What's your butt for?

Question:
What's the biggest muscle in the human body?
Answer:
Gluteus Maximums (your butt muscle)

Question:
What's it used for?
Answer: Running (almost exclusively)

This, from a NYTimes article reporting research that humans are better adapted for distance running than most animals. In part because of the anatomy of our feet and toes, but mostly because we are adapted for efficient cooling, especially compared to hairy animals.

So some quoted research says a well conditioned human can outrun a horse in hot weather at distances of 26 miles.

So why the prevalence of runner injuries:
Hypotheses:
1. Runners start to train as adults, not children
2.Overtraining
3.Running on hard surfaces
4.Runner hunters may have alternated running with walking
5.Overpadded running shoes generate a destructive running technique. (See Tarahumara sandal used for long distance running.)