Free Your Pelvis
Recently a student asked me how to strengthen his side waist muscles. It is a good, and perennial, question, even if his motives are suspect: What most people asking this question really want to know is how to reduce the "love handles" at their side waists. Unfortunately, research has shown that spot reducing just doesn't work. My student's question is still a good one, though, because the side waist muscles (also called the flank muscles), along with the front abdominal, lower back, and buttock muscles, are crucial in supporting and stabilizing the lower back and pelvis.
Sometimes people try to strengthen the flank muscles by weight lifting. Standing and holding dumbbells, they side-bend to the left, use the right flank muscles to lift the torso back up, and then repeat the action to the other side. I'm not very enthused about this exercise because it creates compression in the lower back. With so many people past the age of 40 showing at least the beginning stages of arthritis in the lower back, further compressing it really isn't a good idea.
However, I can enthusiastically recommend strengthening the side waist by the practicing of Trikonasana (Triangle Pose). But, you might ask, isn't Trikonasana a side stretch? Actually, when properly done, no. (At least, not with the Iyengar approach that I teach; some other yoga styles regard Triangle differently.) In fact, the line of the torso from the side waist to armpit should be flat, not rounded up toward the ceiling, and it is the contraction of the flank muscles that keeps it flat.
Engaging the Side Waist Muscles
Let's take a look at the muscles that comprise the flank. The quadratus lumborum sits deep in the back waist, attaching to the top of the pelvis and traveling up to the last rib and the sides of the lumbar vertebrae. When the quadratus lumborum contracts, it pulls the pelvis and rib cage on the same side closer together. The abdominal obliques also help this action. The external obliques originate on the lower ribs and insert on the pelvis and the abdominal connective tissue; the internal obliques originate on the pelvis and insert on the lower ribs and abdominal connective tissue. Some of the obliques' fibers are nearly vertical between the pelvis and ribs, so they perform a similar action to quadratus lumborum except on the front side of the body.
When you bend to one side, your flank muscles on the opposite side must lengthen. To feel this action, stand up and place your hands on your waist. If you bend to the right, you can feel with your right hand that the right waist is shortened so that your ribs and the top of your pelvis nearly touch. You can also feel that the left waist, ribs, and flank muscles are lengthening and curving, and that quite a gap has opened up between your ribs and the top of your pelvis.
As you practice Trikonasana, the two sides of your spine should lengthen nearly evenly, so there is no curve in your torso. For example, if you do Trikonasana to the right, your left ribs should stay flat and the space between your right ribs and right side of the pelvis should stay open, which helps prevent compression of the right side of your lower back.
To keep your left ribs and waist from lengthening and curving excessively when you do Trikonasana to the right, your left flank muscles must contract to pull the ribs and pelvis closer together; this is how these muscles are strengthened in Trikonasana. The quadratus lumborum and the lateral fibers of your obliques bear a large load. To understand how this works, you must consider how gravity pulls on your torso. The weight of your torso is about half of your total body weight. When you are upright, that weight is centered over the bony structures of the pelvis and legs, but as you begin to tip to the side, much more weight must be held up by your flank muscles. And all this good strengthening work is happening without compressing your lower back.
To get the maximum strengthening benefit for the flank muscles, however, you must also create the proper movement of the pelvis. If the bowl of the pelvis stays upright and you bend to the side, all of the bend must come from the spine, and one side of your back will lengthen while the other side shortens. If, on the other hand, the pelvic bowl tips to the side, the spine can actually remain relatively straight as it becomes more parallel to the floor. This tipping movement seems to be a challenge for many students when learning Trikonasana. One reason for this difficulty is that tipping the pelvis to the side is not a movement you use in your everyday activities, so that it's just not in your movement repertoire. Another limiting factor is tightness in the hamstrings, on the backs of the thighs and adductors, on the inner thighs. These muscle groups originate on or attach to the sitting bones. If they are tight and short, the ability of the pelvis to tip to the side will be limited.
If you do have tight hamstrings and adductors, you would be wise to stretch them before working on Trikonasana. An excellent way to open up the range of motion that you will be needing for Trikonasana is to practice a supported version of Utthita Hasta Padangusthasana (Extended Hand-to-Big-Toe Pose) with your leg out to the side, instead of in front of you. Stand with the right side of your body a few feet away from a chair, so that your left leg is directly under your left hip, and put your right foot up on the chair seat. A chair seat is generally about the correct height to use if you have moderately tight legs, but if you are a little more flexible, you might be able to use a higher prop. Whatever height you do use, make sure that your pelvis is level; if you put your foot up too high, your right pelvis will be higher than the left. Also make sure that your foot and your kneecap point to the ceiling. To open your right hip, stand tall and gently rotate your abdomen to the left. Take care that you don't let your right knee turn forward; keep it pointing toward the ceiling. The gentle stretch that you feel in your right hamstrings and/or inner thigh, if you practice this pose regularly, will make it easier to tip your pelvis sideways in Trikonasana.
Extending Your Triangle Pose
Now let's integrate all these awarenesses into Trikonasana. Stand with your back against a wall and position your feet near the wall with your right foot turned out and your left foot turned in. (Using the wall as a prop is not mandatory, but it is a very valuable tool for learning the correct movement of the pelvis.) Your right buttock should be lightly touching the wall, however your left buttock should not. If you force your left buttock to the wall, your ability to tip your pelvis to the side will be severely limited. So allow the left pelvis to stay forward a little during your transition into Trikonasana; this position also helps keep your right knee properly aligned, with the kneecap pointing over the center of the foot. Now place your right hand at the right hip joint, which is the crease at the top of the thigh where it joins the pelvis. Inhale, and as you exhale, apply pressure with your right hand so that your right hip and thighbone slide to the left. You will be able to feel your right buttock slide on the wall, your pelvis will tip to the right, and your hamstrings and adductors will lengthen.
As you are tipping into the pose, it's a good idea to stop, even if your hand isn't on the floor, when you begin to feel a significant stretch in your right hamstrings and adductors. If you continue to move down into the pose, the tight leg muscles will stop the movement of the pelvis and all further downward movement of your upper body will come from compressing the right ribs and waist and rounding the left side of the torso.
So stop when the right-leg stretch becomes significant, then place your hand on your shin, ankle, or a block, and focus on lengthening the right ribs away from the right thighbone. This action will open up the right side of your waist and your lower back; you may also help increase this opening if you visualize your entire spine lengthening from your tailbone to the base of your skull.
As your flank muscles start to become stronger, you can also integrate this balanced lengthening of both sides of the spine into several other sideways standing poses, like Parsvakonasana (Side Angle Pose), and also Ardha Chandrasana (Half Moon Pose).
Although the standing poses are best known for the excellent work they provide for the legs and hips, remember that they can also contribute to the strength of your torso muscles. Because strong torso muscles can help stabilize your lower back and protect it from injury, standing poses can make a critical contribution to your overall health.