Prolonged mechanical and physiological changes in the upper extremity following short-term simulated power hand tool use

This study investigated in-vivo changes in upper limb dynamic mechanical properties and magnetic resonance imaging (MRI) parameters following short-term power hand tool operation. Previous studies have found reduction in mechanical properties following short-term power tool usage at long build-up times. This study advances that work by having participants operate a simulated pistol grip power hand tool and evaluating changes in mechanical properties, strength, discomfort level and MRI prior to tool operation and daily for 3 d after tool operation. Twenty-four participants were randomly assigned to operate a simulated power hand tool for either a high peak reaction force of 123 N (peak torque=8 Nm, build-up time=250 ms) or at a low peak reaction force of 5 N (peak torque=2 Nm, build-up time=50 ms). Subjects operated the tool for 60 min at the rate of six times per min. A reduction in stiffness (27%, p0.05) was observed 24 h after tool operation for the high force group and this change persisted (26%, p0.05) up to 72 h after tool operation. Similar changes were not observed for the low force group. No changes were observed in mass moment of inertia, damping, isometric strength and damping for either group (p0.05). There was a signal intensity increase (12%, CI 19%, 5.06%) in the supinator muscle MRI for both groups 24 h after tool operation but only the high force group remained elevated (10%, CI 13.7%, 0.06%) 72 h after tool operation. Persistent short-term changes in mechanical and MRI parameters at high force levels could indicate increased strain on the upper limb and may negatively affect ability to react during rapid forceful loading of the upper limb. This research can ultimately lead to better ergonomic interventions through quantitative power hand tool design guidelines and work practices based on understanding the damaging effects of exposure to specific levels of reaction force, build-up time and repetition, as well as providing new outcome measures for epidemiological studies.