Sex-Based Differences in Objective Measures of Quadriceps Recovery Following Anterior Cruciate Ligament Reconstruction in Adults: A Systematic Review

Background: Quadriceps strength deficit remains one of the most persistent impairments following anterior cruciate ligament reconstruction (ACLR) and is linked to delayed return to sport and increased reinjury risk. While females sustain ACL injuries at substantially greater rates than males, it remains unclear whether biol ogical sex also influences postoperative quadriceps recovery. The purpose of this systematic review was to determine whether sex-based differences exist in quadriceps strength, power, and morphology following ACLR in adults. Methods: This review was conducted in accordance with PRISMA guidelines and prospectively registered with PROSPERO. A comprehensive search of PubMed, CINAHL, SPORTDiscus, and Embase was performed to identify studies reporting sex-specific objective quadriceps outcomes regarding peak torque, peak torque-to-body weight ratio, rate of torque development, or muscle morphology in adults following primary unilateral ACLR. Risk of bias was assessed using the PEDro scale for randomized controlled trials and the Joanna Briggs Institute checklist for non-randomized studies. Results: Of the 1411 unique citations, seven studies met the inclusion criteria, comprising 552 participants (231 females, 321 males). Peak torque limb symmetry index (LSI) favored males in most studies, and peak torque normalized to body weight favored males in every comparison. When these absolute values are interpreted against sex-specific normative data, the male advantage appears inconsistent at 60°/s but consistent at 180°/s, suggesting a more nuanced pattern than raw strength values alone convey. Quadriceps power (rate of torque development) was assessed in only one study, which found a female disadvantage in isometric RTD limb symmetry. A second study assessed a related but mechanistically distinct outcome—the percentage decline in absolute torque after a fatiguing bout—and likewise found a greater decline in females. A single study assessing the morphology of the quadriceps found no sex difference in quad circumference within the ACLR participants. Conclusions: Sex-based quadriceps recovery deficits after ACLR are not uniform. A pattern consistent with a velocity- and fatigue-dependent female disadvantage was observed, but because only one study assessed true power/rate of torque development, testing velocity was confounded with graft type and other sample characteristics across studies, this pattern should be treated as a hypothesis rather than an established feature of sex differences after ACLR. Clinicians should reference strength outcomes against sex-specific normative data; whether incorporating fatigue- and velocity-sensitive testing into return-to-sport criteria improves outcomes has not been established by this body of evidence and warrants prospective study. Given the limited number of eligible studies, these findings should be considered hypothesis-generating.