Proximal effects could put knees at risk
By Jordana Bieze Foster
Bracing the ankle has kinetic effects at the more proximal joints that could increase the risk of anterior cruciate ligament injury, according to research from Iowa State University presented in August at the annual meeting of the American Society of Biomechanics in Long Beach, CA.
Investigators analyzed 10 healthy volunteers (eight women) as they performed forward and backward horizontal jumps onto a force platform from a 45° angle, landing each jump on one foot. Participants were tested while unbraced and while wearing a rigid stirrup-style ankle brace—the same brace worn prophylactically by all members of the university’s women’s volleyball team at that time.
They found that loading rates for knee extension and knee varus were significantly higher during the braced condition than the unbraced condition, which may suggest an increased risk of knee injury, according to Elizabeth “Kate” Stafford, MPT, a doctoral student in the Department of Kinesiology at Iowa State, who presented her findings at the ASB meeting.
“Loading rate is a measure of how quickly the tissue has to take up the force of landing,” Stafford said. “A faster loading rate usually is not as good, because it means the tissues have less time to adapt to the load. They’re having to stabilize much more quickly when they’re wearing the brace.”
Despite the increased loading rates, the researchers did not observe any increase in joint moments at the knee for the braced condition compared to the unbraced condition. However, they did find that bracing was associated with a significant increase in hip adduction moment.
“We expected to see an increase in knee frontal plane moments because the knee is the next most proximal joint from the ankle,” Stafford said. “But when you think about it, the muscles that control knee abduction and adduction are largely at the hip.”
Although differences between braced and unbraced conditions were not related to jump direction, the researchers were intrigued to find that backward jumps were significantly more likely to result in increased varus and valgus moments at the knee and increased adduction moments at the hip.
An elevated knee valgus moment has been cited by previous studies as a risk factor for ACL injury, but that particular kinetic effect has not been reported in previous studies of the effect of ankle bracing on landing biomechanics. A 2006 study from Indiana University published in the Journal of Athletic Training found that wearing a rigid stirrup-type brace during a drop landing was associated with significant increases in internal-external rotation torque at the knee but not in varus-valgus torque.
In a 2010 study published in the Journal of Sport Rehabilitation, researchers from the University of North Carolina at Charlotte found that knee joint angular displacement during a drop landing was significantly reduced while wearing a semirigid ankle brace compared to an unbraced condition, but that hip angular displacement was not significantly affected. That study also found that bracing was associated with significantly faster time to peak vertical ground reaction force than no bracing.
The Iowa State researchers chose to study horizontal, angled, forward and backward jumps instead of drop landings to more closely mimic the type of athletic situation in which noncontact ACL injuries often occur.
“Rarely are we straight-plane in sports,” Stafford said. “That’s why we did the jumps at a 45° angle and in forward and backward directions, to try to replicate that unpredictability.”
The next step for Stafford and colleagues is to repeat the study and compare the effects of bracing in participants who are unaccustomed to brace wear (like those in the current study) to the effects in habitual brace users (in this case, the Hawkeye women’s volleyball team).