Patellofemoral pain’s subgroup situation just got a lot more interesting, thanks to study findings presented in June at the annual meeting of the National Athletic Trainers Association in Baltimore, MD.
Researchers and clinicians who treat patients with patellofemoral pain (PFP) have long been frustrated by the fact that interventions — specifically designed based on the existing literature on PFP risk factors — seem to be consistently effective in some patients, but not all. This pattern suggests that, even in a PFP population that at first appears homogeneous, there are probably subgroups of patients who differ from each other in ways that affect their response to the interventions that have been tested thus far.
The search for PFP subgroups has been an ongoing theme in the medical literature and in the discussions at the four International PFP Research Retreats (see “Conference coverage: 4th PFP research retreat,” January 2016). In this issue, Lace Luedke, PT, DPT, PhD, nicely summarizes the variable findings in the literature related to lower extremity strengthening in runners with PFP, and reviews her group’s work suggesting that hip and knee weakness in high school cross-country runners is predictive of PFP risk (see “Lower extremity strength and injury risk in runners”). This, in turn, suggests that high school cross-country runners may constitute a subgroup that is more likely to benefit from hip and knee strengthening.
Luedke notes, however, that strength assessed isometrically—as most PFP studies have done—may not correlate strongly with muscular control or kinematics, and therefore may not be the best variable on which to base subgroup-specific interventions. Intuitively, this makes sense: Strength typically is not as clinically relevant as the ability to use it.
That’s where the research presented at NATA comes in. Investigators from Brigham Young University in Provo, UT, analyzed the gait of 30 young adults with PFP. All participants had similar self-reported levels of pain, activity, and kinesiophobia, but half were defined as being “quadriceps functional” or “quadriceps deficient” based on central activation ratio (CAR). And as it turned out, the between-group differences in quadriceps function were associated with differences in gait mechanics, including knee flexion angle, knee extension torque, and vertical ground reaction force.
This study has two important take-home messages. The first is that self-reported pain level should not be the sole determinant of any type of PFP intervention, since individuals with similar self-reported pain scores can differ considerably in terms of quadriceps function and gait mechanics. The second is that, as many have suspected, a “functional strength” measure like CAR may be better than isometric strength for defining PFP subgroups and developing targeted interventions.
Of course, such interventions have yet to be developed and tested. And, given the difficulty of testing for CAR and other activation-based measures outside of a laboratory, researchers will also want to look for correlations with variables that are easier to assess clinically. But it seems likely that this line of investigation will take the PFP subgroup discussion in new—and, hopefully, rewarding—directions.