INTRODUCTION: Anterior cruciate ligament (ACL) injuries are prevalent in athletes, with a re-injury risk up to ~30% following reconstruction and rehabilitation. However, the relationship between measures of leg body composition and muscle force produced during isokinetic and ballistic movements (e.g., jumping) remains unclear during rehabilitation following ACL injury. We aimed to longitudinally evaluate differences in relative muscle functionality (force produced per unit lean mass) between injured (INV) and non-injured (NINV) legs in ACL-reconstructed (ACLR) athletes ~5 and 9 months post-surgery and during rehabilitation. METHODS: 19 ACLR athletes (17 F; aged 17.3±2.6 yrs.; BMI 22.8±2.8 kg/m 2 ) underwent dual X-ray absorptiometry (DXA) scanning to quantify whole-body, leg, and gluteal total mass, lean mass (LM), and fat mass (FM). Unilateral knee extension/flexion peak torque (60, 120°/sec) was measured using isokinetic dynamometry. 3 squat jumps (SJ) were performed on dual force plates to measure peak force of each leg. Two-way analysis of variance (ANOVA; visit x leg) with Tukey’s HSD evaluated changes in total-body, leg, and gluteal composition, isokinetic force, and jump peak force at Visits 1 (V1) and 2 (V2). RESULTS: Total leg mass in the INV leg was lower at V1 (12.34±3.07 kg) vs. V2 (12.70±3.02 kg) (main effect Visit p=0.01, partial eta squared (nP2) = 0.99). Upper leg, anterior upper leg, and posterior upper leg total mass and LM increased from V1 to V2 in both INV and NINV legs (Visit p≤0.023, nP2 > 0.14). Gluteal LM was lower in INV and NINV legs at V2 (INV: 3.64±0.61 kg; NINV: 3.63±0.59 kg) vs. V1 (INV: 3.72±0.59 kg; NINV: 3.98±0.79 kg) (Interaction visit x leg p=0.024, nP2 = 0.99). Normalized to anterior upper leg LM, ACLR athletes’ INV leg had greater isokinetic extension force at both speeds at V2 (60°: 58.0±15.9 Nm/kg; 120°: 47.4±12.5 Nm/kg) than V1 (60°: 42.9±16.0 Nm/kg; 120°: 35.4±12.8 Nm/kg), but was lower vs. the NINV leg at both visits (Visit p 0.47; Leg p≤0.004, nP2 > 0.19). Normalized to posterior upper leg LM, ACLR athletes’ INV leg produced greater isokinetic flexion force at both speeds at V2 (60°: 29.7±5.2 Nm/kg; 120°: 25.2±5.0 Nm/kg) vs. V1 (60°: 25.9±10.0 Nm/kg; 120°: 21.4±9.1 Nm/kg) (Visit p≤0.006, nP2 > 0.14). SJ peak force was higher in the NINV vs. INV at both V1 and V2, despite the INV leg’s increased force at V2 (319.5±76.6 N) vs. V1 (264.8±93.26 N) (Interaction p 0.43). CONCLUSION: Although athletes demonstrate increases in LM amount and improvements in function in the INV leg throughout rehabilitation at 9 months vs. 5-months post-ACLR, the INV leg has continued deficits in strength relative to LM amount compared to the NINV leg throughout rehabilitation. Findings indicate a critical need to investigate mechanisms of continued INV leg muscle functional asymmetries to reduce risk of ACL re-injury. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Doolely et al. (2026) studied this question.