• Locomotion;
  • Comparative morphology;
  • EMG


In order to investigate a correlation between morphological variations of the biceps femoris muscle and its homologues in four primate species (Japanese macaque, spider monkey, white-handed gibbon, and chimpanzee) and each type of species-specific locomotor behavior, I carried out both morphological and functional analyses of these muscles. The description of the level of insertion reveals interspecific variation is in the level of crural attachment, especially in species with a bicipital biceps femoris muscle. Electromyo-grams (EMGs) were induced from both the long and short head of the biceps femoris muscle during four kinds of locomotor behavior (horizontal quadrupedal walking, climbing on an inclined pole, vertical climbing, and bipedal walking). In the case of the monoceptual ischiocruralis lateralis muscle of the Japanese macaque, EMGs were induced from both the one-joint femoral part and the two-joint crural part. Though during horizontal quadrupedal locomotion the crural part of the monocipital-type muscle functioned to maintain the knee joint angle, it functioned to gain propulsive force when the kinematic load became larger, as in vertical climbing and bipedal walking. On the other hand, the long heads of the biceps femoris muscles were active in propulsion regardless of the kinematic load. But in bipedal walking, the long head muscle also acted with the short head muscle to maintain the knee joint angle. These functional features of various biceps femoris muscles of primates correlated with their species-specific locomotor behavior.