The influence of experimental muscle pain on motor unit activity during low-level contraction.

In the present study we compared motor unit (MU) activity in a painful extensor carpi ulnaris (ECU) muscle to that of a pain-free control. According to the pain adaptation model the activity of the painful ECU muscle may be inhibited and its antagonist activity increased during wrist extension performed as a pre-defined low-force ramp. The pre-defined low force may then be maintained by increased activity in the pain-free synergist muscles such as the extensor carpi radialis (ECR) muscle. Nine females (31-47 years old) participated in the study. Maximal voluntary contraction (MVC) of the wrist extensors was performed. A catheter was inserted into the ECU muscle to allow the injection of hypertonic saline to evoke muscle pain, and a concentric needle was inserted for the recording of MU activity. Surface electromyograms were recorded from a synergist and an antagonist (ECR and flexor carpi radialis) to the painful ECU muscle. A force ramp of isometric wrist extensions up to 10% MVC, with a force increase of 1% MVC x s(-1), were performed followed by 60 s of sustained contraction at 10% MVC. The number of MUs recruited was almost identical for baseline and with pain, and no effect of experimental muscle pain was found on the properties of the MUs (amplitude, area) or their firing characteristics (mean firing rate, firing variability) during low-force ramp contraction. During the sustained 10% MVC, no effect of pain was found for concentric or surface EMG of the forearm muscles. At low force levels no pain-induced modulations were found in MU activity, when the mechanical condition was similar to that of a control situation.