Analysis of Two-Handed Pulls Using Free Body Diagrams

¹ From the Department of Anatomy, University of Michigan, Ann Arbor, Michigan

A nude subject made a number of maximum-effort, two-handed isometric pulls—standing, braced and seated. He pulled on a dynamometer that could be attached above, forward, or below. At peak effort he was photographed side view and in 45° mirrors. The subject, as seen in the photograph, may be treated as the rigid body of mechanics. On enlarged photographs, free body diagrams were constructed with emphasis on the dynamometer pull vector, the gravity vector and the reaction force. The first two vectors were measured quantities and the third was derived from the free body diagrams on each photo record The mechanical analyses permitted the defining of the best conditions for two-handed isometric pulls. The method quantified simultaneous bending moments at the several limb joints and pointed to those joints which limited pull maxima. Whenever the body exerts forces on its environment, forming a closed chain system of forces, limb and trunk muscles do not directly affect pull forces; instead, they maintain joint postures which permit body weight to exert an effective moment.