It has been observed that
muscle pain influences force variability and low-frequency (<3 Hz) oscillations in the neural drive to muscle. In this study, we aimed to investigate the effect of experimental
muscle pain on the neural control of muscle force at higher frequency bands, associated with afferent
feedback (alpha band, 5-13 Hz) and with descending cortical input (beta band, 15-30 Hz). Single-motor unit activity was recorded, in two separate experimental sessions, from the abductor digiti minimi (ADM) and tibialis anterior (TA) muscles with intramuscular wire
electrodes, during isometric abductions of the fifth finger
at 10% of maximal force [maximum voluntary contraction (MVC)] and ankle dorsiflexions at 25% MVC. The contractions were repeated under three conditions: no
pain (baseline) and after
intramuscular injection of isotonic (0.9%, control) and hypertonic (5.8%, painful) saline. The results showed an increase of the relative power of both the force signal and the neural drive at the
tremor frequency band (alpha, 5-13 Hz) between the baseline and hypertonic (painful) conditions for both muscles (P < 0.05) but no effect on the beta band. Additionally, the strength of motor unit coherence was lower (P < 0.05) in the hypertonic condition in the alpha band for both muscles and in the beta band for the ADM. These results indicate that experimental
muscle pain increases the amplitude of the
tremor oscillations because of an increased variability of the neural control (common synaptic input) in the
tremor band. Moreover, the concomitant decrease in coherence suggests an increase in independent input in the
tremor band due to
pain.