This study is a subset of a larger body of research that examined shoulder time to fatigue during overhead work in an attempt to reduce the prevalence and impact of work-related musculoskeletal problems in the shoulder associated with overhead work, particularly during automobile assembly. Existing evidence suggests that shoulder injuries are diverse in terms of tissues affected and symptoms presented. Furthermore, the cause of these injuries is multifactorial. The work presented here assumes that musculoskeletal injuries of the shoulder mechanism are at least related to, if not caused by, fatigue localized to the shoulder musculature. While the exact relationship between fatigue and injury has not been clearly established, there is consensus among researchers that fatigue plays and important role. Muscular fatigue, therefore, is viewed as a surrogate measure of risk, and task design to avoid fatigue is seen as a rational method to minimize this risk. An experiment to determine the effects of tool weight on shoulder fatigue and performance during overhead work with work/rest cycles was performed. Times to fatigue were derived based on dependent measures including total task duration, controlled maximum muscle contractions, subjective ratings based on Borg's CR-10 RPE scale, electromyogram behavior (MdPF), and hand force performance measures. Experimental findings indicated that duty cycle (percentage of total task cycle time spent working) significantly affected task duration (p<0.0001), changes in maximum voluntary contraction values for the infraspinatus (p<0.05), and the minimum time for any shoulder muscle to fatigue as determined by changes in the EMG power spectrum (p<0.05). Time to fatigue for the mid deltoid as determined by changes in the median frequency of the EMG power spectrum was shown to change significantly (p<0.05) with change in tool weight. Large intersubject variation was observed for the dependent measures, which showed subjects experiencing different levels of fatigue while performing the same task. Limitations of the study and recommendations for future direction are also discussed. / Master of Science