The effect of knee flexion angles and ground conditions on the muscle activation of the lower extremity in the squat position.

[Purpose] The purpose of this study is to research the most effective knee flexion angle and ground condition in the squat position. [Subjects and Methods] The subjects of this study were 15 female college students who were able to perform squat movements and who had never previously experienced surgery, orthopedic disease, or musculoskeletal impairment. This study was conducted to examine changes of muscle activation of low-extremity muscles at different knee flexion angles of 70°, 90°, and 100°. Balance Pad (Aero Step, TOGU, Germany) was used as unstable ground. Surface electromyogram (4D-MT & EMD-11, Relive, Korea) was used for measuring muscle activation. Measured muscles were vastus medialis, biceps femoris, tibialis anterior, and gastrocnemius. Muscle activation was determined by the root mean square (RMS). [Results] There was a difference in muscle activation of the vastus medialis and tibialis anterior according to the change of the knee flexion angle with the stable ground. However, there was no difference in muscle activation of the lower extremity muscles according to the change of the knee flexion angle with the unstable ground. [Conclusion] These results suggest that changes in the angle of the knee flexion with the stable ground affect the muscle activation of the vastus medialis and tibialis anterior. It was found that as the joint angle increases, muscle activation also increases. However, ground condition does not affect muscle activation.