The effect of added degrees of freedom and handle type on upper limb muscle activity during simulated hand tool use

The human upper limb serves a number of functions ranging from coarse movements such as supporting a load when lifting overhead to the fine motor control required when painting a portrait. However, there are limited data available that address upper extremity function and performance when using hand tools in situations where the tool endpoint is not fixed but can move translationally or rotationally. The goal of this study was to examine variation in arm muscle activity when added degrees of freedom (DOF) were introduced through the use of a force application apparatus with two different handle designs (D-handle or screwdriver). Electromyography of seven forearm muscles and five muscles crossing the shoulder joint were measured to determine relative activity from a reference (0 DOF), most stable condition, to combinations of DOF ranging from 1 to 4. Substantial and statistically significant increases in muscle activity resulted from adding DOF. The screwdriver handle increased forearm muscle activity compared to the D-handle, except in the highest DOF condition. These findings have significance in the planning of work and design of tools because of the potential for increased fatigue that accompanies increased DOF at the tool endpoint. Handle type also influenced the magnitude of the muscular activity.