Background:The aim of this study was to investigate Co-activation of knee flexor muscle during maximal voluntary contraction of the quadriceps muscles at different knee angles. Method and Material: Bipolar surface electromyographic (EMG) signals were simultaneously recorded from knee extensor muscles (vastus medialis, rectus femoris and vastus lateralis) and knee flexor muscles (the medial and lateral heads of the gastrocnemius and the medial and lateral heads of the hamstring) of the right leg during maximal knee extension performed at 90°, 140° and 165° (180° being full extension). Result: The result showed that maximal knee extension force at 90° angle was significantly greater than 140° and at 140° was larger than 165° angle (P< 0.0001). Similarly, EMG amplitudes of the knee extensors at 90° angle was significantly larger than 140° and, at 140° was greater than 165° angle during maximal knee extension contraction (P< 0.05). Conversely, muscle activation for knee flexor muscles were significantly larger at 140° and 165° of knee extension angles in comparison with 90° angle during maximal knee extension contraction (P< 0.05). Conclusion: A greater muscle activation of the knee flexor muscles at 140° and 165° of knee angles produces a high force that acts in opposition to the quadriceps and may partly explain that why knee extension force at the mentioned knee angles are less than 90° of knee angle.

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