MOMENT ARMS AND LENGTHS OF HUMAN UPPER LIMB MUSCLES AS FUNCTIONS OF JOINT ANGLES

Modeling of musculoskeletal structures requires accurate data on anato mical parameters such as muscle lengths (MLs), moment arms (MAs) and t hose describing the upper limb position. Using a geometrical model of planar arm movements with three degrees of freedom, we present, in an analytical form, the available information on the relationship between MAs and MLs and joint angles for thirteen human upper limb muscles. T he degrees of freedom included are shoulder flexion/extension, elbow f lexion/extension, and either wrist flexion/extension (the forearm in s upination) or radial/ulnar deviation (the forearm in mid-pronation). P reviously published MA/angle curves were approximated by polynomials. ML/angle curves were obtained by combining the constant values of MLs (defined by the distance between the origin and insertion points for a specific upper limb position) with a variable part obtained by multip lying the MA (joint radius) and the joint angle. The MAs of the prime wrist movers in radial/ulnar deviation were linear functions of the jo int angle (R(2) greater than or equal to 0.9954), while quadratic poly nomials accurately described their MAs during wrist flexion/extensions . The relationship between MAs and the elbow angle was described by 2n d, 3rd or 5th-order polynomials (R(2) greater than or equal to 0.9904) , with a lesser quality of fit for the anconeus (R(2) = 0.9349). In th e full range of angular displacements, the length of wrist, elbow and shoulder muscles can change by 8.5, 55 and 200%, respectively. Copyrig ht (C) 1996 Elsevier Science Ltd.