In male gorillas and orangutans (and some species of fossil hominin), in which very large chewing muscles are anchored to a relatively small cranial vault, the right and left superior temporal lines not only converge at the midline of the top of the cranial vault (along the sagittal suture), but also require the development of a sagittal crest to provide sufficient attachment area for the temporalis muscles. Flexor digitorum tibialis and Flexor digitorum fibularis both display activity with variable timing during the last quarter of support phase. This is in contrast to humans who recruit tibialis posterior through much of support phase. In apes the superior extent of the temporalis muscles is positioned higher on the cranial vault (because brain size is smaller, and the muscles of mastication are larger than those found in humans), such that the right- and left-side superior temporal lines approach one another at midline. Tibialis posterior is a substantial muscle in chimpanzees but is essentially inactive during bipedal walking. In humans, who have large brains (and hence large cranial vaults) relative to their body size, the temporal muscles occupy a position on the lateral walls of the cranial vault, and extend only about halfway up the vault surface. But now a research team reports that contrary to this belief, chimp muscles’ maximum dynamic force and power output is just about 1.35 times higher than human muscle of similar size, a difference they call modest compared with historical, popular accounts of chimp super strength, being many times stronger than humans. The crest provides a surface for the attachment of the large chewing muscle, temporalis. Sagittal crests are rare in adult male chimpanzees and female gorillas, and are unknown in female chimpanzees, female orangutans, and humans and bonobos of both sexes (and are also absent in juveniles of all species). Chimps possess about twice the amount of 'fast-twitch' muscle fibre. The sagittal crest is a prominent ridge of bone that projects superiorly (upwards) from the cranial vault along its midline, most commonly seen in adult male gorillas and orangutans.
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