The purpose of this study was to analyse the effects of anterior cruciate ligament reconstruction (ACLR) using the semitendinosus (ST) tendon autograft technique on knee flexor maximal torque, rate of torque development (RTD) and electromechanical delay (EMD) in athletes who had returned to competition, and to determine the relationship between knee flexor strength deficits and hamstring muscle cross-sectional area (CSA). Athletes with ACLR and non-injured, sex-matched, and sport-matched control athletes performed maximal voluntary contractions (MVC) of isometric knee flexion and extension to obtain maximal torque (TorqueMax) and maximal RTD (RTDMax) at 30°, 50°, 70°, 90° and 105° of knee flexion. Ultrasound imaging was used to measure muscle CSA of the knee flexors at 25%, 50%, and 75% of the anatomical distance from the knee crease to the ischial tuberosity. Deficits were found at all knee joint angles for Torquemax and more than 70° of knee flexion for knee flexor RTDmax when comparing the ACLR limb to the Contralateral limb. ST muscle CSA was smaller in the ACLR limb compared to the Contralateral limb at all measurement sites and strong to very strong correlations (r=0.6-0.9) were found between ST muscle CSA and knee flexor strength, especially for the distal ST CSA measurement site. No differences in EMD were found across joint angles, and no differences in ST EMD were found between limbs. The EMD was significantly elongated in the lateral hamstring muscles of the ACLR limb. These results suggest the importance of assessing and preserving ST muscle architecture following ACLR as well as restoring BF mechanical properties. Furthermore, graft choice should be made with consideration of the effect of ACLR on the torque-joint angle relationship and the specific strength requirements of the athlete’s sport.