Introduction: Mechanical alignment of lower limb has effect on knee joint stability of athletes. Researches show high pronation of ankle is effective in happening anterior cruciate ligament (ACL) injury. So the objective of this research is investigating knee flexion angle and Quadriceps to Hamstring Co-activation in athletes with ankle pronation deformity and healthy.

Materials & methods: In this research 15 athletes with increased pronation of ankle and 15 normal athletes selected purposeful. Electromyographic signals and knee flexion angle recorded with using surface electrode from four muscles (rectus femoris, vastuslateralis, medial hamstring and lateral hamstring)and electro goniometer respective. Statistical analysis method of T- independent was used for investigating between variables.

Findings: Results showed there is significant different between two groups in knee flexion angle (P=0/034) and quadriceps to hamstring Co-activation (P=0/001).

Discussion & Conclusion: Increased pronation of ankle with change in quadriceps to hamstring Co-activation andknee flexion angle can be a risk factor in ACL injury incidence.