The mechanisms behind the persistent Quadriceps weakness observed after an Anterior Cruciate Ligament reconstruction (ACLr) remain debatable. We described the time course of changes in Quadriceps thickness, strength, and thigh circumference over six months following an ACLr. We also studied the causal relation between Quadriceps thickness, total thickness and strength. Quadriceps thicknesses, Quadriceps strength, and thigh circumference were measured preoperative, 3, and 6 months post-ACLr surgery in 103 patients (77 men and 26 women). Limbs and time were compared with repeated-measures ANOVA and causal relations through mediation analysis (α = 5%). From 0 to 3 months post-surgery, Quadriceps strength (p<0.05), and VI (p<0.05) and VL (p<0.001) thicknesses was reduced. From 3 to 6 months, Quadriceps strength (p<0.05), total Quadriceps thickness (p<0.001), and VI (p<0.001) and VL (p<0.05) thicknesses increased. VM and RF thicknesses increase to 3 months after ACLr (p<0.05). Quadriceps strength, circumference at 5 cm, VI, VL, VM, and total Quadriceps thickness remain lower than the contralateral limb 6 months following an ACLr (p<0.05). The VI thickness determine indirectly (p<0.001) through the total Quadriceps thickness the Quadriceps strength pattern, while the VL thickness directly determined it (p=0.034). Quadriceps thickness adapts heterogeneously, with VI and VL atrophying for up to three months, explaining the post-ACLr quadriceps weakness. By 6 months, VI does not recover its thickness proportion, and VI and VL do not return to pre-surgery conditions. Both vastii muscles primarily determine Quadriceps strength changes, eliciting the VI thickness as a key biomarker for Quadriceps weakness following ACLr.

The authors have declared no competing interest.

This study did not receive any funding

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IRB of Meds Clinic gave ethical approval for this work (#122024)

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