This is an excerpt from Pilates-3rd Edition by Rael Isacowitz.
I like to think of the head as a ball balancing on a pin, or a large sunflower on a long, straight stem. None of the muscles of the neck should be strained; instead they should all act as cables connecting to the head to keep it balanced. None of the cables should have to overwork, but rather they should continuously make micro-adjustments to maintain balance. Adjusting the position of the head and the direction of the gaze of the eyes are among the most common and important corrections I give when I teach. I view the head as simply a large vertebra (or a large ball encompassing the first vertebra), an integral part of the spine, and as such it should follow the line of the spine. Deviations from this alignment, particularly because of the head’s relatively heavy weight, inevitably result in neck tension and strain and can be aesthetically unpleasing. This situation tends to progressively deteriorate with time and age. It is even more pronounced when a person has a long neck, which essentially acts as an extended lever arm. For instance, if someone has forward head posture (the head is carried anterior to the plumb line), the neck extensors will become tight and overworked and the flexors relatively weak and inactive. The head weighs 12-14 pounds (approximately 5 kilograms), so shifting it away from the base of support has a significant effect on the musculature, an effect that grows exponentially as the head moves farther away from the plumb line with a longer lever (neck).
During abdominal exercises using forward flexion such as the Mat: Chest Lift, the trunk lifts up and forward, and the spine, including the head, must follow the natural curve (often referred to as the chest lift position). When forward flexion is inadequate, the head lies farther from the body’s center of gravity, creating a longer lever. The result is often neck strain and tension. This inevitably happens when weak abdominals, tight lower back muscles, or both restrict the forward flexion of the trunk, and is often exacerbated by attempts to maintain a neutral pelvis when the body is inadequately prepared or able to do so (discussed in the next section). Contrary to this, people sometimes pull on the neck with the hands and push the chin to the chest. This also creates pressure on the neck and promotes excessive use of the neck flexors. To understand correct positioning, visualize the body in forward flexion with the sun shining directly overhead. The shoulder girdle would cast a shadow on the ground below, slightly larger than the actual size of the shoulder girdle. This shadow marks the base of support of the upper girdle; the head should be held within this area. Establishing a good position of the head and spine in this exercise is extremely important and lays the foundation for many of the abdominal exercises that follow, both on the mat and on the apparatus (figure 2.8a).
The Mat: Roll-Up offers a good illustration of spine and head alignment and a perfect example of how the art and science of human movement merge. In the sitting phase of this exercise, many people place the head down between the arms. I prefer that the head follow the natural C curve of the spine and be held above the arms, with the arms remaining parallel to the floor (figure 2.8b). Not only does this present a longer, more continuous line, which is visually pleasing, but it also encourages better placement of the head, shoulders, and scapulae, and a balanced interplay between the spinal flexors and extensors—leaving the body devoid of tension and lessening the load on the spine.