This is one of the standing poses in which the pelvis faces forward, parallel to the front plane of the body. It is the natural progression from warrior II, with one hand placed on the floor and the other stretching over the head.
Pelvis and legs
The back leg buttock muscle extends the hip and turns it outward. The group of muscles on the inside of the thigh extend and draw the leg towards the midline stabilizing the back foot on the floor. The tensor fascia lata and gluteus medius turn the hip inward. This balances the strong outward pull by the large buttock muscle. The quadriceps, straighten the back knee. The muscle along the front of the shin draws the ankle towards the shin, which stretches the calve muscles and the muscles along the outside of the calf. The psoas and pectinius muscles bend the hip of the front leg. The muscle crossing diagonally over the midline of the thigh refines this action. The quadriceps of the front legs activates to support the body weight. The muscles along the outside of the front leg calve turn the ankle slightly outwards. The foot is pressed firmly against the floor by flexing the ankle.
The lower side abdominal oblique and transverse muscle draws the trunk towards the bent leg, stretching the same muscles on the upper side of the trunk. On the lower side the muscles along the spine and those in the lower back bend the trunk to the side stretching the corresponding muscles on the upper side.
Shoulders and arms
The muscles attached to the side of the ribs and the shoulder blade straightens the lower side arm drawing the same side shoulder blade away from the midline and pressing the hand to the floor. The triceps straighten the elbows the back portion of the deltoid on the lower arm draws the arm away from the midline. Since the hand in fixed on the floor this action turns the chest upwards and opens it. On the lower side the muscles that wrap the kidneys and lower arms ribs opens the chest through the closed chain contraction. This occurs because the muscles of the upper arm remains fixed with the origins on the lower back moving, lifting the chest. Muscles over the shoulder blade turn the arm outward. The lower section of the trapezius draws the shoulders away from the ear freeing the neck.