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. Therefore, a geometric model of the calf of mammals was developed to predict the length of the gastrocnemius muscle-tendon complex from knee and ankle angles.
METHODS: The model requires three skeletal length measurements (radius of femoral condyle, ankle lever, and tibia length) to predict muscle-tendon length. The model was tested on the hopping mouse (Notomys alexis) by comparing polynomial fittings with geometrical fits of muscle length-joint angle measurements (i.e., the equation of 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 vivo) joint-angle-muscle length-force relationships.
RESULTS: It appeared that small and, on average, statistically nonsignificant length adjustments of the skeletal lengths were needed for the geometrical fit. Combinations of joint angles that normally occur during locomotion yielded muscle lengths close to optimum (i.e., with the highest isometric force potential).
CONCLUSIONS: By relying on the geometry of the animal's leg, the calculated moment arms of the model appeared more reliable than those calculated from the polynomial fit. It was concluded that the architecture regarding length-force properties of the gastrocnemius muscle in both hopping mouse and rat is well adapted for the locomotion patterns.