Healthy Pediatric Athletes Have Significant Baseline Limb Asymmetries on Common Return to Sports Performance Tests

Abstract

Objectives: Return to sport (RTS) after anterior cruciate ligament (ACL) reconstruction in children is associated with a higher risk (∼30%) of subsequent ACL injury than in adults. Most RTS testing protocols use Limb Symmetry Indices (LSI) on physical performance tests (PPTs) to assess an athlete's readiness for sport. This assumes that in a healthy state, both lower extremities are and should be equal. We hypothesized that in the pediatric population, baseline limb asymmetry exists, limiting the clinical utility of LSIs. Since LSI > 90% is often used as a clinical cutoff for RTS, we defined a test as valid if 80% of healthy volunteers had an LSI > 90% on that test. Methods: This study included healthy, uninjured volunteers (n = 63) between the ages of 6 and 18 [mean age = 10.7 +/- 3.2 years; 34 females (54%)]. Pubertal Maturity Observation Scores (PMOS) were calculated for all individuals. Demographic data including sex, chronologic age, Pubertal Maturity Observational Score (PMOS), height, weight, and body mass index (BMI) were collected. Subjects performed ten common PPT's including the lower quarter Y-balance, stork balance, stork balance on BOSU, single leg squat (SLS), SLS on BOSU, clockwise and counterclockwise quadrant single leg hop (SLH), forward SLH, timed SLH, and triple crossover SLH. Subjects performed the 10 PPT's in the same order (listed above), however we randomly assigned the starting test and the starting limb to avoid practice effects. We conducted two trials on each leg for each test with the exception of the forward SLH, timed SLH, and triple crossover SLH, which were performed in triplicate. For analysis, we developed a composite score for each limb by averaging trials. We then calculated the absolute value of the side-to-side difference, and normalized this difference to the test mean to obtain a percentage side-to-side difference for each test (%STS). Multivariable linear regression analysis was performed to assess the effect of age on limb symmetry while correcting for body mass index (BMI), PMOS and sex. Results: All 63 subjects successfully completed testing. %STS were not normally distributed for any of the PPTs, therefore data were reported as medians and interquartile ranges. All PPTs showed baseline limb asymmetry, and none met our definition of validity (Figure 1). The most symmetric PPT was the clockwise quadrant hop test (%STS in females, median = 9.85, interquartile range = 4.63-18.7; %STS in males, median = 6.9, interquartile range = 3.64- 14.04). The stork balance on BOSU test had the greatest limb asymmetry (%STS in females, median 41.4, interquartile range 10.1 - 71.3; %STS in males, median 47.6, interquartile range 18.2-66.7, Figure 2). PMOS was strongly correlated with chronologic age (Pearson's ρ = 0.83), therefore PMOS was excluded as a predictor variable in regression models to avoid multicollinearity. Age was an independent predictor of %STS only for the stork test (β = -1.15, 95% CI = -1.92 to -0.38, p = 0.004), with older subject having less limb asymmetry. Conclusion: Healthy children ages 6 - 18 years have significant baseline limb asymmetries on PPTs that are commonly used for RTS decision making after ACL reconstruction. None of the PPTs evaluated in this study met our definition of clinical validity. Limb symmetry was typically not affected by subject age. In light of these results, limb symmetry indices (LSI) should be utilized with caution in this population. Functional recovery may be better assessed by comparison to age and sex-specific norms.