Workplace tasks that involve multidimensional demands, such as physical and cognitive workloads, may increase the injury risks posed on workers. To aid in designing ergonomic guidelines for workplace safety, it is thus important to understand the interactive nature of these combined demands. The goal of this study was to investigate the effects of said multidimensional demands on the muscles of the upper extremity. Seventeen healthy participants performed isometric upper extremity exertions at five levels of physical intensities (5%, 25%, 45%, 65%, and 75% maximum voluntary contraction (MVC)) in the presence and absence of a mental task. Mean shoulder muscle activity and co-contraction indices (CCI) of the upper extremity were measured using electromyographic signals. Force fluctuations were measured using a force transducer to evaluate joint steadiness. In general, shoulder muscle activity (mean activity and CCIs) increased with increasing levels of physical exertions. A decrease in mean anterior and posterior deltoid muscle activity and CCI of the shoulder was observed in the presence of the mental task. However, these changes were more prominent at higher levels of physical workload compared to the lower levels. Force fluctuations were lowest at 25% MVC, and highest at 85% MVC. Mental workload significantly increased force fluctuations at 85% MVC, which may have been a result of decreased muscle output to maintain the loads in a steady posture. Overall, results from this study suggest that certain task demands are more susceptible to interference by mental workload than others. It will provide a better understanding of combined exposure levels in occupational settings and serve as a guideline to determine task demands for different occupational settings.