Some­thing strange in Usain Bolt’s stride

Miami Herald - - FRONT PAGE -

Among those ques­tions: Does even­ness of stride mat­ter for speed? Did Bolt op­ti­mize this ir­reg­u­lar­ity to be­come the fastest hu­man? Or, with a more bal­anced stride dur­ing his prime, could he have run even faster than 9.58 sec­onds at 100 me­ters and 19.19 sec­onds at 200 me­ters?

“That’s the mil­lion-dol­lar ques­tion,” said Peter Weyand, di­rec­tor of the SMU lab

The SMU study of Bolt, led by An­drew Ud­ofa, a doc­toral re­searcher, is not yet com­plete. And the ef­fect of asym­met­ri­cal strides on speed is still not well un­der­stood. But rather than be­ing detri­men­tal for Bolt, the con­se­quences of an un­even stride may ac­tu­ally be ben­e­fi­cial, Weyand said.

It could be that Bolt has nat­u­rally set­tled into his stride to ac­com­mo­date the ef­fects of sco­l­io­sis. The con­di­tion curved his spine to the right and made his right leg half an inch shorter than his left, ac­cord­ing to his au­to­bi­og­ra­phy.

Ini­tial find­ings from the study were pre­sented last month at an in­ter­na­tional con­fer­ence on biome­chan­ics in Cologne, Ger­many. Most elite sprint­ers have rel­a­tively even strides, but not all. The ex­tent of Bolt’s vari­abil­ity ap­pears to be un­usual, Weyand said.

“Our work­ing idea is that he’s prob­a­bly op­ti­mized his speed, and that asym­me­try re­flects that,” Weyand said. “In other words, cor­rect­ing his asym­me­try would not speed him up and might even slow him down. If he were to run sym­met­ri­cally, it could be an un­nat­u­ral gait for him.”

It was once widely as­sumed that the swiftest run­ners achieved top speed by swing­ing their legs more rapidly than slow run­ners while repositioning their limbs be­tween take­off and land­ing.

In a 2000 study, Weyand, then work­ing with a team at Harvard, de­ter­mined that elite sprint­ers did not swing their legs ap­pre­cia­bly quicker through the air. In­stead, they gained max­i­mum speed by strik­ing the ground with a greater force than oth­ers in re­la­tion to their body weight, and for a shorter pe­riod of time.

For Olympic-cal­iber sprint­ers, that peak force can equal five times their body weight, pro­vid­ing lift and propul­sion to be­gin the next stride. In Bolt’s case, his peak force can sur­pass 1,000 pounds.

Peak im­pact force is de­liv­ered within 0.03 sec­onds of strik­ing the track. It is one of the most crit­i­cal mo­ments of sprint­ing. Less force put into the ground means less pop back into the air. Lau­rence Ryan, a physi­cist in the SMU lab, calls that pe­riod “30 mil­lisec­onds to glory.”

In other words, Weyand said, “You win your medal or you’re out of the run­ning based on that short du­ra­tion.”

Sprint­ers like Bolt land just be­hind the ball of the foot, which strikes the ground at an an­gle of about 6 de­grees. His lower leg de­cel­er­ates abruptly, ab­sorb­ing 16 Gs of force. His heel drops for only 0.02 sec­onds — the equiv­a­lent of an inch — be­fore ris­ing again. The to­tal time spent on the ground with each stride is about 0.09 sec­onds.

In ef­fect, there is one biome­chan­i­cal way for world­class sprint­ers to run ex­tremely fast.

“They’re ma­chine-like,” Weyand said. “It’s in­cred­i­ble the ex­tent to which they do the same thing.”

The SMU re­searchers did not know that one of Bolt’s legs was longer than the other when they be­gan their study six months ago. They were test­ing a new mo­tion­based tech­nique, called the two-mass model, which al­lows them to de­ter­mine ground forces by us­ing high­speed video of races in­stead of spe­cially equipped tread­mills in the lab.

Ud­ofa, the lead re­searcher, ex­am­ined 20 steps apiece taken by Bolt and three other elite 100-me­ter sprint­ers, us­ing video from a race in Monaco in 2011.

On av­er­age, Bolt struck the ground with 1,080 pounds of peak force on his right leg and 955 pounds on his left leg. Be­cause his right leg is shorter, it has a slightly longer drop to the track, con­tribut­ing to a higher ve­loc­ity for that step.

“The log­i­cal thing to think is, well, you want both legs to de­liver as much force as pos­si­ble and if one is not de­liv­er­ing as much force, if it de­liv­ered more force, he’d go faster,” Weyand said. “But that su­per­fi­cial logic doesn’t re­ally flesh out.”

A nat­u­ral adap­ta­tion for Bolt has been to keep his left leg on the ground for slightly more time with each step — 0.97 sec­onds, com­pared with 0.85 sec­onds for the right leg. This gives him slightly more time to gen­er­ate force with the left leg, Weyand said, pro­vid­ing greater lift off the ground.

Ralph Mann, a pi­o­neer­ing biome­chan­ics re­searcher in the United States, said he could de­tect a kind of gal­lop in Bolt’s un­even stride. But a vari­abil­ity of 13 or 14 per­cent was sur­pris­ing, Mann said, given that his con­sult­ing work with USA Track and Field gen­er­ally found an asym­me­try be­tween zero and 7 per­cent among elite sprint­ers.

“That’s a huge num­ber; when you see that you’re go­ing to find a phys­i­cal ab­nor­mal­ity,” said Mann, who won a sil­ver medal in the 400me­ter hur­dles at the 1972 Mu­nich Olympics. But by strength­en­ing the so-called weaker leg, he added, “I can think of no rea­son why that’s not go­ing to im­prove per­for­mance.”

There is one per­son who ap­par­ently does not find the SMU re­search par­tic­u­larly in­ter­est­ing. That is Bolt him­self, ac­cord­ing to his agent, Ricky Simms, who said in an email, “He isn’t the kind of per­son who stud­ies this type of thing.”

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