Tuesday, November 3, 2009

Using Video Overlay to Compare Two Athletes' Shuttle Runs

A shuttle run is often used in professional sports combines to determine the fitness of a prospective athlete. A shuttle run consists of an athlete running back and forth between two lines approximately 30 feet apart, touching each line before heading back the other direction. The videos below were taken at a basketball court that already has a series of lines marked out. We used those lines to mark the beginning and end points of the run, rather than measuring out 30 feet.

This exercise is a way to evaluate an athlete's speed, quickness, and agility. According to the National Academy of Sports Medicine, “speed” is defined as the ability to move the body in one intended direction as fast as possible, “quickness” is the ability to react and change body position with maximum rate of force production, in all planes or directions of motion, from all body positions, during functional activities, and “agility” is the ability to accelerate, decelerate, stabilize, and change direction quickly, while maintaining proper posture.

Most testing sites simply use the athlete’s overall time as a way to make this evaluation, as shown in the first two parts of the video below (the first subject recorded a time of 10.377 seconds, and the second subject recorded a time of 10.010 seconds). But knowing the overall times only tells us which subject completed the trial the fastest, not why or how one was faster. In this week’s article, I’d like to discuss the value of using the video overlay feature to determine the point or points in the run where one subject gains an advantage over the other.

Video overlay can be used for a wide variety of comparisons. For example, it can be used to compare:
• A single athlete, before and after training (as a way to determine the effectiveness of training)
• A single athlete, before and after rehab.
• Multiple athletes

Video overlay and athlete comparison works best when the subjects are moving from left to right, or right to left,(sagittal plane) across the camera’s view, as it’s much easier to view when the separation between the athletes occurs during the trials. During the 2009 NFL combine, the television coverage tried to use this technology for the 40 yard dash, but they overlaid too many athletes’ trials (10 or more) on top of each other; and since there was very little difference in the athletes’ times, the end video looked like a bit of a mess. Therefore if you plan to use video overlay to compare athletes, don’t overlay more than two or three subjects at the same time.

To make things interesting for our recordings, we chose to make each shuttle run turn facing the camera, so there are two right turns and one left turn in each trial.

Take a look at the third part of the video (showing the overlaid athletes), to see where the separation between these two subjects occurs.

We see that the subject in the gray shirt (we’ll call him Subject 2) gets off to a slightly faster start than Subject 1 (blue shirt), and reaches the far cone first. After making this right turn, the distance between the subjects remains the same until they return to the starting cone, so we can assume they both came out of the first right turn in the same amount of time, and that they ran with the same speed between the far cone and the starting cone. Coming out of the left turn, Subject 2 starts to break away a little more, so it’s possible he is faster pivoting off his left leg. And after making the final right turn, the distance between the two subjects remains about the same after the left turn through the finish.

So what were the turning points in this race? It seems Subject 2 got off to a faster start, and was quicker coming out of the left turn; so it may be useful for Subject 1 to work on his start (see http://videosportsanalysis.blogspot.com/2009/08/video-analysis-of-sprinting-from-start.html for details on how to start fast)> We also see that Subject 1 lost some time in his second turn and acceleration from there. He may also work on strengthening his left leg, to help him make the left turn a bit faster. We also happen to know that Subject 1 has had rehab on his left knee in the past and therefore may have a weakness or instability that still needs work.

By using the overlaid video, we were able to get a lot more information about these trials than just who finished first, and can use this information to make specific recomendations to an athlete's training regimen or to assess the rehabilitation of an athletes injury.

As you can see video overlay can be a powerful tool when evaluating an athlete’s performance. We value your comments and would be happy to answer any questions.

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