinsic and extrinsic finge r muscles from 15 experimental sessions was decomposed into the potentials of single MUs. The behaviour of 62 intrinsic and 30 extrinsic MUs in the mo tor task was quantified. Most MUs displayed a positive correlation between firing rate and grip force. Compared to MUs in extrinsic muscles, intrinsic MUs had steeper regression lines with negative intercepts indicating highe r force sensitivity and higher recruitment thresholds. A cross-correlation analysis was performed for 69 intra- and 166 intermuscular MU pairs while s teady grip force was exerted at the three force levels. Synchronisation, fo r at least one force level, was found in 78% of the intra- and 45% of the i ntermuscular pairs. The occurrence of synchronisation was not stable over t he force range tested. Factors influencing the fluctuations in occurrence a nd strength of synchronisation were investigated. Force increase was not pa ralleled by increased synchronisation; in contrast, in most MU pairs, espec ially intermuscular pairs, synchronisation occurred preferentially at the l ower force levels. The recruitment threshold appeared to play a determining role in synchronisation: the more similar the thresholds of two MUs, the g reater the probability of them being synchronised at this force level. Sync hronised MUs fired on average at a lower frequency than non-synchronised on es. Finally, synchronisation at the multi-unit EMG level does not indicate that all underlying MUs are synchronised, nor does the absence of temporal coupling at the multi-unit level indicate that none of the MUs is synchroni sed.