The wrist extensor muscles have been implicated in a work-related upper limb disorder referred to as non-specific arm pain (NSAP), which has an unknown pathophysiology. The primary objective of this thesis was to perform an electrophysiological evaluation of NSAP to gain a better understanding of the underlying pathophysiology. Secondary objectives were to determine the utility and reliability of the decomposition-based quantitative electromyography (DQEMG) system used to examine NSAP. The utility of the DQEMG system was first tested to determine whether physiological changes in muscles of healthy individuals performing low-level fatiguing contractions could be detected using this approach. Next, the reliability of the outcome variables produced through this system was tested on healthy individuals performing low-level non-fatiguing contractions. A case-control study was then performed using DQEMG to determine whether there were measurable changes in electrophysiological variables that suggest whether NSAP is myopathic or neuropathic in nature. Finally, the case control study was repeated using a less invasive approach of electrophysiological evaluation to determine if this method might be equally useful in determining the pathophysiology of NSAP. Results revealed DQEMG can be effectively and reliably used to detect changes in the physiological characteristics of motor units that accompany fatigue. Specifically, decreases in mean motor unit firing rates along with increases in amplitude, duration, and area parameters of needle- and surface-detected motor unit potentials (MUPs) suggest that recruitment is a main cause of increased electromyographic amplitude parameters with fatigue. Results of the reliability study suggested that DQEMG provides sufficiently consistent results to allow it to be effectively used for quantitative electromyographic (QEMG) analysis. In the first case control study, the QEMG parameters suggested that the underlying pathophysiology in NSAP may be myopathic in nature; specifically, QEMG findings for the NSAP group revealed smaller MUPs compared to the other groups. Lastly, the case control study using spike shape analysis across different levels of isometric wrist extension contractions was deemed to be useful in determining differences among the groups. This research suggests that NSAP may be myopathic in nature, since the NSAP group showed significantly lower mean spike amplitude and mean spike slope values compared to healthy subjects. / Thesis (Ph.D, Rehabilitation Science) -- Queen's University, 2008-08-27 14:53:55.892