Hop Distance Symmetry Does Not Reflect Biomechanical Symmetry in Adolescents Post-ACL Reconstruction

Abstract

PURPOSE: To assess symmetry and biomechanics of young athletes with anterior cruciate ligament reconstruction (ACLR) during a single-leg hop. METHODS: 39 patients with unilateral ACLR (62% female; age 13-18 years; 5-12 months post-surgery) and 29 controls (58% female) performed a single-leg hop for distance and were classified as asymmetric if hop distance on the operative or control limb with the shorter distance was <90% of the contralateral limb. Lower extremity landing biomechanics were compared among operative, non-operative and control limbs. RESULTS: 10/29 controls (34%) and 12/39 patients (31%) were classified as asymmetric. Asymmetric patients hopped a shorter distance on the operative side compared with non-operative and symmetric control limbs (op: 1.3 leg lengths, non-op and control: 1.6 LL, p≤0.04). Symmetric patients tended to hop a shorter distance on both sides (1.4 LL, p=0.17) with lower peak ground reaction force (op and non-op: 2.8 body weights; control: 3.1 BW, p<0.10). Compared to controls, asymmetric patients landed more plantarflexed (op: -18°, control: -2°, p=0.002) with greater pelvic drop (op: -13°, control: -10°, p=0.06) and less knee varus (op: 0°, control: 3°, p=0.05). Operative limbs had lower knee flexion moments (p=0.004) and greater power absorption at the ankle (p=0.05) with a trend of higher dorsiflexion moments (p=0.08). Symmetric patients had greater bilateral hip flexion compared with controls (op: 71°, non-op: 68°, control: 55°, p≤0.001) and less varus at initial contact on the operative side (op: 1°, control: 3°, p=0.03). This resulted in higher hip flexion moments (p≤0.002) and power absorption (p≤0.02) and lower knee valgus moments on both sides compared with controls, as well as lower knee flexion moments on the operative side (p<0.001). CONCLUSIONS: A similar percentage of patients and controls were classified as asymmetric based on single-leg hop distance suggesting hop distance symmetry may not reflect single leg function and return to sport readiness. Both symmetric and asymmetric patients demonstrated biomechanical differences compared with controls but employed different movement strategies. Asymmetric patients offloaded the knee to the ankle, while symmetric patients offloaded the knee to the hip and decreased task performance on the non-operative side.