Effects of Mass on Wrist Velocities and Accelerations

Abstract

Upper Extremity Cumulative Trauma Disorders (UECTD) have been linked to risk factors such as frequency, forcefulness, posture and time for rest/recovery. More recently, wrist and forearm movement velocities and accelerations have been identified as significant correlates to UECTD in industrial tasks. In earlier work with biomechanical analysis of Sign Language Interpreting (SLI), we found that SLI involves frequencies, velocities and accelerations of wrist and forearm motions that greatly exceed those noted as UECTD high risk jobs. A primary difference between the workers in the industrial studies and our studies of SLI is that industrial workers typically hold objects in their hands, while in SLI, the hand is empty. In SLI, we have found the maximum velocities and accelerations reach as much as 70 to 80% of the maximum possible values.This study looks at the effect of holding graduated masses on the maximum velocities and accelerations for the wrist joint for the flexion/extension and radial/ulnar deviation and forearm pronation/supination motions. Five subjects were used. Each subject repeated a maximum voluntary motion three times in each configuration, starting from a stationary neutral position, with the forearm held stationary for the wrist movements. Four different masses were used (9.5 125, 250 and 500 grams) to assess the effect of mass. Each mass was a cylinder of the same external dimensions. As a control, a hollow cardboard tube (9.5 grams) was used to measure the velocity and acceleration for essentially a no-load condition. Thus the hand was always in the same grip configuration.The results show that for the masses and hand-grip configurations used, the effect of the masses was not statistically significant for forearm pronation/supination, but was for wrist movements. The effect was greatest for flexion/extension, with about a 20% decline in velocity and acceleration at 500 grams versus the 9.5 gram weight.