The Chimp Crawl

A new report due out in PNAS measured the efficiency of bipedal versus quadripedal gaits in chimps. The authors find that in individual chimps, bipedal walking was more energy efficient than differed in energetic cost from quadripedal walking, with individual differences in anatomy and gait determining the more efficient method. These findings suggest that biomechanical changes and the accompanying energetics may have spurred the development of human bipedalism, which is far more efficient than either chimp gait.

So that's why the bear crawl is so painful...