HOW MUSCLE ARCHITECTURE AND MOMENT ARMS AFFECT WRIST FLEXION-EXTENSION MOMENTS

The purpose of this investigation was to determine how the moment arms and architecture of the wrist muscles influence their isometric momen t-generating characteristics. A three-dimensional computer graphics mo del was developed that estimates the moment arms, maximum isometric fo rces, and maximum isometric flexion-extension moments generated by 15 muscles about the wrist over a range of wrist flexion angles. In combi nation with measurements of muscle strength, we used this model to ans wer three questions: (1) why is peak wrist flexion moment greater than peak extension moment, (2) why does flexion moment vary more with wri st flexion angle than does extension moment, and (3) why does flexion moment peak with the wrist in a flexed position? Analysis of the model revealed that the peak flexion moment is greater than the peak extens ion moment primarily because of the larger (110%) summed physiologic c ross-sectional area of the flexors. The larger variation of flexion mo ment with flexion angle is caused mainly by; greater variation of the moment arms of the major wrist flexors with flexion angle. The locatio n of the peak flexion moment is determined by the wrist flexion moment arms (which tend to increase with wrist flexion) in combination with the force-length characteristics of these muscles. (C) 1997 Elsevier S cience Ltd.