In this difficult pose, the forward-bending action in the front leg/pelvic half combines with the backward=bending actions in the back leg pelvic half to create the challenge of keeping the spine balanced between those two contradictory actions.

Because of the opposing action of the legs and pelvic halves, the forward=bending aspect of this pose is more difficult than a forward bend done with both legs, and vice versa. This is due to the opposite action of the other leg, which prevents the spine from doing the flexing or extending. Thus, all the movement in the lower half of the body must come from the sacroiliac joint, hip, and leg.

Because there is generally more hip range of motion in flexion than in extension, the movement. of the back leg draws the spine into extension. This is also why more work is often felt in the extensors of the font leg than in the flexors of the back leg.

In a way, this is a bound pose because of the way the action in each leg is limited by the opposing leg. It is therefore possible to over mobilize vulnerable areas (hamstring attachments are especially at risk in this pose). This concern is greatly compounded inf the pose is done passively.

If hanumanasana is done more actively, with attention to the eccentric action of the lengthening muscles, the “stretch” of the pose can be distributed over several joints—stabilizing the mobile ones and mobilizing the fixated areas. Neuromuscularly, this eccentric activity also stimulates the muscles’ spindle position antagonistic muscles (for example, contracting the quadriceps can also activate the “reciprocal innervation” stretch reflex, which stimulates the hamstrings to release further.

In this pose, many people allow the back leg to externally rotate to get it “all the way down.” Letting the back leg roll outputs twisting pressure into the lumbar spine and/or the sacroiliac joint of the back leg— not to mention a twisting pressure into the back knee. It also puts more pressure into the adductors of the back leg (longus, brevis, pectineus, and gracilis) without the eccentric support of the iliacus and psoas major of rectus femoris. As a result, the groin gets overstretched, and the usually overtight rectus femoris doesn’t get much stretch as it could. It’s much more rigorous to do this correctly, and much safer for the legs and pelvis.