Importance of the theme
In any pose we want to begin by organizing the base. As the quadriceps are the largest muscle group in the legs, it makes sense to devote a fair amount of attention to them, especially at the beginning.
When the legs are straight in standing poses, such as Utthita Trikonasana (Extended Triangle Pose) or Parshvottanasana (Intense Side Stretch Pose), or in forward extensions such as Adho Mukha Shvanasana (Downward Facing Dog Pose) and Upavishtha Konasana (Seated Angle Pose), activating the quadriceps will stabilize and energize the legs. In such cases, engaging the quadriceps causes the end of the muscle, at the knee, to draw up towards the hip. Given that the idea in yoga is always to be creating space in the body, we do not generally want to be thinking in terms of words like “grip,” “contract” or “tighten.” Better to think of lengthening in the direction of action. In this case, think of lifting up out of the base towards the heavens, hence the idea of lifting the quadriceps.
Utthita Trikonasana (Extended Triangle Pose)
Parshvottanasana (Intense Side Stretch Pose)
Adho Mukha Shvanasana (Downward Facing Dog Pose)
Upavishtha Konasana (Seated Angle Pose)