Early Neuromuscular and Co-activation Patterns After Anterior Cruciate Ligament Reconstruction In Males: A Case-control Exploratory Feasibility Study

Background Arthrogenic muscle inhibition (AMI) after anterior cruciate ligament reconstruction (ACLR) is a major barrier to muscle recovery, but early (within the first ~2 weeks after surgery) neuromuscular profiles remain poorly quantified. Excessive hamstring co-activation, a common compensatory response to quadriceps inhibition, may further compromise knee mechanics and rehabilitation outcomes. Hypothesis/Purpose: The primary objective of this study was to compare early quadriceps activation and quadriceps/hamstring co-activation patterns between individuals following ACLR and matched healthy controls (CTRL). The secondary objective was to determine whether these neuromuscular alterations were present bilaterally, affecting both the operated and non-operated limbs. It was hypothesized that individuals post-ACLR would demonstrate reduced voluntary quadriceps activation and increased hamstring co-activation compared to CTRL, as well as bilateral neuromuscular alterations within the ACLR group. Study Design Cross-sectional case–control study. Methods Thirty males (15 primary unilateral ACLR with hamstring autograft, 15 healthy controls) were assessed 15.3 ± 0.7 days after ACLR (pre-injury Tegner Activity Scale: 7 ± 1). Inclusion criteria required a Stroke Test score < 1+ and no passive extension deficit, defined by SANTI (Scientific Anterior Cruciate Ligament Network International) classification of AMI type 1-2. Participants performed three 5-second maximal voluntary isometric knee extensions (MVIC) in supine position with the knee fully extended; no sEMG normalization was performed, and raw RMS values were used for analysis. Surface electromyography (sEMG) was recorded from vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and medial hamstrings (MH) at 1000 Hz; mean Root Mean Square (RMS) values were then computed over a 3-s stable plateau window of each MVIC and used for analysis. Co-activation Indices (CoI) were calculated for the agonist–antagonist pairs VL–BF, VM–BF, VL–MH, and VM–MH. ACLR group versus CTRL was via independent-samples t-tests, and intra-individual ACLR group comparisons (operated vs. non operated limb) via paired-samples t-tests. Effect sizes (Hedges’ g for between-group, Cohen’s d for paired comparisons; rank-biserial correlation for non-parametric tests) were reported with 95% confidence intervals. Results Significantly lower mean values were found in the operated limb of the ACLR group compared to CTRL for VL (4.15 ± 2.24 vs 7.89 ± 1.50 μV, p < 0.001), VM (4.07 ± 2.25 vs 9.07 ± 1.24 μV, p < 0.001), and BF (3.73 ± 2.64 vs 5.79 ± 3.56 μV, p = 0.011). In the operated limb, all CoI were significantly higher (all p < 0.001) in the ACLR group than in CTRL for VL–BF (47.77 ± 9.92% vs 32.07 ± 9.32%), VM–BF (48.83 ± 8.98% vs 29.10 ± 5.45%), VL–MH (49.83 ± 11.58% vs 31.08 ± 6.78%), and VM–MH (48.17 ± 9.38% vs 29.11 ± 5.69%). Intra-individual and contralateral comparisons showed that the ACLR group presented with significantly higher CoI in the non-operated limb compared to CTRL for VL–MH (36.1 ± 5.1% vs 29.9 ± 6.0%, p = 0.018) and VM–MH (35.7 ± 5.9% vs 28.4 ± 4.9%, p < 0.001). No significant between-group differences were found for MH activation (p = 0.250). Conclusion Within the first 15.3 ± 0.7 days after ACLR, subjects who had undergone ACLR showed markedly reduced voluntary quadriceps activation associated with increased, bilateral hamstring co-activation, in both the operated and non-operated limb. Level of Evidence Level 3, cross-sectional case–control study.