A new approach to motion capture technology is offering fresh insights into tennis injuries – and orthopedic injuries in general. Researchers from Ohio State University studied three types of tennis serves, and identified one in particular, called a “kick” serve, which creates the highest potential for shoulder injury. The results could aid sports training and rehab, said Alison Sheets, assistant professor of mechanical engineering.

With further development, she added, doctors could use her “markerless motion capture” technique to diagnose patients. Traditional motion capture technology works by attaching markers to a subject’s skin or clothing and tracking them as the subject moves. The markers can emit an electronic signal or reflect light, and the associated wiring and other equipment can limit or otherwise influence people’s movement. Moreover, the tracking has to take place in a laboratory setting, where lighting and background are carefully controlled.

A computer program combines the images to identify the 3D volume and shape of the person in each video frame. By comparing this shape to precise body measurements of the person under study, researchers can pinpoint the parts of the body that engage for a particular action, such as serving a tennis ball.

“The potential for markerless motion capture in medicine is vast and exciting, because it can quantify how a person moves without the need to attach electronic markers or other equipment to their body. People can move naturally, and in a natural setting outside of a laboratory,” Sheets said.

The serve is the most often performed stroke in the game, so researchers focused on gauging the effects of three common types of serves on the players’ back, arm, and shoulder joints. The “kick” serve requires the player to brush the ball upwards from underneath, giving it a lot of topspin. The kick serve sends the ball sideways along a wide arc. Ideally, it drops into the opponent’s service box from high above, and produces an equally steep bounce.

Sheets is working with OSU Sports Medicine, which study ACL injuries – a tearing of a ligament in the knee joint. She also has a new project with D. Michele Basso, professor of physical medicine and rehabilitation, to use the technique with animals in order to develop more effective spinal cord injury rehabilitation protocols. Sheets envisions that tennis coaches could use motion capture analysis of their players – both to prevent injury and improve performance.

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