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The aim of this study was to test the mechanical advantage (MA) hypothesis in multifinger torque production tasks in humans: fingers with longer moment arms produce greater force magnitudes during torque production tasks. There were eight experimental conditions: two prehension types determined by different mechanical constraints (i.e., fixed- and free-object prehension) with two torque directions (supination and pronation) and two torque magnitudes (0.24 and 0.48 N·m). The subjects were asked to produce prescribed torques during the fixed-object prehension or to maintain constant position of the free hand-held object against external torques. The index of MA was calculated for agonist and antagonist fingers, which produce torques in the same and opposite directions to the target torques, respectively. Within agonist fingers, the fingers with longer moment arms produced greater grasping forces while within antagonist fingers, the fingers with shorter moment arms produced greater forces. The MA index was greater in the fixed-object condition as compared with the free-object condition. The MA index was greater in the pronation condition than in the supination condition. This study supports the idea that the CNS utilizes the MA of agonist fingers, but not of antagonist fingers, during torque production in both fixed- and free-object conditions.
Jaebum Park, Brian S. Baum, and You-Sin Kim are with the Department of Kinesiology, University of Maryland, College Park, MD. Yoon Hyuk Kim is with the Department of Mechanical Engineering, Kyung Hee University, Global Campus, Korea. Jae Kun Shim (Corresponding Author) is with the Department of Kinesiology, the Department of Bioengineering, and the Neuroscience and Cognitive Science Graduate Program, University of Maryland, College Park, MD, and with the Department of Mechanical Engineering, Kyung Hee University, Global Campus, Korea.