Effects Of ACL Reconstruction On In Vivo Quadriceps Contractile Behavior During Weight Acceptance In Walking

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Appropriate knee loading during walking is essential for optimal health of mechanosensitive joint tissues and is largely governed by quadriceps muscle forces. However, individuals with anterior cruciate ligament reconstruction (ACLr) often exhibit quadriceps muscle dysfunction conventionally measured via reduced peak knee extensor moments (pKEM). Recent advances in ultrasound imaging provide a unique opportunity to determine if quadriceps dysfunction also manifests as altered contractile behavior between those with ACLr and uninjured controls. PURPOSE: Determine differences in quadriceps contractile behavior during weight acceptance in walking between ACLr, contralateral, and control limbs. METHODS: Six individuals to date with unilateral ACLr (4 females, 20±2 yrs, BMI: 25.3±1.8, months post-surgery: 7.1±0.7) and 11 uninjured controls (6 females; 24±3 yrs, BMI: 22.0±2.0) walked for 2 min on an instrumented treadmill. We collected motion capture and ground reaction force data and recorded cine B-mode ultrasound images of the vastus lateralis (VL). We quantified pKEM, knee flexion excursion (KFE), and VL fascicle length change during weight acceptance (i.e., heel-strike to the instant of pKEM). We report effect sizes (ES) for all comparisons. RESULTS: pKEM was 25% lower in the ACLr limb (0.18±0.18 Nm/kg) than the contralateral limb (0.24±0.11 Nm/kg, ES=0.40) and 75% lower than for uninjured controls (0.74±0.19 Nm/kg, ES=3.03). Similarly, the ACLr limb exhibited 21% less KFE (11.4±3.4°) than the contralateral limb (14.5±2.2°, ES=1.08) and 32% less KFE than in uninjured controls (16.8±3.5°, ES=1.57). In uninjured controls, VL fascicles shortened by 0.13±0.23 cm during weight acceptance despite 1.21±0.26 cm of muscle-tendon-unit lengthening, alluding to a predominant role of tendon elongation. VL fascicles in the contralateral limb of ACLr subjects also exhibited shortening during weight acceptance (0.07±0.33 cm). Conversely, we observed fundamentally different behavior in the ACLr limb, for which VL fascicles lengthened by 0.10±0.14 cm (vs controls, ES=1.21). CONCLUSION: ACLr alters quadriceps contractile behavior during weight acceptance in walking. Fascicle lengthening unique to the ACLr limb may be a functional consequence of quadriceps dysfunction relevant to altered knee loading.