To describe the changes in quadriceps and hamstrings muscle strength in the uninvolved limb of male professional and recreational athletes during rehabilitation after anterior cruciate ligament reconstruction (ACLR) and compare to preoperative strength values. Prospective longitudinal study. During rehabilitation, 665 participants who underwent unilateral ACLR performed a strength test preoperatively and every 6 weeks after surgery for up to 9 months. Isokinetic quadriceps and hamstrings strength of the uninvolved limb were measured at an angular velocity of 60°/s and normalized to body weight (N·m/kg). Quadriceps and hamstrings strength of the uninvolved limb gradually increased during rehabilitation until 6 months post-ACLR, and plateaued thereafter. Postoperative quadriceps' strength significantly exceeded preoperative values by 3 months for professional (2.99 N·m/kg; 95% confidence interval [CI]: 2.93, 3.04; = .007) and by 6 months for recreational athletes (2.77 N·m/kg; 95% CI: 2.71, 2.83; <.001), and hamstrings exceeded presurgery levels by 4.5 months (1.71 N·m/kg; 95% CI: 1.67, 1.74; <.001 and 1.43 N·m/kg; 95% CI: 1.40, 1.46; = .002, professional and recreational athletes respectively). In male professional and recreational athletes, uninvolved limb quadriceps and hamstrings strength gradually improved after ACLR, until 6 months after surgery. The uninvolved limb's strength may present as a "moving target" that requires consistent monitoring during rehabilitation. .