The influence of muscle pain and fatigue on the activity of synergistic muscles of the leg

The purpose of this study was to investigate the effect of experimentally induced muscle pain on the motor-control strategies of synergistic muscles during submaximal fatiguing isometric contractions. The root mean square (RMS) and median frequency (MF) of the surface electromyographic (EMG) signal from synergistic plantarflexors and dorsiflexors were assessed to exhaustion. Ten subjects performed sustained dorsiflexions and plantarflexions at two contraction levels, 50% and 80% of maximum voluntary contraction, with or without muscle pain, induced by injection of 6% hypertonic saline in one synergist. In the painful contractions, the RMS of the EMG signal was decreased compared to the control condition in the initial phase of the contraction, in the muscles where pain was induced as well as in the nonpainful synergists. Moreover, the EMG signal MF decreased faster during muscle pain than in the control condition. The endurance time was shorter during muscle pain, and some of the nonpainful synergists showed increased compensatory activity at the end of the contractions to maintain the target force. The decreased EMG activation during pain was coupled with significantly decreased torque levels during the painful condition that would partly explain the results. However, the ratio between force and EMG amplitude was decreased for both the painful and nonpainful synergists, so other mechanisms might explain the present findings. This study shows that localized muscle pain can reorganize the EMG activity of synergists where no pain is present. These findings may have implications for the understanding of manifestations seen in relation to painful musculoskeletal disorders.