THE EFFECTS OF POWER HAND TOOL DYNAMICS AND WORKSTATION DESIGN ON HANDLE KINEMATICS AND MUSCLE-ACTIVITY

Reaction force and workstation design aspects of right angle nutrunner operation were studied in order to better understand their effects on handle kinematics and muscle activity. Tool reaction force factors in cluded spindle target torque (25 Nm and 50 Nm) and joint hardness (35 ms and 900 ms build-up time). Workstation factors included orientation (horizontal and vertical) and operator distance (10 cm and 35 cm) fro m the tool. Dependent variables included handle displacement and veloc ity, work done on the tool-hand system, power involved in doing work, and EMG activity in the forearm and upper arm muscles. Isometric and e ccentric strength corresponding to exertions against the tool for velo cities of 0 m/s, 0.084 m/s, 0.251 m/s, 0.503 m/s and 0.754 m/s were me asured and the relationship between strength and handle kinematics dur ing tool operation was studied. Six inexperienced volunteers (four mal es and two females) participated. Subjects operated a 58.5 cm long, 3. 6 kg right angle nutrunner on a fastener 120 cm off the floor using th e right hand while standing. The handle was most stable (defined as mi nimum average peak velocity and displacement) when torque was 25 Nm, w hen vertical workstations were closest (10 cm) to the operator, or whe n horizontal workstations were farthest (35 cm) from the operator. Gre ater handle stability was observed for the horizontal workstation than for the vertical workstation. The hard joint (35 ms build-up) resulte d in 307% greater peak handle velocity and 195% greater average power acting against the operator compared to the soft joint, however total work against the operator was 134% less for the hard joint. Little cor relation was observed between static or dynamic strength and handle ki nematics. EMG latency was measured from the onset of torque build-up. The average latency was 38 ms for the hard joint and 171 ms for the so ft joint.