Fig. 5. Electromyographic activity of gluteus medius in spider monkeys (Ateles sp.) and chimpanzees (Pan troglodytes) during terrestrial quadrupedalism, terrestrial bipedalism, and climbing a large vertical support. The data for the spider monkey are from Fleagle et al. (1981), and for the chimpanzee from Stern and Susman (1983). The graphs follow the approach of Stern et al. (1980). The x-axis represents stance and swing phase. The y-axis represents activity (expressed as a percentage of maximum muscle recruitment) that occurred 75% of the time during the respective activity. Muscular recruitment increases in both magnitude and duration from quadrupedalism to bipedalism. The recruitment patterns during bipedalism and vertical climbing are similar to each other. The same pattern is found for the orangutan (Pongo pygmaeus) for all three behaviors and for the gibbon (Hylobates lar) during bipedalism and vertical climbing (Stern and Susman, 1983).

Return to article