Abstract
Background:
Quadriceps strength is a primary focus of rehabilitation following anterior cruciate ligament reconstruction (ACLR), yet its relationship to functional knee mechanics during dynamic tasks like running remains unclear. The strut index is a joint-level metric that reflects how much a joint resists compression during stance. Higher values may indicate stiff or bracing-like function, which could be maladaptive post-ACLR. Understanding how quadriceps strength relates to the knee strut index may help refine return-to-running criteria.
Hypothesis:
We hypothesized that involved limbs would exhibit lower knee extensor strength and higher strut indices than uninvolved limbs, and that greater knee extensor strength would be associated with lower strut index, potentially with limb-specific differences in this relationship.
Methods:
Fifteen adolescents post-ACLR (mean age 15.9 ± 1.5 years; 80% male) ran while 3D kinematics and ground reaction forces were collected simultaneously using an 8-camera motion capture system (150 Hz; Vicon, Oxford, UK) and a dual-belt force-instrumented treadmill (1200 Hz; AMTI, Watertown, MA). Data were processed and analyzed using Visual3D (C-Motion, Germantown, MD) and MATLAB (MathWorks Inc., Natick, MA). Strut indices were calculated for each limb based on previously published methods (Kuhman, 2019). Peak isokinetic knee extensor torque at 60 deg/sec was normalized to body mass. Linear mixed-effects models with subject-level random intercepts evaluated: (1) limb differences in torque, (2) limb differences in strut index, and (3) the relationship between torque and strut index, including an interaction term for limb to test whether this association differed between limbs.
Results:
Involved limbs had significantly lower knee extensor strength (β = -0.67, p < 0.001) and higher strut index (β = 7.24, p = 0.004) than uninvolved limbs, supporting both hypotheses. However, knee extensor strength did not significantly predict strut index (β = 3.08, p = 0.418), and there was no significant torque-by-limb interaction (p = 0.432).
Conclusion:
Adolescents post-ACLR showed both reduced quadriceps strength and altered joint behavior in the involved limb, as indicated by elevated strut index during running. However, the lack of a direct relationship between quadriceps strength and strut index suggests that altered running mechanics may reflect compensatory strategies rather than strength deficits alone. These findings highlight that strength and function may recover along separate trajectories. Joint-level metrics like the strut index may provide valuable, independent markers of recovery and merit further investigation in larger cohorts.