Background Limb asymmetries have been identified as a significant predictor of injury risk. However, the natural boundaries of variation in a non-injured population using clinically relevant and reliable measures has not been determined.

Objective To assess clinical measures of limb symmetry and determine the natural boundaries of symmetry present in a non-injured population.

Design Prospective cohort study.

Setting Controlled laboratory research.

Participants 39 non-injured participants, 12 male, 27 female (23.2±2.9 years, 170.2±9.1 cm, 68.4±15.6 kg).

Assessment of Risk Factors Knee PROM, effusion, endfeel, knee extension, flexion, and hip abduction strength, dynamic balance, single leg hop, and IKDC Subjective rating were measured.

Main Outcome Measurements In measures demonstrating excellent reliability (ICC≥0.75, Kappa≥0.81), limb symmetry index (LSI) and variability scores were calculated.

Results All clinical measures, except hip abduction strength, demonstrated excellent reliability. Knee extension PROM was 6.0±5.0°, flexion PROM was 154.0±5.0°. 100% of participants had normal knee extension endfeel, and no knee effusion. 95% had normal flexion endfeel. LSI scores for each measure ranged between 96.6–100.0%. Using minimal detectable change (MDC) scores for each measure, boundaries of limb symmetry were calculated for knee extension PROM (100±1.0°), knee flexion PROM (100±4°), knee extension strength (100±21.0%), knee flexion strength (100±24.0%), dynamic balance (100±6.0%), single leg hop (100±9.1%), and IKDC Subjective rating (100±3.3%).

Conclusions Most measures of limb symmetry used in the study demonstrated appropriate reliability for clinical use. Although strength measures had high degrees of limb symmetry, there was significant variability present in non-injured participants. Conversely, high levels of limb symmetry with very little variation were observed in knee PROM, dynamic balance, single leg hop, and IKDC Subjective measures. Strength may not be the best measure of limb symmetry while PROM, dynamic balance, single leg hop, and the IKDC Subjective form appear to have more utility to identify limb asymmetries.