Researchers are using a new motion capture technique to study the causes of shoulder injuries in tennis. Here, a varsity tennis player serves the ball. On the left is a still frame from a video, and on the right is a computer model based on the motion capture of the video, which enables researchers to gauge stresses on different parts of the body. Images courtesy of Alison Sheets, Ohio State University.
A new approach to motion capture technology is offering fresh insights
into tennis injuries – and orthopedic injuries in general.
studied three types of tennis serves, and identified one in particular,
called a “kick” serve, which creates the highest potential for shoulder
The results, published in a recent issue of Annals of Biomedical Engineering,
could aid sports training and rehab, said Alison Sheets, assistant
professor of mechanical engineering at Ohio State University.
With further development, she added, doctors could use her “markerless motion capture” technique to diagnose patients.
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,” Sheets
said. “People can move naturally, and in a natural setting outside of a
motion capture technology works by attaching markers to a subject’s
skin or clothing and tracking them as the subject moves, she explained.
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
Sheets and her colleagues are working to do away with the markers and take motion capture out of the laboratory.
a project at Stanford University—where Sheets was a postdoctoral
researcher before coming to Ohio State—she was part of a team that
designed a system of eight video cameras that record a person’s
movements at the same time, each shooting from a different angle.
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.
coauthor Marc Safran, MD, an expert in shoulder surgery at Stanford,
proposed the project in order to investigate why he’d seen an uptick in
the number of tennis players with shoulder injuries. Safran’s own 2005 research
had shown that tennis injury rates had risen to as high as 20 injuries
per 1,000 hours played, with most injuries to the upper body.
“To understand the cause of these injuries, we wanted to study how players move in a real, game-like situation,” Sheets said.
recruited seven members of the Stanford men’s varsity tennis team for
the study. 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.
tennis serve is analogous to a baseball pitch, in that a player must
deliver the ball to particular location. In baseball, it’s the “strike
zone,” and in tennis it’s the “service box,” an in-bounds section of the
opposite side of the court. And—just like a pitcher—a server artfully
delivers the ball to make it harder to hit.
first serve in the study, the “flat” serve, is a straight shot down the
center of the court—similar to a pitcher’s fastball. The second, the
“slice” serve, requires the player to brush the racquet against the ball
sideways during the hit. That gives the ball a slight spin that makes
it skid across the ground and take an unpredictable bounce—somewhat
analogous to the unpredictable flight of baseball’s knuckle ball.
last, the “kick” serve, requires the player to brush the ball upwards
from underneath, giving it a lot of topspin. Just like a pitcher’s curve
ball, 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 draws one big difference between a tennis serve and a baseball pitch, however.
baseball, a pitcher uses the arm like a whip. The movement is
sequential—shoulder, elbow, wrist. But in tennis, the shoulder and elbow
move together toward the ball, followed by the elbow alone and then
finally the wrist, so that the racquet is moving the fastest right
before ball impact.”
the flat serve, the player puts almost of all of the energy into moving
the racquet in the forward direction,” she continued. “But for the kick
serve, the player is also trying to precisely time upwards and sideways
racquet movements to generate that characteristic spin.”
study examined the difference in body positioning for the three serves.
Researchers measured the distance between the vertical center line of a
player’s body and the hitting surface of the racquet when the player
hit the ball. For the kick serve, players swung the racquet closer to
the center—about 21 cm (8 inches) and 16 cm (6 inches) closer than for
the flat serve and slice serve, respectively. The players also extended
the racquet farther behind them for the kick serve: 8 cm (3 inches)
farther than for the flat serve.
measurements suggest that the kick serve generates larger forces on
muscles crossing the shoulder joint than the other two serves, which
could promote injury, Sheets said.
now working with Ajit Chaudhari and Timothy Hewitt of OSU Sports
Medicine, who 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
envisions that tennis coaches could use motion capture analysis of
their players – both to prevent injury and improve performance. She
doesn’t think the kick serve itself is going to change anytime soon,
can see how motion capture might change training and rehab, but I don’t
think it’ll change how the game is played,” she said. “In tennis as in
all sports, you do whatever it takes to keep your opponent on their
coauthors on the tennis study include Geoffrey D. Abrams and Thomas P.
Andriacchi of Stanford and Stefano Corazza of Mixamo Inc. in San
Francisco. The research was funded by the BioMotion Laboratory at
Kinematics Differences Between the Flat, Kick, and Slice Serves Measured Using a Markerless Motion Capture Method