Investigation of Triggering Force by the Index Finger for Optimal Trigger Design Using a Cadaver Experiment

Abstract

This study determined the optimal trigger grip span for the index finger and contact location between the index finger and trigger, using a cadaver experiment. Three fresh-frozen right male cadaveric hand specimens without any medical problem were employed, and the index finger motion simulator (IFMS), consisting of 1) support frame, 2) force measurement system, 3) motion delivery unit, and 4) operation system, was developed to support and control the specimens. The experiment consisted of two phases: triggering force was observed as a function of 1) grip spans (40, 50, 60 mm) and 2) three contact locations on the middle of the distal to the middle of the medial phalange. Also, three levels of total tendon forces (FDP + FDS: 40, 70, and 100 N) were applied to both phases. As a result, at Phase I, the maximum triggering forces, 8.9, 15.0, and 20.0 N, were obtained at 50 mm grip span for total tendon forces respectively. At Phase II, The contact location on the middle of the medial phalange showed the maximum triggering forces, 10.1, 18.2, and 28.2 N, for the total tendon forces respectively. Force efficiency, triggering force to tendon force ratio, showed approximately 10 to 30% of internal tendon force was converted into external trigger force. On the basis of the results of this study, 50 mm grip span and the contact location on the middle of the medial phalange were recommended for optimal trigger design.