Study simulates pulling on athlete's jersey to predict noncontact ACL injuries
LAWRENCE — Contact sports frequently see athletes go down with ACL injuries, but they most commonly do not result from direct contact to the knee. To better understand indirect contact knee injuries, a University of Kansas study has simulated pulling on an athlete’s jersey while jumping to determine which types of contact are most risky for such injuries, finding pulling from behind is the most dangerous and that upper body strength is more important in preventing injuries than perhaps thought.
In sports like football, basketball, volleyball and soccer, competitors frequently suffer knee injuries. To better understand how the injuries happen and how to prevent them, the KU study had participants jump while a strap connected to their torso dropped a weighted “slam ball” to pull on them from either the right or left side or from the posterior. The pull simulated injuries where contact to the body but not the knee results in ACL injuries.
“Studies have shown that sports with contact have a more than five times higher chance of ACL injuries than noncontact sports,” said the study's lead author Yu Song, assistant professor of health, sport & exercise sciences and director of the Biomechanics Laboratory at KU. “We are looking at where most of that contact happens. Most of the time it’s in the trunk region when a player is jumping, cutting, planting their foot or changing direction that contributes to injury. So, we’re working to quantify in a lab setting how that trunk contact affects the knee.”
For the study, the research team recruited 31 participants who are active in sports and had no prior history of knee injuries. After warming up, the subjects performed a series of jumps where they jumped from both feet and landed on one. For all jumps, the slam ball was connected to their torso via a strap that did not impede movement. While attempting to touch a basketball fixed above them, researchers dropped the slam ball either front or rear of the subject to simulate someone pulling on their jersey forward or backward. Whole body kinematics and ground reaction forces were measured using optoreflective cameras that measured movements of joints and trunk angles and force plates that measured force applied during takeoff and landing.
Posterior pulling proved to be the most significant in the effect it had on subjects. Jumpers landed with impact of more than two times their body weight during the condition. It also resulted in the smallest peak trunk and knee flexion angles, resulting in subjects landing in such a way that applied more stress to their joints, increasing risk for injury. Anterior pulling saw the highest peak trunk flexion and smallest peak knee extension.
The results help illustrate the importance of the torso in knee injuries. Research has shown that most ACL injuries do not result from direct contact to the knee or leg.
“If we show that being pulled back is more dangerous, which this study indicates, what can we do? We don’t say you can’t play sports, but it suggests that developing your trunk is very important and effective,” Song said. “We can suggest it’s important for athletes not only to do strength training, but to focus on the core and do things like neuromuscular training where you have resistance or pulling.”
The study, co-written with Zhichen Feng, Kareem Mersal and Lauren Salsgiver of the University of Wyoming, Kaden Van Valkenburg of the University of Utah and Boyi Dai of the University of Vermont, was published in the Scandinavian Journal of Medicine & Science in Sports.
In addition to the published work, ongoing research projects continue to strengthen these efforts within the Biomechanics Laboratory at KU. Song praised several undergraduate students who contributed to ongoing studies. The students, not only exercise sciences majors (Anne Jordan, Thanh Nguyen, Nawfal Malik, Lexi Dillon, Lexi Rasmussen), but also from majors ranging from biology to mechanical engineering (Kristina Lincoln, Phoebe Lane, Hammad Javed) at KU, as well as Haskell Indian Nations University in Lawrence (Elijah Savala) conducted data gathering and research for the Biomechanics Laboratory in KU’s Department of Health, Sport and Exercise Sciences, and they recently presented the preliminary findings at KU's Undergraduate Research Symposium.
The study builds on Song and colleagues’ work to better understand both risk factors for knee injuries and previous research on how single-leg hopping can predict recovery from ACL surgery. Future research will expand on the study by pulling on subjects when they start a jump or when planting a foot to see if one is more dangerous. They also plan to see if a subject knowing which direction a pull is coming from changes their kinematic reactions.
While the studies can help better understand causes of knee injuries, Song emphasized that no subjects were injured while performing the jumps. Taken in total, the research can help coaches, trainers, athletes and medical personnel better understand ways to anticipate and prevent what can often be devastating injuries.
“We know being pulled during competition is dangerous. This helps us understand from a biomechanical standpoint what happens to the body when it occurs and what we can do to help prevent it,” Song said. “It also helps show that you don’t only want to look at the knee, but up at the trunk and what’s happening throughout the body.”