Psychologists tell us that the symmetry of a person’s face influences whether he or she is perceived as attractive. But when it comes to rehabilitation of anterior cruciate ligament (ACL) injuries, some types of symmetry are more desirable than others.
One of the primary criteria used to clear athletes for return to sports after ACL reconstruction is between-limb symmetry for quadriceps strength. Because quadriceps strength deficits have been reported in ACL-injured limbs, the reasoning is if quadriceps strength is found to be similar in both limbs, that indicates strength in the injured limb has reached a normal level, which in turn should be protective against the risk of reinjury.
But research from The Ohio State University, presented in February at the annual Combined Sections Meeting of the American Physical Therapy Association, found that quadriceps strength symmetry in athletes who had been cleared to return to sport after ACL reconstruction was not necessarily associated with symmetry between limbs in terms of single-leg landing mechanics. Even in athletes whose quadriceps strength limb symmetry index (LSI) was greater than .90, knee flexion excursion LSI and knee extension moment LSI both differed significantly from those of uninjured controls.
That study did not look at actual reinjury rates, but another study presented in the same session did. Researchers from the University of Delaware found that nine of 90 athletes suffered a second ACL injury after returning to sports, and seven of those had demonstrated between-limb symmetry for quadriceps strength and single-leg hop testing before being cleared.
One big problem with using between-limb symmetry as a criterion for return to sports is that we can’t actually assume that symmetry means the injured limb has become stronger; it might just as often mean that the uninjured limb has become weaker—a common pitfall of rehabilitation programs that focus solely on the injured limb.
To address this potential problem, the Delaware researchers suggest that return-to-sports criteria could be based on comparing the injured limb post-rehab to the uninjured limb shortly after injury, instead of comparing the post-rehab limbs to each other. Of the nine athletes in their study who suffered second ACL injuries, only three would have been cleared to return to sports based on the suggested Estimated Preinjury Capacity (EPIC) criteria.
To be sure, significant between-limb asymmetry is not a good thing. Asymmetries can lead to malalignments and compensatory movement patterns that can have negative ramifications throughout the kinetic chain. And given that 41 of the 93 athletes in the Ohio State study had between-limb quadriceps strength LSIs of less than .85—despite having been cleared to return to sports—that’s something practitioners could pay more attention to during ACL injury rehabilitation.
But just because asymmetry is a risk factor for injury doesn’t mean that symmetry will magically make that injury risk go away. Between-limb symmetry must be considered in the context of other contributing factors, including the status of the uninjured limb. The EPIC system may not prove to be the ultimate solution, but it’s a step in the right direction.