Triggering force and measurement of maximal finger flexion force

Abstract

Long-term isometric and repetitive contractions including tripping a trigger as well as supporting the arms and the tools can be observed when electrically-powered tools are operated. The objective of the present laboratory study was to analyze the magnitude and ratios of the contribution of triggering force by finger II or fingers II + III as well as the contribution of holding force by fingers III + IV + V or fingers IV + V when all digits were flexed simultaneously at the maximum exertion level. The forces' contributions were studied according to: (1) grip span and (2) center mass of the tool. This study supports the idea that the index finger should not be used as the only digit for effectively tripping a trigger when tasks are performed forcefully. The isometric finger flexion strength indicated that additional help from the middle finger would double the triggering force. The highest forces, independent of triggering type, were obtained when the grip span was 50 mm, the forces decreased when the span decreased (40 mm) or increased (60 mm). A non-linear profiling of the handle fitted to optimal finger grip spans would be advantageous to increase the holding force. The resultant torque at the wrist can be lower when the center mass of the tool falls back toward the handle of the tool. On the basis of the results of this study, we recommend that the task demands for tripping a trigger for single and double finger-triggering should be smaller than 2 kg and 4 kg, respectively.