Antagonist. 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.
The prime movers in hip flexion are the psoas, which originates on the lumbar vertebrae and inserts on the inner upper femur (thighbone), and the iliacus, which originates on the inner bowl of the pelvis and attaches on the upper femur. (These muscles are often grouped together and referred to as the iliopsoas, because they perform the same action and converge to attach to the thighbone via the same tendon.) The psoas and iliacus are assisted in hip flexion by several other muscles, most notably the rectus femoris. Part of the quadriceps, the large muscle group on the front of the thigh, the rectus femoris originates on the pelvis near the ASIS and attaches with the other three parts of the quadriceps just below the knee.
The hip flexors fold the thigh and torso closer together. They are in a shortened position when you're seated, and they will shrink and tighten if they're never stretched out to counteract long hours of daily sitting. If your hip flexors have become short and tight, your iliopsoas and rectus femoris will continue to pull forward and down on the front pelvis and lumbar (lower) spine even when you are standing. This pull creates an anterior pelvic tilt, which in turn contributes to an increased curve in the lower back.
If the hip flexors and extensors are balanced in terms of both strength and flexibility, they will support the pelvis in a neutral position, which helps maintain the normal spinal curves and keeps the weight of the upper body centered over the hips when you sit and over the legs when you're on your feet. If the hamstrings are comparatively short and the hip flexors comparatively long, the pelvis will be pulled into a posterior tilt and the lower back curve will be flattened, which can contribute to back strain and, more seriously, disk injuries. On the other hand, if the hip flexors are comparatively short and the hamstrings comparatively long, the anterior tilt contributes to an overarched, compressed lower back. This compression can cause not only short-term discomfort but also a wearing down of the cartilage of the facet joints along the back of the lumbar vertebrae, contributing to arthritis of the lower spine.
Undoing Bad Habits
Any imbalance between your hip flexors and hip extensors can negatively affect your yoga poses in many ways. Fortunately, conscious practice of pelvic and lumbar alignment during your practice can improve the balance between your hip flexors and hip extensors.
Tadasana is a good pose for increasing your pelvic awareness and balance. If you have tight hamstrings, a posterior pelvic tilt, and a tendency toward a flat lower back, you need to release any gripping in the hamstrings and buttocks in Tadasana; this will allow your tailbone to lift up and move back slightly. Moving your inner upper thighs backward will also help release the hip extensors' downward pull on the sitting bones. Also soften any gripping in your abdominal muscles and allow your breath to move gently in the belly. All of these actions and releases will help reestablish the normal lumbar curve.
If you tend toward a posterior tilt, you must move slowly and consciously to avoid reinforcing this bad habit and creating further problems when you practice poses that stretch the hamstrings deeply or require a lot of hamstring flexibility. In seated forward bends, for example, tight hamstrings will pull the sitting bones toward the knees, placing the pelvis in a posterior tilt. If you then reach forward to grab your toes, the movement will come from the lumbar spine, which shifts into a reverse of its normal curve. If you come into this position forcefully or repetitively or hold it for prolonged periods, you can strain or damage the muscles, ligaments, and disks in your lower back.
To avoid injury in these poses, I recommend that most of your hamstring stretching be in poses that make it easier to maintain a normal lumbar curve. These include Supta Padangusthasana (Reclining Hand-to-Big-Toe Pose) and a variation of Utthita Hasta Padangusthasana (Extended Hand-to-Big-Toe Pose) in which you place your foot on a chair, a ledge, or some other support. In both poses, position your stretching leg so you can keep a mild anterior pelvic tilt and a normal lumbar curve. That means not putting your foot up so high in the standing version that the pelvis moves into posterior rotation. Most students with tight hamstrings should start with their foot no higher than the seat of a chair. In the reclining version, keep your buttocks on the floor and use a small rolled towel under the back of your waist to support the normal curve of your lower back. Whether you are lying down or standing, rotate the tailbone and sitting bones toward the back of your body while keeping the knees straight.
With daily work in these poses, your hamstrings will gradually become more flexible, and you'll be able to work on seated forward bends without risking injury to your lower back.
If you have the opposite imbalance—flexible hamstrings and tight hip flexors—make sure you regularly integrate hip flexor stretches into your practice. Such poses include lunges, Virabhadrasana I (Warrior Pose I), and quadriceps stretches. Make sure that you emphasize a posterior tilt, lifting up your ASISs and lengthening the lumbar spine to decompress the vertebrae. Take this same posterior tilt awareness into Tadasana: Lift the ASISs and feel the hamstrings pull down on the ischial tuberosities, but don't grip the buttocks or push the pelvis in front of the line between your shoulder and ankles. Then lift your rib cage away from your waist (especially the back of your waist) and, without gripping your abdominals or restricting your breathing, move your navel toward your spine.
If your hamstrings are really flexible, they may be weak as well, and Setu Bandha Sarvangasana (Bridge Pose) is good for strengthening and teaching them to engage. Make sure to stretch the hip flexors, strongly lifting the tailbone while drawing the knees and ASISs in opposite directions. Maintaining these actions, lift one foot just a few inches off the floor; when you do this, the hamstrings in the standing leg will have to engage to help lift the pelvis and tailbone.
By focusing on the right poses, you can correct muscular imbalances between your hip flexors and extensors, and thus help prevent future back injuries. And with a little mindfulness as you learn to balance the antagonist muscles in your hips, you might also learn a lot about moving from adversarial relationships, in which the strong overpower the weak, to teamwork, in which all parties work together for the good of the whole.
A licensed physical therapist and certified Iyengar Yoga teacher, Julie Gudmestad runs a private physical therapy practice and yoga studio in Portland, Oregon. She regrets that she cannot respond to correspondence or calls requesting
personal health advice.