Meeting Abstract
Moment arms are integral to understanding how muscle forces power limb movements. Two common approaches for measuring moment arms are the tendon excursion (TE; the distance a tendon moves past a joint, as a function of joint angle) and the geometric method (GM; the linear distance between the tendon and the center of joint rotation). Although TE has the advantage of circumventing the difficulty of identifying the joint center, it may not adequately account for complex joint geometries. For instance, if the path of a tendon in a toe is altered during flexion-extension in ways that increase travel length without actually contributing to the phalangeal in-lever, then TE and GM may not measure the same thing. We used TE and GM to measure (in 3D from CT) the moment arms of the combined M. flexor hallucis and digitorum longus tendon at the tarsometatarso-phalangeal (TMTP) joints of an Emu foot. The TE-derived moment arms were similar across all toes, despite the apparent variation in TMTP joint condyle diameters. Conversely, the GM-derived moment arms reflected differences in condyle size. Although both methods agreed for the middle toe, TE produced considerably larger moment arm estimates than did GM for the inner and outer toes. The oblique orientations of the TMTP condyles of the side toes, coupled with the effects of cartilaginous pulleys at the bases of the proximal phalanges, dynamically change the positions of the flexor tendon branches relative to the joints. As a result, tendon excursion may be increased proximal to the TMTP joints, without any ostensible effect on the moment arms measured just distal to them. We further discuss this phenomenon, and explore the potential functional implications of the apparent similarity in TE, and discrepancies in GM, moment arms across toes.
