Biceps Insertion Distance Influences Elbow Flexor Isometric Force at 900 Joint Angle

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Lever arm distances significantly affect the external force that can be generated by flexing a limb (Nordin & Frankel, Basic Biomechanics of the Musculoskeletal System, 2001), but this effect is seldom considered in strength testing. PURPOSE To examine variability in the distance from the insertion of the biceps brachii to the elbow joint and its effect on elbow flexor isometric strength score. METHODS We measured the distance (DB) from the radial insertion of the biceps brachii to the center of the axis of rotation of the elbow joint directly in 8 cadavers (CAD), and in 54 subjects using magnetic resonance imaging (MRI). DB estimated from height (DBest) was used to calculate peak true force of the elbow flexors in a test that measured external force transmitted by the wrist pressing against a padded force detector, fixed at 28 cm from the elbow joint, during maximal isometric contraction of the elbow flexors at a joint angle = 90° in 700 subjects. RESULTS Actual DB varied from 3.8 to 7.5 cm. Standing body height predicted DB in both the MRI and cadaver groups: Height was used to estimate DBest in the 700 subjects, whose height ranged from 52.8 to 76.8 inches. Based on height, DBest ranged from 4.1 cm to 7.1 cm. Biomechanically, the elbow joint and forearm function as a third class lever: in subjects with a greater DB, the same elbow flexor force (true force) generates a greater measured external force at the wrist. Thus elbow flexor force measured at the wrist 28 cm from the elbow, was found to overestimate true flexor force by 10% per 4 inches of height. CONCLUSIONS The findings illustrate the mechanical advantage afforded by height, expressed through DBest, on a common strength test in which force is measured at the wrist. For adult subjects who vary in height, investigators can use body height to estimate DBest to obtain true force of the elbow flexors on an isometric force test when the elbow angle is 90°.