By Julie Gudmestad
Published in Yoga Journal

In Part 3 of our ongoing series on basic anatomy, learn the principles of rotation and the joints that govern this motion.

To help deepen your understanding of anatomy in the yoga poses, my first two columns covered the traditional language of movement. This language simplifies and clarifies the description of human movement, which helps you discuss, analyze, and teach the poses with greater accuracy. In this column, we'll take a look at joint rotation and its anatomical underpinnings. And, because rotation is frequently a contributor to injuries to joints and the ligaments, tendons, and muscles that support them, we'll discuss which joints can rotate safely.

The Third Plane

The language of movement is based on the three cardinal planes. The first two planes, which we previously studied, are both vertical. The Sagittal plane bisects the body from front to back, and the movements that occur within it are either flexion or extension. The second plane, the Frontal or Coronal, bisects the body from side to side. If you stand with your back to a wall, the Frontal plane is parallel to the wall, and the Sagittal plane is perpendicular to the wall. The movements that occur in the Frontal plane are abduction (the body part in question is moving away from the midline of the body) and adduction (the body part is moving toward the midline).

In order to discuss rotation, you'll need to understand the third plane, the Horizontal or Transverse. This plane cuts across the body in line with the horizon. It is parallel to a tabletop, but at any level of the body. For example, it could cut across the body at knee, hip, shoulder, jaw, or any other level. The movements that occur in the Horizontal plane are rotation and horizontal abduction and adduction. This column will focus solely on rotation.

Rotation in the Horizontal Plane

To understand how rotation occurs in this plane, picture the plane cutting across the body at shoulder level. The shoulder can rotate internally and externally. Standing upright with arms by your sides, and looking at the front of your body, the shoulder rotates internally when the front of your upper arm turns in and the crease of your inner elbow faces the side of your waist. It rotates externally when the front of your upper arm turns out and the elbow crease faces forward and out. An arrow showing the direction of either rotation would be sitting on the Horizontal plane.

The joints with the greatest, and most frequently activated, rotation are the hips, shoulders, and spine. The hips and shoulders are ball-and-socket joints, and each allows about 90 degrees of internal (also called medial or inward) rotation and 90 degrees of external (lateral or outward) rotation, if it has normal range of motion. In yoga poses, you make use of all of the possible rotation at these joints, although they can be injured by over-rotation. Most poses, including most standing poses and cross-legged sitting poses, require external rotation of the hips. The shoulders, too, primarily perform external rotation in yoga poses-with a few exceptions, such as the "down" arm in Gomukhasana (Cow Face Pose) and the arm that wraps around the knee(s) in Ardha Matsyendrasana (Half Lord of the Fishes Pose) and Pasasana (Noose Pose).

The spine, including the cervical spine in your neck, rotates hundreds of times daily in your normal activities and your yoga practice. Spinal rotation is allowed by the malleability of the intervertebral discs and the facet joints, which are the small joints, one on each side, between each two neighboring vertebrae. While the facets allow movement, they also limit it, so that each segment only contributes a few degrees to the overall rotation of the spine. Spinal rotation can be limited by tightness of the muscles in your back, chest, and/or abdomen, but repetitive or excessive rotation is a frequent contributor to back strain and disc injuries. Interestingly, you can rotate sections of your spine in different directions, as you do in sitting twists when you rotate your torso (the thoracic and lumbar spines) in one direction, and then turn your head, rotating the cervical spine, in the other.

Other Rotating Joints

There are a few other joints in your body that allow some less obvious rotation. Your ankles and forearms are rotating when you turn your palms or soles of your feet up (supination), and when you turn your palms down or collapse the arches of the feet (pronation). The knee has about 70 degrees of rotation, but this is best explored when it's bent and not bearing any weight. Sitting on the floor with your knee bent and heel on the floor, you can feel the sharp front edge of your shinbone rotating under your fingertips as you rotate back and forth on your heel. On the other hand, twisting while bearing weight on a straight knee is likely—sooner or later—to result in significant injury to the meniscus (cartilage) or ligaments of your knee. So while you perform a lot of twists in your yoga practice, don't put big leverage on any one joint, even if it can rotate; and never, ever, put a twist on a weight-bearing knee.