Does the relative muscle activation of the vastus medialis, rectus femoris, and vastus lateralis, during the various activities, change in relation to the quadriceps angle?

[Purpose] The purpose of this study was to examine the correlation between the quadriceps angle and muscle-activation ratios for the vastus medialis, rectus femoris, and vastus lateralis during various activities. [Subjects and Methods] Seventeen heathy females were recruited. The quadriceps angles were measured with long-arm goniometers. The muscle activity of the vastus medialis, rectus femoris, and vastus lateralis were measured using electromyography under four different activity settings: walking, squatting, step-up, and sit-to-stand. The muscle activation ratios were calculated and their correlations with the quadriceps angles were analyzed. [Results] The activation ratio of the rectus femoris to the vastus medialis (and, although less significant, of the vastus lateralis to the vastus medialis) was positively correlated with the quadriceps angle during the step-up and sit-to-stand. A similar tendency was also seen during squatting. The activation ratio of the vastus lateralis to the rectus femoris was negatively correlated with the quadriceps angle during walking. [Conclusion] The relative muscle activity among the muscles composing the quadriceps was correlated with the quadriceps angle. During activities involving deeper knee flexion like the step-up, sit-to-stand, and squatting, the relative activity of the lateral muscles tended to increase as the quadriceps angle increased. Meanwhile, during walking the activity of the medial muscles seemed to increase with a larger quadriceps angle.