Motion analysis helps soccer players get their kicks

A video-based motion analysis study has uncovered significant differences in how males and females go about kicking a soccer ball — differences that may help explain why women are more susceptible to a common knee injury, suggests a sports medicine researcher at Washington University in St. Louis.

“Prior to this kick study, there had been very little motion and EMG analysis of the soccer kick,” said Robert Brophy, M.D., lead investigator and an assistant professor in the department of orthopaedic surgery at Washington University in St. Louis.

“We know that female soccer players face a greater risk of ACL injury and patellofemoral problems and male players are more at risk for sports hernia. We used motion analysis to determine if the two types of players have different lower extremity alignment and muscle activation that might correlate to the injury patterns.”

Brophy, who also serves as head team physician for the St. Louis Athletica professional women’s soccer club, said the study provides clues about underlying causes for certain common sports injuries that fall into gender-based patterns. Eventually, these findings may lead to better treatment, or even prevention for present and future soccer stars.

The study was conducted at the Leon Root Motion Analysis Laboratory at the Hospital for Special Surgery in New York City, where Brophy served as an orthopaedic surgery resident and sports medicine fellow. Data from the study was presented at this year’s American Orthopaedic Society for Sports Medicine meeting in Keystone, Colo.

With the use of video motion analysis, Brophy and colleagues from the Hospital for Special Surgery were able to examine the dynamics of the kicking motion, including differences in soccer kick dynamics based on gender

The researchers found that male and female players do differ in both of these areas. Male players have more activation in the hip flexors of their kicking leg and in their hip abductors of the supporting leg compared to women.

“The hip abductor may be protective against ACL injury and it is interesting that its activation was markedly diminished in women,” said Brophy.

In addition, the hip of the supporting leg in female players collapses more than in males. These two differences, low activation of the hip abductor and the altered hip position, could be a factor in the increased Anterior Cruciate Ligament (ACL) injuries seen in female soccer players.

Female players also have lower activation of the medial quad muscles in their standing leg, one way male players could be protecting their patellofemoral joint from injury. The increased hip flexor activation, however, in the male kicking leg could correlate to the higher rate of sports hernia seen in male soccer players.

With more than eighteen million people playing soccer in the United States, according to the latest FIFA (Fédération Internationale de Football Association) survey, understanding the body mechanics specific to the sport could potentially prevent a large number of injuries from occurring.

“It is a logical extension to think about how our data could be connected to injury patterns,” said Brophy. “And what we found tells us that the role of muscle activation and leg alignment during kicking definitely warrants further investigation.”

During motion analysis, the kick is looked at in two different ways simultaneously. The activity of each of the muscles involved is measured based on electrical impulse.

“But you also need to know what the body is doing when you are taking the readings, is it kicking? Standing? And so on,” said Sherry Backus, PT, DPT, M.A., an advanced clinician at the motion analysis lab and co-author on the paper.

“To record what the body is doing while the muscles are working, we attach bright surface markers to different parts of the body,” said Backus. “Around the room there are eight to 10 cameras that are trained on the markers, recording what the body is doing. We merge the images from all of the cameras and create a three dimensional picture of the person going through the kicking motion. From there we can match up, frame by frame, the electrical signals we get from the muscles with what the person is doing.”

For the most part, Hospital for Special Surgery uses motion analysis in a clinical setting to analyze children who don’t walk normally. Clinicians and doctors identify, before surgery, which muscle is pulling a joint. The analysis allows the doctors to look closely at where motions are coming from so the surgical plan is strategic.

But as motion analysis technology has improved, Hospital for Special Surgery researchers say motion analysis can now be used to look at and analyze faster and more complex movements like the soccer kick.

“In investigating the dynamics of kicking, our hopes are to help soccer players reduce their injury risk and to assess the impact of a specific injury on a player’s return to play,” said Riley J. Williams, M.D., senior author and orthopedic surgeon in the Sports Medicine and Shoulder Service at Hospital for Special Surgery.

“By understanding the mechanics of a soccer kick, the long term goal would be to recommend small changes in training, coaching or strengthening that could protect both female and male athletes from the most common injuries,” continued Dr. Williams, who is also the head team physician for the New York Red Bulls soccer club.

Brophy played collegiate and professional soccer in the United States during the 1990s before turning his passion for sports into a passion for sports medicine. He graduated from Washington University School of Medicine in 2001.

During residency training, his focus on orthopedics and sports medicine was a natural fit and he was accepted for a prestigious fellowship in sports medicine and shoulder surgery at the Hospital for Special Surgery in New York City. In July 2007, he returned to Washington University School of Medicine to join WUSTL Orthopedics and jumped into an active sports medicine practice.

Andrew P. Kraszewski, M.S.; Barbara C. Steele, M.D.; Yan Ma, Ph.D.; and Daniel Osei, M.D., from the Hospital for Special Surgery also contributed to the research.