Introduction: After anterior cruciate ligament reconstruction (ACLR), individuals often experience chronically impaired ability to activate and contract the knee-extensor muscles (1). Whilst quadriceps inhibition up to 12 months post-ACLR has been attributed to attenuated motor unit (MU) discharge activity in the vastii muscles (2,3), it remains unclear whether these adjustments persist >1 year post-surgery and whether their magnitude varies with contraction level. Furthermore, whilst long-term quadriceps inhibition post-ACLR has been shown to be more pronounced at extended compared to flexed knee positions (4), it is unknown whether this is reflected at the MU level. This study aimed to compare vastii MU discharge properties of the long-term ACL reconstructed leg to the contralateral leg and uninjured controls at different isometric contraction levels and knee-joint angles. Methods: Twelve participants 3.1±1.3 years (range: 1.2-5.1) post a primary, unilateral ACLR with a hamstring tendon autograft and twelve pair-matched (for sex, body mass, and physical activity level) controls performed unilateral isometric knee-extension contractions at 25, 55, and 85° of knee flexion (0° = full extension). Maximal voluntary contractions (MVCs) to assess maximal voluntary torque (MVT) were followed by submaximal trapezoidal and triangular contractions performed at four (10-70% MVT) and two contraction levels (30-50% MVT), respectively. Quadriceps voluntary activation (VA) during MVCs and potentiated resting twitch torque (Qtw) were assessed via percutaneous femoral nerve stimulation. High-density surface electromyography decomposition was used to acquire discharge timings of individual MUs in the vastus lateralis and medialis muscles. Maximal and mean discharge rates were quantified for individual MUs active during MVCs and the plateau phase of trapezoidal contractions, respectively. The magnitude of persistent inward currents (PICs), which modulate spinal motoneuron gain, was estimated by calculating the onset-offset discharge rate hysteresis (ΔF) of MUs active during triangular contractions using paired MU analysis (5). Legs of the ACLR-control participant pairs were matched by dominance, and linear mixed-effects models were used to assess whether group × leg interactions and their interactions with knee-joint angle or contraction level affected the outcome variables. Results: No group × leg interactions were noted for MVT, VA and Qtw (p≥0.197), indicating no deficits in global neuromuscular function of the ACLR compared to the contralateral leg. However, the overall vastii MU discharge rates during both maximal (p=0.002) and submaximal (p<0.001) contractions were lower in the ACLR compared to the contralateral leg, independent of the knee-joint angle (p≥0.433) or contraction level (p=0.672). Although no overall side-to-side differences were noted for ΔF in either participant group (p=0.692), ΔF modulation with contraction level differed between legs in the ACLR group (p=0.009). Specifically, ΔF increased with contraction level (p<0.001) in both control legs and the contralateral leg but remained unchanged in the ACLR leg (p=0.063). Conclusion: Despite restored knee-extensor strength, ACLR individuals may exhibit long-term alterations in motoneuronal output to the vastii muscles, regardless of knee-joint angle or contraction level. This may be due to altered regulation of intrinsic excitability of spinal motoneurons, potentially from chronically increased inhibitory feedback from the injured knee or decreased descending drive to the vastii muscles.