The Sanskrit word vriksha means tree, A tree is always in a standing position, remains stable and grows upward. The same stages involves in this asana therefore, it is called Vrikshasana.
Position of Readiness
Stand with feet together on the floor. Keep hands at your sides, eyes looking forward. Breathe normally.
Stand up firmly on the left leg and fold the right leg at the knee, raise the right thigh and bring the sole of the right foot as high up the inside of the left thigh as possible. Right foot will be twisted slightly pressing its side against left thigh. do not press the thigh with the heel but with its sides. Balance on the left foot and raise both arms over the head keeping the elbows unbent and joining the palms together. The whole body right from the heels to the fingers of the hands is stretched upward. Hold the posture while breathing gently through the nostrils for about 10 complete breaths. Release the pose exhaling, lower the arms and right leg and return to the tad-asana, standing position with feet together and arms at the sides. Rest for a few seconds and repeat on the opposite leg. Daily PracticeRepeat it two or three times on each leg. Do not practice more six times a day.
In this asana all the muscles are stretched and so it tones up the muscle of the ankles, toes, knees, hip joints, shoulder joints, elbows, hands and fingers. Overall it brings flexibility to the body and removes all the strains activating all the joints of the body.
Also see Vrikshasana Tree Yoga Pose वृक्षासन