POLK, J.D.: Limb length, body mass and musculoskeletal design

The design of the musculoskeletal support system in terrestrial vertebrates is strongly influenced by body mass, with limb postures becoming more extended to moderate increased joint moments. Adaptive differences in limb proportions (e.g., between cursors and noncursors) should also influence joint moments and animals with longer limbs were predicted to use more extended joint postures to attenuate these moments. In order to experimentally investigate the effects of limb proportions on joint moments and postures, 3D kinematic and kinetic data were collected for 6 closely related, semi-terrestrial monkeys (vervets, patas, and olive baboons). The subjects ranged in body mass from 3-25kg and the species differed in body proportions. All data were collected at the SUNY Stony Brook Primate Locomotion Laboratory. Body proportion effects were revealed through comparisons between 3 monkeys with the same body mass (15kg) but different limb proportions, as well as by multiple regression analyses (to distinguish the effects of body mass and body proportions) across the entire sample of monkeys. In the body proportion comparisons, predicted differences were obtained such that animals with longer limb segments used more extended limb postures and used lower shoulder and hip angular excursions than those with shorter limb segments. The multiple regression analyses across the entire sample revealed that 30% of the variables showed significant limb proportion effects. These results suggest that adaptive differences in limb lengths play a significant role in determining joint postures as well as the moments that must be resisted by limb bones and and muscles.