Ardha Bhekasana (Half Frog Pose) is a lovely little backbend that stretches the quadriceps and shoulders while opening the chest and upper back. This variation is the perfect preparation for the full pose, Bhekasana (Frog Pose), a challenging posture present in the Intermediate Series of Ashtanga yoga.
Ardha Bhekasana primarily lengthens the quadriceps, but it can also stretch the psoas if you engage the glutes to extend the hip further as if you were to lift the thigh from the mat and point the knee up (this doesn't need to happen to get the stretch. Simply engaging in this way is enough). In the upper body, the anterior deltoid and pectoralis are opened while the thoracic spine extends.
Come into Ardha Bhekasana lying on your belly. Bring one forearm to the floor beneath your chest with the elbow aligned directly under the shoulder and the forearm parallel with the short edge of your mat. Separate the legs to hip width. Bend one knee and reach back with the same-side hand to take hold of the inner edge of the foot, then swivel the hand so that the elbow points up to the sky and the fingers point down toward the ankle. Be aware of any sensation in your knee as you do this. Strengthen around the ankle and do not allow the foot to rotate outwardly as you bring your hand into position to prevent strain on the inner knee.
Anchor your pose by engaging the straight leg and pressing the top of the foot into the mat. Lift up out of the supporting shoulder as you square the chest forward. Deepen your expression by pressing the heel toward the outer hip, using the opposing forces of the arm against the leg to intensify your sensation. You may leave the hand there with the fingers pointed down, or take it a step further by pivoting on the heel of the hand to point the fingers forward over the toes. This will require a degree of flexibility in the shoulder. Always move with awareness and honor your body's limitations.