Shortness in the latissimus dorsi will interfere with lifting the arms overhead. Overly arching the lumbar spine or overly flexing the hips can happen because of a collapse into gravity. Using the hamstrings to draw the ischial tuberosities (sitz bones) forward or using the psoas minor to lift the pubic bone can prevent tipping the pelvis too far forward without necessarily affecting the alignment of the spine.
The knees are very mobile in this position because they are partly flexed. Gravity, rather than muscles working against each other, should be the main source of resistance in the pose. This is an interesting pose to explore a balance between effort and release.
Images from the book Yoga Anatomy by Leslie Kaminoff & Amy Matthews displayed on this website are used under license.