Imaging-based estimates of moment arm length in intact human muscle-tendons.

The muscle-tendon moment arm length, i.e. the perpendicular distance from the muscle-tendon action line to the rotation centre of the joint that the muscle-tendon spans, is responsible for transforming muscle force and linear displacement to joint moment and rotation. In this paper, previous work on in vivo measurements of human muscle-tendon moment arms at rest and during isometric maximal voluntary contraction (MVC) is reviewed. The results obtained by actual measurements on 2-D magnetic resonance images indicate that the moment arm lengths of the Achilles and tibialis anterior tendons increase during MVC compared with rest by between 22% and 44%, due to (1) joint displacement, (2) muscle thickening and (3) stretching of collagenous structures mediating the action of tendon. However, moment arm length calculations based on the virtual work principle fail to show the above effect. Potentially severe mechanical limitations of the latter method as adapted under in vivo conditions raise questions about its validity during muscle contraction.