Firing rate interactions among human orbicularis oris motor units

Groups of human motor units from the same muscle exhibit joint fluctuations in firing rate during voluntary muscle contraction. In an effort to determine whether similar behavior would be observed in a muscle lacking muscle spindles, motor unit firing behavior was examined in the human orbicularis oris inferior (OOI) during mild voluntary effort. Motor unit activity was recorded with a quadrifilar needle inserted in the OOI. Firing occurrences were identified using a motor unit decomposition procedure. Cross-correlation of motor unit firing rates revealed a tendency for motor unit firing rates to covary, although the effect was somewhat more variable than that observed previously in other skeletal muscles. There was also a statistically significant tendency for pairs of motor units to fire at simultaneous or near-simultaneous (+/- 5 ms) intervals (synchronization). Firing rate variability in OOI motor units was not significantly different (p > .05) from that observed in the FDI. Thus, the present results suggest that the common drive of human motor unit activity may not depend on the presence of muscle spindles.