Many different things are said about the amount of external or internal rotation of the back leg in warrior I. What is consistently true is that the back leg is extended and to some degree adducted (in comparison to warrior II, where the back leg is extended and abducted).
We suggest that the back leg be organized from the spiral of the foot upward, and that the bones of the foreleg, thigh, and pelvis orient themselves to create a clear pathway from foot to spine. If the back leg is organized in this way, the amount of internal or external rotation in the hip joint varies from person to person, but the joint spaces can be balanced and the back leg is a strong support for the weight of the torso. This also takes some of the effort of this position from the front leg.
In the back foot, the subtalar joint and the joints between the tarsals and metatarsals need to articulate so that the back part of the foot supinates so the calcaneus can clearly connect to the floor and the forefoot pronates so the toes can clearly connect to the floor. If the foot doesn’t articulate in this way, the outer ankle can be overmobilized and weakened. The amount of rotation needed in the spine depends on how articulate the SI joints and hip joints are—the less mobile the lower limbs are, the more rotation is needed in the spine to orient the chest forward.
Images from the book Yoga Anatomy by Leslie Kaminoff & Amy Matthews displayed on this website are used under license.