Deficits in quadriceps voluntary activation following anterior cruciate ligament reconstruction do not improve over time despite increases in quadriceps strength: A systematic review with meta-regression analyses of temporal trends

BACKGROUND: Quadriceps weakness is common following anterior cruciate ligament reconstruction (ACLR). Voluntary activation (VA) deficits may contribute to persistent weakness, but their recovery trajectory remains unclear. This systematic review examined temporal changes in quadriceps strength and VA following ACLR. METHODS: Six databases and 2 clinical trial registries were searched to March 2025. Studies reporting quadriceps strength and VA following ACLR were included. Random-effects meta-analyses and meta-regression examined temporal trends. Risk of bias was assessed using JBI tools and certainty of evidence using GRADE. RESULTS: Fifty studies (1772 participants; 91 effect sizes) were included. Three-level meta-regression demonstrated that VA showed no significant improvement over time (0.03% per month, 95% confidence interval (95%CI): -0.25% to 0.29%; mean = 86.8%, 95%CI: 83.9-89.8%, p = 0.876). Quadriceps strength improved nonlinearly (Q = 31.53, p < 0.001), reaching 2.63 Nm/kg (95%CI: 2.00-3.27) by 69 months. Participant age (p = 0.60), sex (p = 0.50), assessment position (p = 0.44), graft type (p = 0.32) or primary vs. secondary reconstruction (p = 0.56) did not moderate VA. Studies that measured VA with the central activation ratio overestimated VA compared to the interpolated twitch technique by 6.5% (95%CI: 2.4-10.6%, p = 0.002). Evidence certainty was rated as very low. CONCLUSION: Following typical rehabilitation, quadriceps strength improved over time, whereas VA showed no detectable recovery. Strength recovery may therefore occur despite ongoing impairments in neural activation, suggesting strength symmetry alone may overestimate neuromuscular recovery. Given the very low certainty of evidence, these findings should be interpreted cautiously.