Objective The isokinetic muscle strength indicators and surface electromyography (sEMG) signals of the healthy side lower limb of patients with non-contact anterior cruciate ligament injury (ACLD) in the acute phase were compared with those of healthy individuals, so to provide a theoretical basis for the prevention of secondary ACL rupture in the healthy side lower limb of ACLD patients. Methods 21 patients with ACLD in the acute phase (ACLD group) and 21 healthy individuals (control group) were subjected to isokinetic muscle strength tests for hip flexion, extension, adduction, abduction, and knee flexion, and sEMG signals of the vastus medialis, vastus lateralis, semitendinosus, biceps femoris, and gluteus maximus were collected during the single-leg jumping movements. The differences between the two groups in relative peak torque, antagonist muscle ratio, root mean square amplitude, and co-contraction index were analyzed by independent sample t-test. Results The extension of the knee (P=0.040) and hip flexion strength (P=0.041) were lower in the healthy side lower limb of ACLD patients. In the single-leg landing action, there were lower activation of the vastus medialis muscle in the pre-activation phase (P=0.014), lower activation of the vastus medialis (P=0.010) and gluteus maximus (P=0.002) in the reaction phase, and also lower co-activation levels of the biceps femoris-semitendinosus (BF-ST) (P=0.020). Conclusions At the early stage of injury, the function of the healthy side lower limb of ACLD patients changes, which may indicate why ACLD patients face a greater risk of contralateral lower limb injury.