Running Stride
“While all these other runners had long, trailing legs, his foot was coming right up to his butt,” Salazar recalled. “I thought, Is that just coincidence? Or could that perhaps be part of why he’s so good?” Salazar called the retired sprinter Michael Johnson, a four-time Olympic gold medalist who now heads the Michael Johnson Performance Center, in Dallas. “I told him, ‘Hey, Michael, I’m watching this race. . . ’ And when he heard what I was saying he laughed. He said, ‘Alberto, that’s Sprint 101 biomechanics!’ ”
According to Johnson, sprinters retract their trailing leg quickly for two reasons: it generates power, and it means that the foot has a shorter distance to travel before it arrives back in position for another stride. Salazar said, “One thing sprinters say is: Go to the ground. Don’t wait for the ground to come to you.” By “going to the butt,” he believes, Bekele can take more strides per minute—which gives him greater speed.
The fastest finishers had a higher thigh drive, for one thing; at its
apex, their femur bone was almost parallel to the ground, like the
front legs of a bounding deer. They also slapped the ground so quickly
with their forefoot that the contact seemed almost incidental.
According to Walker, the short slap transfers force more efficiently,
shooting it from the ground forward into the pelvis, rather than
allowing it to dissipate in the flex of the foot. The effect, Walker
says, is like “a pogo stick with a stiff spring.” He explained, “You
want the chain of force to travel from the ground through the body
with minimal energy loss. That’s what it means to run efficiently.”
“His hips are directly under his body, which is directly above his
foot. So all that force is going up through his legs and hips into his
upper body, to propel him forward. There’s nothing being lost there.”
Read more http://www.newyorker.com/reporting/2010/11/08/101108fa_fact_kahn#ixzz19SwZY118