The challenge of this pose is not one of flexibility, but instead of how to maintain the neutral alignment of the spine and legs and the simple position of the arms against the action of gravity. The asymmetrical relationship to gravity means that muscles have to work asymmetrically to create a symmetrical alignment of the body—essentially tadasana turned on its side.

There are many ways that gravity pulls the body out of tadasana in this pose: The spine may twist, the hips may fall forward or the shoulders may fall back (or vice versa), the bottom scapula and bottom leg may both adduct, and the pelvis may fall to the floor. It’s easy to overcompensate by lifting the hips too high or to create lateral flexion of the spine in either direction, by either giving in to gravity or over resisting it.

Overall, the side plan pose is very simple, but not very easy.