gluteus maximus

http://jeb.biologists.org/cgi/content/abstract/209/11/2143

The human gluteus maximus is a distinctive muscle in terms of size, anatomy and function compared to apes and other non-human primates...

The results indicate that the gluteus maximus is mostly quiescent with low levels of activity during level and uphill walking, but increases substantially in activity and alters its timing with respect to speed during running...

Evidence for when the gluteus maximus became enlarged in human evolution is equivocal, but the muscle's minimal functional role during walking supports the hypothesis that enlargement of the gluteus maximus was likely important in the evolution of hominid running capabilities.