Lisa Walford's reply:
Moving quickly into Chaturanga Dandasana (Four-Limbed Staff Pose), as in sun salutations, can punish the shoulders, possibly leading to injury. So until you have the strength to hold Chaturanga for a few breaths, I highly recommend that you practice it step by step rather than in the context of a sun salutation.
It is common to think that we need upper body strength to do Chaturanga Dandasana (Four-Limbed Staff Pose). And while this is true, notice the full name of this posture. Dandasana is translated as "staff" or "stick." When you make the rest of your body compact, it is easier for you to coordinate the shoulder and arm muscle strength. So let's start there.
Before bending the elbows for Chaturanga, we often practice the Plank Pose. Come into Plank Pose with a block placed on the floor underneath your hips. In Plank your thighbones should move toward the hamstrings to prevent your hips from sagging toward the floor. Likewise, firm the abdominal muscles to support the weight of the torso. Move the tailbone toward the pubis so that you do not arch the lower back. Finally, the shoulder blades should release down your back to keep the trapezius muscle from rolling forward.
Once you find a place of steadiness and alignment in plank, begin to lower yourself down onto the block. While maintaining the compactness in the torso, reinforce the alignment of the upper body—the shoulder blades move down the back, away from the ears. The upper arms should be parallel to the floor. To do this, the shoulder heads must lift up off the floor and the sternum should project forward toward the chin, rather than closing the collarbones and collapsing the chest. At first it helps to lightly squeeze the elbows into the side ribs to lengthen the front of the body.
If the shoulder heads drop toward the floor, then all the weight goes into the ligaments and tendons or the rotator cuff--which can lead to an unhappy ending in the form of an injury. Instead the triceps muscles (the outer upper arm), the serratus anterior (give yourself a big hug and squeeze your back muscles, this is the serratus), and the latissimus dorsi all contribute to supporting your body weight. You must strengthen these and hold the body as a compact staff or stick to distribute the weight throughout the body.
To reinforce the alignment of the arms and upper body, you can also practice this at the wall. At first stand a few feet away, place the hands shoulder height on the wall, make the body compact by firming your muscles and bend the elbows. Remember to lengthen the front of the body.
In addition, many of the standing poses will strengthen the arms and back if you practice them properly. Remember to draw the shoulder blades down the back, firm the triceps, and really lengthen the arms from the shoulder blades. Keep the sternum well lifted and the collarbones broad.
Lisa Walford is a senior intermediate Iyengar Yoga instructor and has been teaching for more than twenty years. She is one of the directors of the Teacher Training Program at Yoga Works, in Los Angeles. She has served on the faculty of the 1990 and 1993 National Iyengar Yoga Conventions and studies regularly with the Iyengars.