Sprinters are bigger and brawnier than marathoners because they're built for speed.

Researchers from Rice University and the Texas Medical Center's National Center for Human Performance have found that greater body mass helps sprinters hit the ground harder - and go faster.

Past studies of distance runners and the limited running ability of large animals such as elephants had convinced scientists that being big meant being slower. But kinesiologist Peter Weyand found that when speed rather than endurance is the goal, different body mass rules apply.

The researchers looked at body mass - which measures body size based on height and weight - to determine what body size would maximize the performance of runners who race both short and longer distances.

The study appears in the July 15 issue of the Journal of Experimental Biology.