Ergonomic evaluation of two-handed torque exertion in using two types of hand tools

Abstract

Despite enhancing automation, hand tools are still the primary interface for operators. Using both hands is sometimes required for better task control or higher torque exertions while using hand tools. This descriptive- analytical, experimental, and applied study consists of two parts. In part one, the effects of handle diameter (3/4/5 cm), between-handle distance (0.5/1/1.5 shoulder span (SP)), workpiece orientation (transverse/frontal), working height (shoulder/elbow/knuckle), and exertion direction (clockwise (CW)/counter-clockwise (CCW)) while using T-shaped handles in 20 participants and in part two, the effects of tool grip span (4.5/5.5/6.5 cm), workpiece orientation (transverse/sagittal), moving direction (CW/CCW), and working height (shoulder/elbow/knuckle) while using locking pliers in 22 participants on isometric two-handed maximum voluntary exertion (MVE), usability, comfort, and discomfort were investigated. Each participant performed 36 experimental conditions. MVE followed the standard recommended protocol: the participant increased his force gradually over a 2-3s period, exerted his maximum force for 3s (steady-state of exertion), and then gradually reduced it to zero over a 2-3s period. Two repetitions for each combination were recorded, and they were considered valid if the highest scores were within 10% of each other. The average value of these two measures were calculated and used in subsequesnt analyzes. The comfort questionnaire for hand tools, discomfort upper extremity map, and usability scale were filled out after finishing each experimental condition. The results of part one showed that the handle diameter had no significant main effect on MVE, although the 3 cm diameter handle caused better usability and comfort compared to other options. Higher MVE values were recorded with between-handle distance equal to 1.0 and 1.5 SP besides in frontal plane, in shoulder and knuckle heights, and in CCW direction. The between-handle distance equal to SP had better comfort and higher usability than other conditions. The results of part two showed no significant main effect of tool grip span on MVE but the locking pliers with 4.5 cm handle grip span led to more comfort and better usability. The MVE was significantly higher in sagittal plane, and in CW direction. The highest values of MVE in sagittal and transverse planes were exerted in knuckle and elbow heights, respectively. Significant interactions were also found between the variables in both parts. The present study indicated that for detecting the optimal hand tool and workstation design features, investigating each type of torque exertion separately is needed. Moreover, the existed ergonomic guidelines (e.g. working at elbow level) are not always recommended. The findings can be used to develop guidelines and recommendations with regard to daily and occupational activities which require the use of both hands for torque exertion with manual hand tools.