The legs in this pose are performing the same actions as in warrior II, and similar muscle groups are active. In this pose, however, the weight of the torso falls more over the front leg, and the muscles of the front leg need additional strength, length, and stamina.
While the position of the upper arm alongside the head is similar to that of the arms in utkatasana and virabhadrasana III, different muscles are required to maintain the arm position in this pose because of the different relationship to gravity. The action is also more eccentric than concentric, again because of the relationship of the weight of the arm to gravity.
Images from the book Yoga Anatomy by Leslie Kaminoff & Amy Matthews displayed on this website are used under license.