Moment arms of hip abductor and adductor muscles measured in vivo by computed tomography.

Anatomical data on the length of moment arms of hip muscles is required to evaluate the load on an endoprosthesis or a joint. We investigated 20 subjects and measured the moment arms of eight hip muscles with respect to their action in the coronal plane, i.e. in abduction or adduction. For this purpose computed tomography was used. Consecutive transverse scans were taken, three-dimensional coordinates recorded and moment arms calculated. The length of the moment arms was slightly longer in comparison with earlier data. It was also shown that several muscles produce moments in more than one plane, which probably contributes to joint stability. It is therefore concluded that one should consider contribution from muscles other than those traditionally classified as abductors, otherwise the forces at the hip will tend to be understimated.