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Researchers use image processing to study Usain Bolt’s stride. Does scoliosis make him faster?

Posted by Lisa Harvey,

The New York Times article, Something Strange in Usain Bolt’s Stride, asked the question: Is Usain Bolt the fastest sprinter ever in spite of — or because of — an uneven stride that upends conventional wisdom?

Bolt, an eight-time Olympic gold medalist, holds the world record in the 100 m and 200 m race, as well as of part of the 4 x 100 m relay. Bolt is also the only sprinter to win Olympic 100 m and 200 m titles at three consecutive Olympics (2008, 2012 and 2016), a feat referred to as the “triple double.” Yes, he’s that fast. But he also has scoliosis. In fact, his right leg is 1/2 inch shorter than his left leg, per his autobiography.

 

Usain Bolt in the 200m heats, August 7, 2012. Image Credit: Wikipedia CC license.

 

Understanding the mechanics of sprinting

Researchers from Southern Methodists University (SMU) are experts in the biomechanics of sprinting. The SMU Locomotor Lab, led by Peter Weyand, accessed Bolt’s running mechanics to understand what makes him so fast.

They knew from previous studies of world class sprinters to expect peak forces nearing 1000 pounds for each stride. They also understood and that sprinters’ feet are in contact with the ground less than 1/10th of a second. They knew that Bolt was taller than his competitors, taking longer strides. Bolt typically finishes a 100 m race in 41 strides versus his competitors’ 45. What they didn’t know was that Usain Bolt has an uneven stride.

According to The New York Times article, the team did not know that one of Bolt’s legs was longer than the other when they began their study. They discovered that instead of the typical one to three-percent difference in stride, Bolt’s left leg is in contact with the ground 14% longer than his right leg. There was also a significant difference between the force created by his left foot and that of his right foot. His right foot created a peak force of 1080 pounds, whereas the force created by his left foot hitting the track was calculated to be 955 pounds. The difference in peak force is clearly seen in the video below:

 

Video Credit: SMU Locomotor Performance Laboratory

SMU Research News reported, “Running experts generally assume asymmetry impairs performance and slows runners down.” Could this asymmetry be a benefit? Does it help Bolt’s speed?

Image analysis to understand mechanics of gait

The SMU team’s research was recently published in the Journal of Experimental Biology. They applied a two-mass model of the runner’s body, where one mass is the lower limb contacting the ground, and the second mass is the rest of the runner’s body. From this, they created vertical force-time waveforms patterns.

Previous studies utilized runners on a treadmill and measures of force for each stride. The two-mass model requires only body mass and three stride-specific measures as inputs: Contact time, aerial time and lower limb acceleration. Since these three variables can be acquired from a video, the team was able to generate the running ground reaction force waveforms without a requiring the athlete to run on the specially designed treadmill. They didn’t have to measure the force of each strike since they created a model for the value.

 

Lower limb motion and force during ground contact. (A) A stick figure illustration of mass segment m1 motion (a–d) during the foot–ground portion of a running step. The red circle represents the axis of rotation of the ankle joint. (B–E) Corresponding schematic graphs for the vertical position (B), velocity (C), acceleration (D) and force (E) of lower limb mass m1versus time during the ground contact phase. Image Credit: Kenneth P. Clark, Laurence J. Ryan, Peter G. Weyand via Journal of Experimental Biology.

 

Usain Bolt did not set foot in the SMU lab. Instead, the researchers used their two-mass model to analyze a high-speed video of Bolt’s race. They extracted three-dimensional marker coordinates from multiple, high-quality camera videos. The procedure corrected image distortions and calibrated the three-dimensional space and digitizing the markers. The calibration and digitization routines used functions from the MATLAB Image Processing Toolbox.

“The manner in which Bolt achieves his impulses seems to vary from leg to leg,” said Andrew Udofa, a biomechanics researcher at SMU Locomotor Performance Laboratory. “Both the timing and magnitude of force application differed between legs in the steps we have analyzed so far.”

 

Peter Weyand, director of a performance laboratory at SMU. Image Credit: Cooper Neill for The New York Times.

 

Gait optimized for speed

“Our observations raise the immediate scientific question of whether a lack of symmetry represents a personal mechanical optimization that makes Bolt the fastest sprinter ever or exists for reasons yet to be identified,” Udofa said.

Peter Weyand said the consequences of Bolt’s uneven stride may actually have been increased Bolt’s speed.

“Our working idea is that he’s probably optimized his speed, and that asymmetry reflects that,” Weyand stated. “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.”

 

Usain Bolt winning the 100 m final 2008 Olympics. Image Credit: Wikipedia CC.

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