Leg Muscle Architecture in Primates and Its Correlation with Locomotion Pattern

Bone biomechanical studies indicate that leg bone structure can be related to different locomotor patterns. The osteological correlates of extant primates’ locomotion patterns and substrate use are important to consider when estimating corresponding behaviors of extinct primates. Here, we test if these same patterns are seen in the differences in leg muscular architecture. Muscle mass, fascicle lengths (FL), physiological cross-sectional area (PCSA), reduced PCSA (RPCSA) and tendon-tomuscle belly ratio were studied in 33 primate species (6 strepsirrhines, 14 platyrrhines and 13 catarrhines). Muscles were grouped into toe and ankle flexors and extensors and studied for phylogenetic and functional signals. All variables strongly correlate with body mass: strength variables (mass, PCSA and RPCSA) scale with positive allometry, whereas the speed/stretch measure (FL) trend toward negative allometry. Thus, larger primates are relatively stronger than smaller species, but they have relatively shorter leg muscle fibers than smaller primates. The strongest functional signal emerged when comparing belly-muscle tendon unit (MTU) length ratio in leaping and non-leaping primates. Leapers show significantly smaller plantarflexor belly-MTU ratio. Surprisingly, no significant results reflect a correlation between muscle architecture and substrate and locomotor groups. However, several trends suggest that a larger sample and more fine-grained defined categories could produce significant results. These results show the complex relation between leg bone biomechanics and muscle architecture and demand for further studies on this topic.