GASTROCNEMIUS-MUSCLE LENGTH IN RELATION TO KNEE AND ANKLE JOINT ANGLES - VERIFICATION OF A GEOMETRIC MODEL AND SOME APPLICATIONS

Background: For understanding the relationship between skeletal muscle architecture and muscle function in vivo, the development of accurate geometric models relating muscle length to joint angles is crucial. T herefore, a geometric model of the calf of mammals was developed to pr edict the length of the gastrocnemius muscle-tendon complex from knee and ankle angles. Methods: The model requires three skeletal length me asurements (radius of femoral condyle, ankle lever, and tibia length) to predict muscle-tendon length. The model was tested on the hopping m ouse (Notomys alexis) by comparing polynomial fittings with geometrica l fits of muscle length-joint angle measurements (i.e., the equation o f the geometric model was used for least square fitting of the data). The model was applied to the hopping mouse and the rat to study (in vi vo) joint-angle-muscle length-force relationships. Results: It appeare d that small and, on average, statistically nonsignificant length adju stments of the skeletal lengths were needed for the geometrical fit. C ombinations of joint angles that normally occur during locomotion yiel ded muscle lengths close to optimum (i.e., with the highest isometric force potential). Conclusions: By relying on the geometry of the anima l's leg, the calculated moment arms of the model appeared more reliabl e than those calculated from the polynomial fit. It was concluded that the architecture regarding length-force properties of the gastrocnemi us muscle in both hopping mouse and rat is well adapted for the locomo tion patterns. (C) 1997 Wiley-Liss, Inc.