THE EFFECTS OF TASK ROTATION AND ORDER ON DELTOID MUSCLE FATIGUE Sachin Raina 1 and Clark Dickerson 1 1 University of Waterloo, Waterloo, ON, CAN E-mail: smraina@uwaterloo.ca INTRODUCTION Job rotation is an industrial practice that is often applied with the intention of reducing muscle fatigue, exposure, or workload. It is believed that this reduction can help lower the risk of developing or exacerbating work- related musculoskeletal disorders (WMSDs). Recent survey results indicated that 42.7% of 178 manufacturers evaluated in the Midwest United States used job rotation as part of their administrative work design (Jorgensen, 2005). Despite its popularity, the specific effects of job rotation on the musculoskeletal system are not well known (Jorgensen, 2005; Kuijer, 2005). While it is intuitive that rotating between jobs that involve different regions of the body over a period of time may pose less risk of developing a WMSD than performing one job over this same period of time, the same cannot be said for jobs that involve similar muscles. In the shoulder, for instance, many muscles are frequently active, albeit differentially, in movements that are apparently unique (Sigholm, 1984). The effect job rotation would have in this scenario is not obvious. This study aimed to determine: 1) the impact of rotating between two tasks involving the same principal muscles; and 2) the importance of the order of task rotation, both with respect to muscle fatigue. METHODS Ten healthy subjects (5 male, 5 female, mean age = 25.7) took part in this study. Two repetitive, unloaded arm movements were used: forward shoulder flexion (A) and shoulder abduction in the frontal plane (B). Subjects completed four task combinations, generating four conditions: AA: four continuous minutes of task A; AB: two minutes of task A, then two minutes of task B, and their counterparts, BA and BB. Maximum voluntary force (MVF) for shoulder elevation was measured at the hand immediately before and after each trial. Electromyographic (EMG) signals from the anterior, middle, and posterior deltoids were measured during the trials, as were ratings of perceived exertions (RPE). Raw EMG was full-wave rectified and low pass filtered with a cut off frequency of 3Hz. EMG amplitude (AMP) was normalized to 100% maximum voluntary contraction (MVC). Because EMG AMP varies during a movement with joint angle (Potvin, 1997), peak AMP values were used for comparisons. Integrated EMG (iEMG) values were calculated using a trapezoidal method on the linear enveloped data. Mean power frequencies (MPF) were also used to assess muscle fatigue. One way ANOVAs were used to compare AMP, iEMG, MPF, and RPE between conditions for each muscle part. Changes in MVF post- exercise were also evaluated. RESULTS AND DISCUSSION Overall Indicators of fatigue: While each condition caused an MVF decrease (mean = 16.8 ± 9%, p<0.05) after exercise, there were no differences between the magnitude of the decrease across conditions. Further,