Experimental assessment of effectiveness of arm-supporting exoskeleton for overhead work

Abstract

It is well known that overhead work is associated with musculoskeletal disorders in the upper extremities. Arm-supporting exoskeletons (ArmExos) help to reduce mechanical load to the shoulder joint and subsequently risk of injury in the area. The ArmExos are adopted rapidly by industries such as car and airplane manufacturers, although there lack studies examining the effectiveness of the ArmExos in these industry settings as the associated overhead tasks often involve use of power hand tools. To simulate overhead tasks with use of power hand tools, an electromagnetic shaker was hung from the ceiling and produced a random vibration spectrum modified from the ISO 10819. In this posture the ArmExos exerted the highest torque to the upper arm when it was flexed 90 degrees. As comparison, the shaker was also placed in front of the body, in which the ArmExos produced minimum torque to the upper arm when it was hanging down along the body. Vibration transmissibility along the arm and the spine was monitored using accelerometers. Activity of the shoulder muscles was obtained using surface electromyography. The grip force was assessed in the shaker handle while the push force was assessed using a force plate placed under subject’s feet. Live feedback was shown on a computer monitor for the subjects to maintain an average grip force at 30 N and an average push force at 50 N. The data demonstrated that wearing ArmExos didn’t alter vibration transmissibility along the body. Wearing ArmExos led to lower shoulder muscle activities. The agonist muscle activities in the overhead posture were higher when compared to the front-of-body posture. Antagonist muscle activities tended to increase with vibration turned on. The existence of vibration significantly increased the peak grip force and push force, indicating a higher mechanical load to the shoulder. These findings suggest that the impact of ArmExo use in overhead tasks involving power tools is complex. Shoulder joint load analysis using advanced musculoskeletal models is recommended to understand the effectiveness of ArmExos in such industry settings. work posture, vibration, muscle activity.