BACKGROUND: The single-leg squat (SLS) is a common strengthening exercise after anterior cruciate ligament reconstruction (ACLR) surgery. The multi-joint nature of squatting may allow patients to shift the demand from the target muscle group to its synergies, thus reducing the strengthening effect of the SLS exercise. OBJECTIVE: To compare the muscular contribution in the involved and uninvolved side of the ACLR patients and both sides of the ACLR patients with the matched control group during SLS exercise. METHOD: Seventeen athletic men with unilateral ACL reconstruction and 16 athletic healthy men participated. Integrated Electromyography was used to calculate the relative contributions of, Vastus Medialis (VM), Vastus Lateralis (VL), rectus femoris (RF), Gluteus maximus (GMax) and Soleus (SOL) muscles to SLS performance and squat peak force was measured with a Biodex System 3 isokinetic dynamometer. RESULTS: The VM muscle contribution was significantly lower and the VL and GMax contribution was higher on the involved side of the ACLR patients compared to their uninvolved side and the control group. The SLS peak force was significantly lower on the involved side compared to the uninvolved side. CONCLUSION: The increased VL and GMax muscle contribution might be a compensatory mechanism for the reduced VM contribution on the involved side.