Abstract
Anterior cruciate ligament reconstruction (ACLR) restores passive joint stability, yet athletes who meet strength benchmarks may still land stiffly, hesitate to push off, and re-injure. Muscle stiffness governs how rapidly force is transmitted to the joint, and shear wave elastography (SWE) quantifies it muscle by muscle, but SWE findings after ACLR have not been synthesized.
Scopus, PubMed, SportDiscus, and CINAHL were searched through November 2025 following PRISMA 2020. Eligible studies compared SWE-derived lower-extremity muscle stiffness after ACLR with contralateral limbs or controls. Two reviewers independently screened, extracted data, and appraised quality (Downs and Black). Findings were synthesized narratively with Hedges' g (95% confidence intervals) where data permitted.
Six studies (173 ACLR participants, 79 controls; quality poor to good) were included. Reduced passive vastus medialis shear modulus was consistently observed in operative limbs (g − 1.10 to −1.21 versus controls) and was associated with quadriceps strength, knee function, and landing biomechanics. Semitendinosus stiffness was reduced after hamstring tendon harvest (g − 1.29 to −1.64) but not at mid-belly after return to sport. Longitudinal deficits persisted at three months; active-contraction findings conflicted. No study sampled the distal vastus medialis obliquus or the distal hamstring musculotendinous region.
SWE detects muscle-specific mechanical deficits that relate to function beyond strength: a compliant vastus medialis offers a mechanical explanation for the strong patient who lands stiffly, and reduced semitendinosus stiffness marks donor-site morbidity. Standardized protocols that sample clinically relevant regions and follow patients to return to sport are needed before SWE can guide rehabilitation.
•Vastus medialis stiffness is reduced after anterior cruciate ligament reconstruction.•Lower vastus medialis stiffness accompanies weaker quadriceps and stiffer landings.•Semitendinosus stiffness falls after hamstring graft harvest when sampled mid-belly.•Passive and active elastography protocols give different, non-interchangeable results.•Distal vastus medialis and distal hamstring regions remain unmeasured after surgery.