The relationship of the moment arms of muscle-tendon units to limb orientation in animals

PAYNE, R. C. *; HUTCHINSON, J. R. ; ROBILLIARD, J. J. ; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK; Royal Veterinary College, London, UK: The relationship of the moment arms of muscle-tendon units to limb orientation in animals.

The moments (torques) generated about animal limb joints are the product of muscle-tendon unit (MTU) force times MTU moment arm. Many studies have focused on the variable force-generating capacity of MTUs, but most biomechanical studies that include moment arms treat them as constant values; few studies have directly considered how MTU moment arms might depend on the joint angles assumed. We have studied how MTU moment arms vary with limb orientation (joint angles) in a variety of animal species ranging from primates (humans, common chimpanzees, bonobos, gibbons, orang-utans, gorillas) to horses, birds, and even models of large dinosaurs. Here we focus on extant taxa and on limb extensor moment arms, as extensor MTUs are crucial for producing the moments that support body weight during standing and moving.

We typically find wide variation in moment arms with limb orientation, whether assessed in vivo with ultrasound imaging, in vitro with tendon travel methods, or in silico with computer models. However, it is not yet clear how much of this variation relates to the techniques used to measure moment arms, or to variation among individual species, subjects, muscles, or joints. Some muscles do exhibit near-constant moment arms, but these cases seem relatively few; the more common case is large variation of moment arms with limb orientation. The maximum amount of moment arm variation with a particular joint angle throughout its range of motion can be over 200%, and is frequently >50%. Hence MTU moment arms are best treated as constant values only when empirical evidence demonstrates such constancy.

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