Motor unit action potential rate and motor unit action potential shape properties in subjects with work-related chronic pain.

The objective of this study was to investigate differences in motor control of the trapezius muscle in cases with work-related chronic pain, compared to healthy controls. Ten cases with chronic pain and 13 controls participated in the study. Electromyographic (EMG) signals were recorded from the upper trapezius during five computer work-related tasks. Motor control was assessed using global root-mean-square value (RMS(G)), motor unit action potential (MUAP) rate (number of MUAPs per second, MR) and two MUAP shape parameters, i.e. root-mean-square (RMS(MUAP)) and median frequency (FMED(MUAP)). MR and FMED(MUAP) were higher for the cases than for the controls (P < 0.05). RMS(MUAP) showed a trend for higher values in the chronic pain group (P < 0.13), whereas RMS(G) did not show a significant difference between the groups. The higher MR, FMED(MUAP) and the trend for higher RMS(MUAP) suggest that more high-threshold MUs contribute to low-level computer work-related tasks in chronic pain cases. Additionally, the results suggest that the input of the central nervous system to the muscle is higher in the cases with chronic pain.