Work-related musculoskeletal disorders (WMSDs) are a major source of lost productivity and revenue in the workplace and disability in workers. There is strong evidence for a relationship between physical risk factors, such as repetitive motions and excessive force, and the development of WMSDs; yet there are unexplained discrepancies in determining which workers are more at risk. Researchers hypothesize that non-physical factors in the workplace, or psychosocial factors, may contribute to the development of WMSDs. The following study examined the effects of two psychosocial factors, mental workload and time pressure, on perceived workload and physiological reactions of the lower arm and wrist during typing activity by measuring muscle activation patterns, wrist posture and movement, key strike forces, and subjective assessments of overall workload. The results indicate that increases in time pressure lead to increases in lower arm muscle activation, key strike forces, and wrist deviations. Key strike forces may increase with higher mental workload levels, but other effects of mental workload were not clear. Perceived overall workload (time load, mental effort load, and stress load) increased with mental workload and time pressure, and typing performance decreased. The evidence from this study suggests that these psychosocial factors (mental workload and time pressure) mediate physical risk factors to increase risk for WMSD development in the upper extremities. The results illustrate the need for those designing jobs and work tasks to consider both physical and psychosocial aspects of the working environment to prevent injuries in employees.