Task dependency of motor adaptations to an acute noxious stimulation

This study explored motor adaptations in response to an acute noxious stimulation during three tasks that differed in the number of available degrees of freedom. Fifteen participants performed three isometric force-matched tasks (single leg knee extension, single leg squat, and bilateral leg squat) in three conditions (Control, Pain, and Washout). Pain was induced by injection of hypertonic saline into the vastus medialis muscle (VM; left leg). Supersonic shear imaging was used to measure muscle shear elastic modulus as this is considered to be an index of muscle stress. Surface electromyography (EMG) was recorded bilaterally from six muscles to assess changes in neural strategies. During tasks with fewer degrees of freedom (knee extension and single leg squat task), there was no change in VM EMG amplitude or VM shear elastic modulus. In contrast, during the bilateral leg squat, VM (-32.9 ± 15.8%; P < 0.001) and vastus lateralis (-28.7 ± 14.8%; P < 0.001) EMG amplitude decreased during Pain. This decrease in activation was associated with reduced VM shear elastic modulus (-17.6 ± 23.3%; P = 0.029) and reduced force produced by the painful leg (-10.0 ± 10.2%; P = 0.046). This work provides evidence that when an obvious solution is available to decrease stress on painful tissue, this option is selected. It confirms the fundamental assumption that motor adaptations to pain aim to alter load on painful tissue to protect for further pain and/or injury. The lack of adaptation observed during force-matched tasks with fewer degrees of freedom might be explained by the limited potential to redistribute stress or a high cost induced by such a compensation.