265 Motor Control, 2012, 16, 265-283 © 2012 Human Kinetics, Inc. Vafadar is a PhD Student at McGill University, School of Physical and Occupational Therapy, Montreal, QC, Canada. Côté is an Associate Professor, McGill University, Department of Kinesiology and Physi- cal Education, Montreal, Canada. Archambault is an Assistant Professor, McGill University, School of Physical and Occupational Therapy, Montreal, QC, Canada. The Effect of Muscle Fatigue on Position Sense in an Upper Limb Multi-joint Task Amirhossein K. Vafadar, Julie N. Côté, and Philippe S. Archambault The purpose of this study was to estimate the extent to which muscle fatigue can impact on the position sense in the upper limb. Twelve healthy volunteers were asked to do a reaching task while grasping a wooden block and match the block’s position with a corresponding target displayed on a lat screen, without vision. Following that, subjects performed resistive exercises with Thera-band strips until fatigue was induced and then the position sense task was repeated. A signiicant change in the endpoint position was observed after fatigue, in the up/down direction (p ≤ .001). The variability of endpoint positions in up/down direction was also signiicantly increased after fatigue (p ≤.03). There was no signiicant change in endpoint orientation but there was a signiicant fatigue × orientation effect on endpoint rotational variability. In a follow-up experiment, a group of subjects repeated the same protocol, but with a period of quiet rest between the two position sense tasks. In that group, there were no differences in endpoint position, orientation or variability. Muscle fatigue is an important factor that should be taken into consideration during the treatment of musculoskeletal injuries as well as athletic training. Keywords: electromyography, neuromuscular control, position sense, reaching task, shoulder joint The glenohumeral joint is the most mobile joint in the human body, and is inherently unstable (Williams & Warwick, 1986). However, static (ligament and capsule) and dynamic (muscle) stabilizers around this joint can compensate for the lack of stability (Wilk et al., 1997). For stability to be achieved, muscles around the joint should function in a coordinated manner. This is fully dependent on the commands sent from spinal and higher levels of the nervous system (Wilk & Arrigo, 1993). Proprioception was originally deined by Sherrington (1906) as the affer- ent information arising from mechanoreceptors located in the muscles, tendons, ligaments and skin around the joint. Among these, muscle spindles are considered as important proprioceptive receptors found in skeletal muscles (Baldissera et al.,