The purpose of this study was to analyze the effects of anterior cruciate ligament reconstruction (ACLR) using the semitendinosus (ST) tendon autograft technique on knee flexor maximal torque and rate of torque development (RTD) in athletes who had returned to competition, and to determine the relationship between knee flexor strength deficits and hamstring muscle cross-sectional area (CSA). Fifteen athletes with ACLR (1.9 ± 0.6 years post-surgery) and non-injured, sex-matched, and sport-matched control athletes performed maximal voluntary contractions (MVC) of isometric knee flexion to obtain maximal torque (Torque max) and maximal RTD (RTD max) at 5 knee joint angles. CSA of the knee flexors was measured using ultrasound imaging. Deficits were found at all knee joint angles for Torque max and more than 70° of knee flexion for RTD max when comparing the ACLR limb to the Contralateral limb (P < .05). ST muscle CSA was lower in the ACLR limb compared with the Contralateral limb at all measurement sites (P < .01). Correlations (r = 0.6-0.9) were found between ST muscle CSA and knee flexor strength. These results suggest the importance of preserving ST muscle architecture following ACLR. Further, graft choice should be made with consideration of the specific strength requirements of the athlete's sport.