Electromyography of the gluteal muscles in <i>Hylobates, Pongo</i>, and <i>pan</i>: Implications for the evolution of hominid bipedality

TLDR

Primates’ gluteal musculature has been extensively studied, with debate over whether gluteus medius changed its function from pongids to hominids while gluteus superficialis remained unchanged, and the earliest hominid is thought to have been part biped and part climber. The study aimed to clarify the functions of gluteus medius, gluteus superficialis, and tensor fasciae latae during ape locomotion. We performed a telemetered electromyographic study of these muscles in two gibbons, one orangutan, and four chimpanzees while they walked bipedally and quadrupedally on ground and horizontal trunk, and climbed a vertical trunk. The results show that ape gluteus medius acts mainly as a medial rotator rather than a thigh extensor, providing side‑to‑side trunk balance during bipedality just as in humans; the hominid lateral balance mechanism evolved osteologically, preserving muscle function with an extended thigh, increasing stride length, reducing demands on other muscles for anteroposterior balance, and suggesting that vertical climbing may be preadaptive to bipedalism.

Abstract

Abstract The gluteal musculature of primates has been a focus of great research interest among those who study human evolution. Most current theorists agree that gluteus superficialis (= maximus) need not have changed its action in the step from pongid to hominid, but dispute has arisen over a purported change in action and role of the gluteus medius. To clarify the functions of gluteus medius, gluteus superficialis, and tensor fasciae femoris during ape locomotion, we conducted a telemetered electromyographic study of these muscles in two gibbons, one orangutan, and four chimpanzees as they walked bipedally on the ground and on a horizontal tree trunk, walked quadrupedally on the same substrates, and climbed a vertical tree trunk. The results indicate that the gluteus medius of apes is not, as has been previously suggested, primarily an extensor of the thigh; its action is chiefly that of medial rotation. The role of the gluteus medius during bipedality is the same in apes and humans–to provide side‐to‐side balance of the trunk at the hip. The change in the hominid lateral balance mechanism can be viewed as primarily osteological, allowing preservation of the same muscle function with an extended thigh. As a result, the stride length is increased and there occurs a diminution of the demands placed on other muscles to maintain anteroposterior balance at the hip and knee. Our data also support the view that vertical climbing may be specifically preadaptive to bipedalism. One may picture the earliest hominid as part biped, when on the ground traveling between scattered food trees, and part climber, when moving from the ground to food.