Sirsasana or headstand is a pose that predominantly uses isometric contraction as the muscles contract to stabilize the body. In this pose the prime mover is the serratus anterior, shown in the image in red. This muscle originates from the ribs and inserts in the scapula with contraction drawing the scapula away from the spine and allowing the hands to extend above the head. The serratus anterior combines with the lower trapezius to depress the scapula. At the same time the rhomboids contract to prevent the scapula from rotating too much. The latissimus dorsi help prevent the winging of the scapula, but are also stretched.
The arms are flexed above the head and slightly externally rotated. The pectorialis major flexes arm up while the infraspinitus assists in external rotation of shoulders. Engaging medial triceps brachii prevents movement of elbows out to sides. The triceps stabilize arms and shoulders. Forearms help to balance body, hands slightly supinated. Rectus abdominus slightly engaged to ensure neutral, or slight posterior tilt of the pelvis. Other core muscles such as the transverse abdominus also provide stability and protect the spine.
The gastrocnemius and soleus contract to point the foot while the tibialis anterior stretches.
The inverted position of sirsasana has effects on a number of systems in the body. If a student is calm and relaxed in sirsasana the inverted position will increase the stroke volume, that is, the volume of blood ejected by each ventricle during a single contraction. This is because gravity assists in the blood leaving the ventricles rather than pushing against it as it does when we are standing. If the student is calm their heart rate will should lower as more blood is leaving the heart so fewer stroke per minute are required. This takes some of the stress off the heart. The inverted state also makes it easier for blood from the toes to return to the heart.
According to yoga theories sirsasana is also one of the best poses for benefiting the pituitary gland which regulates many of the bodies hormones, particularly those relating to the thyroid, affecting our metabolism; adrenal cortex stimulating the release of glucocortoids and thus increasing blood glucose; hormones such as follicle stimulating hormone, luteinizing hormone, prolactin and oxytocin which are involved in reproduction and human growth hormone which stimulates our growth as the inverted position directs a greater quantity of blood to the head.