Experimental pain has a greater effect on single motor unit discharge during force-control than position-control tasks.

Objective: When matching target force during pain, single motor unit (SMU) discharge is modified in a manner thought to redistribute load in painful tissue. This adaptation might not be appropriate when maintaining joint posture against an external load. We compared changes in SMU discharge rate of knee extensor muscles in a force-control and a position-control task during pain.

Methods: Thirteen healthy adults (31±6years) performed position- and force-control contractions using matched loads in non-pain and pain states. Pain was induced by injection of hypertonic saline into the infrapatellar fat pad. Intramuscular and surface electromyography of knee extensor and flexor muscles was recorded.

Results: When considering the discharge of a select population of SMUs that were recorded across all conditions performed on the same day, there was a decrease in mean discharge rate, and this was smaller in the position- than force-control task for the same SMUs. A similar tendency was observed for SMUs recorded on different days. However, gross agonist muscle activity (which incorporates SMUs that are not included in the discharge rate analysis because they were not present in all conditions) increased in both tasks during pain. Gross antagonist muscle EMG only increased in the force-control task.

Conclusion: The effect of pain on muscle activity appears unique to the contraction type, with less influence during position- than force-control tasks.

Significance: Simplistic theories of pain adaptation of movement during voluntary efforts cannot be extrapolated to more postural functions. This has implications for understanding movement changes that may underpin persistence/recurrence of pain and the management of musculoskeletal pain.