Of all reported cases, muscular pain in the arms is the second main cause of work-related musculoskeletal disorders (WMSDs) in European workers. The most popular prevention method for shoulder-related WMSDs are passive shoulder exoskeletons, as of their light weight and ease of use. Yet, due to their static force-angle characteristic, passive shoulder exoskeletons show a limited load reduction effect for dynamic tasks. Active shoulder exoskeletons show better performance in the reduction of loads for dynamic tasks, but are not accepted in industrial environments due to their high weight and complexity. In this study, the advantages of both passive and active shoulder exoskeletons have been combined in the design of a mechanism that can actively alter the force-angle characteristic of the Skelex 360-XFR passive shoulder exoskeleton, increasing its load reduction performance, and hence making it a more effective prevention method to WMSDs. By experiments using a prototype, it has been shown that a drivable parallelogram linkage capable of inducing angular changes between an in-going lever arm link and an out-going link to the user’s arm, can effectively induce proportional phase changes of the conventional force-angle characteristic, and that the model predicting the optimal stiffness of the spring used in the implemented force-balancing mechanism is reliable. With the experimental findings, recommendations have been made regarding the minimum capacity in output torque of the actuator, 0.38 Nm, and the stiffness of the balancing spring, 1752 N/m, to end up with optimal weight, size, and power consumption of the semi-passive shoulder exoskeleton.