External finger forces in submaximal five-finger static pinch prehension

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Abstract
Small conductive polymer force sensors were attached to the distal phalangeal pads for measuring individual finger forces exerted during submaximal static pinch. A linear force summing strain gauge dynamometer for measuring resultant five-finger pinch force was grasped vertically using a neutral wrist posture. Individual finger forces were measured at fixed total pinch force levels of 10%, 20%, and 30% of maximum voluntary exertion using pinch spans of 45 mm and 65 mm. Total pinch force and individual finger forces were also measured while similarly grasping the dynamometer and supporting fixed weights for 1·0 kg, 1·5 kg, and 2·0 kg loads using pinch spans of 45 mm and 65 mm. The index and middle fingers exerted more than 3 N greater average force than the ring and small fingers for the fixed total pinch force task. No significant individual finger force differences were observed at the 10% maximum voluntary exertion level, however both the index and middle fingers exerted more than 5 N greater force than the ring and small fingers at the 30% maximum voluntary exertion level. The average contribution of the index, middle, ring, and small fingers were 33%, 33%, 17%, and 15%, respectfully, for the fixed total pinch force task. As exertion level increased from 10% to 30%, the contribution of the middle finger was not constant increasing from 25% to 38%. Total pinch force increased from 15 N to 30 N when the load weight increased from 1·0 kg to 2·0 kg. The index finger exerted more than 1 N greater average force than the middle finger, and more than 3 N greater average force than the ring and small fingers during the static lifting task. The average contribution of the index, middle, ring, and small fingers were 35%, 26%, 20%, and 19%, respectfully, for the static lifting task. As load weight increased from 1·0kg to 2·0kg, the contribution of the index finger was not constant, decreasing from 38% to 30%.