Joint function in marmosets and tamarins: Insights from computational modeling of hip extensor muscles.

Analyses of the musculoskeletal system of callitrichid primates contribute to the understanding of the specializations of an apparently highly conserved body plan exhibited by this radiation of New World primates. This pilot study provides data from computational modeling of muscle function of five hip extensor muscles in four species of Callitrichidae to identify potential adaptations to previously documented differential leaping behaviors. Based on microCT scans of fresh cadavers, we reconstructed the muscle topology to inform the modeling of instantaneous muscle moment arms (MMAs) contributing to hip extension and accompanying muscle strains. Generally, muscle properties of the four species were surprisingly similar despite documented differences in leaping behavior. However, all extensors of Goeldi's marmoset (except for the semimembranosus) had the longest instantaneous MMAs. This may result in a greater capacity to generate hip torques in these marmosets (assuming identical force provided by the muscles), beneficial to their specialization in long-distance trunk-to-trunk leaps. The shorter instantaneous MMAs of the extensors of the three other studied species indicate specialization toward more rapid hip extension. Strain analysis showed that, in all four species, the two glutei optimally generate force during the entire extension of the hip from a strongly crouched leg position to take off with an almost entirely extended leg. For the other three muscles (biceps femoris, semimembranosus and semitendinosus), we found optimal strains for force generation only at 50°-140° hip extension. We tentatively conclude that a relatively generalized musculoskeletal system for hip extension, coupled with moderate biomechanical adaptations favoring either joint torque or rotational speed, enables callitrichids to achieve remarkable locomotor versatility within highly intricate arboreal environments.