Spinal TrapAntagonist. When most people hear the word, they think of an opponent or adversary. But when anatomists, medical professionals, or scientists talk about antagonist muscles, they are referring to a much more complicated relationship.
Although muscles that are antagonists do control opposite actions, they often cooperate to support a joint or body part. Consider the muscles that flex the wrist and those that extend it: When you do Handstand, both groups contract to help stabilize your wrist. The antagonist muscle groups around the ankle similarly stabilize that joint in Tadasana (Mountain Pose) and other standing poses.
This teamwork by antagonist muscles, called co-contraction, is also important in supporting the spine, especially when you're in an upright posture. Ideally, the muscles of the torso are in balance, so they sustain the normal curves of the spine. But if one set of muscles overwhelms the other, postural problems and pain can result. For example, if the abdominal muscles (which flex the spine into a forward-bending position) overpower their antagonists, the erector spinae (the long, parallel muscle groups along either side of the vertebrae that extend the spine into backbends), the abs will pull the spine into a slumped position and the normal curve of the lower back will be flattened. On the other hand, if the erector spinae overpower the abdominals, the lower back curve becomes excessive. Either of these imbalances can contribute to strain on other muscles, pressure on the spinal disks, and many other problems.
Another important set of antagonists, the hip flexors and the hip extensors, also plays an important role in spinal alignment. These muscles help control the tilt of the pelvis, which forms the foundation for the spinal curves. In turn, these curves affect the position of other parts of the skeleton and the balance of muscular actions used in performing many movements.
When the top front part of your pelvis drops down and forward as the sitting bones and tailbone lift up, the pelvis moves into an anterior tilt. When the top front part of your pelvis lifts up and back and the tailbone drops down and moves forward, the pelvis moves into a posterior tilt. Fortunately, your pelvis includes handy reference points that help you understand which way it is tilting. These reference points are called the anterior superior iliac spines (ASISs). To find them, put your right and left index fingers at your navel, then draw them out to the sides of your abdomen and down about two inches. If you are sitting on the front edge of a chair, you can feel the ASISs drop forward toward the fronts of your thighs as you move into an anterior tilt. If you then use your fingers to lift the ASISs and roll back onto your tailbone, you will move into a posterior tilt. You may also notice that when you tilt to the front, the curve of the lower back increases, and when you tilt to the back, that curve tends to flatten.
The hip flexors and extensors help control the tilt of the pelvis by rotating it over the hip joints. The hip extensors attach to the posterior pelvis and help the hip move from flexion, in which the thigh and abdomen fold toward each other, to a position in which the thigh is in line with the torso. The hip extensors also help tilt the pelvis back. The major players in this movement are the hamstrings, which originate on the ischial tuberosities (often called the sitting bones, the tuberosities form the bottom back part of the pelvis), and the gluteus maximus, which originates on the back of the pelvis and the sacrum. The gluteus maximus attaches on the outer upper thighbone, and the hamstrings attach on the lower leg bones just below the knee; both muscles use these anchors to pull down on the pelvis.