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  • Changing Moment Arms in Yoga Poses to Va

    Diane Bruni asked me to write an article about Moments (or Force Moments

    Moments are created by forces acting in such a way to create a rotation around an axis. The moment arm is the perpendicular distance (or radius) between the force and the axis it is acting on. If you apply a force to the rim of a wheel (and assuming that the wheel has some resistance at its axis), the moment is the product of the component of the force acting at a tangent to the rim of the wheel and the radius of the wheel.

    In terms of yoga poses, understanding moments can be used to add or subtract "weight" from a yoga pose.

    In front triangle (parsvotanasana) you can bend forwards and place your hands on the floor or on yoga blocks or a chair. With your arms helping to support the weight of your upper body, you body weight is then supported by both your legs and your arms. If you lift your arms and reach them back, then the combined center of gravity of your arms and torso is close to your pelvis. The moment acting at the hips is thus the mass of your upper body and arms, accelarated by the force of gravity multiplied by the distance of your center from your pelvis.

    If you reach your arms forwards you shift your center further away from your pelvis. Thus the moment acting at the hips is now greater because of the greater distance of your upper body's center from your hips.

    Transitioning from hands back to hands forwards, this increase in moment translates to having to do more work to keep your torso lifted.

    You could do the same thing in triangle pose. Initially you might rest your hand on your shin, the floor, a yoga block or chair. Here your weight is supported by your arm and your legs.

    Lift your hand and reach your hands past your hips, then your center is quite close to hips and so the moment is mass of upper body times gravity times the distance between center and hips. Reach your arms past your head and you shift your center further away from your hips increasing the moment acting at your hips. here again the experience is that your legs and waist have to do more work to maintain this position. You could think of this as a way of strengthening your waist and legs.

    Want to reduce the weight, reach your arms past your pelvis. Increase the weight slightly by reaching one arm. Increase it more by reaching both arms.

    One of my favorite poses is one leg forward bend. If you do warrior 3, with arms reaching back your center will be close to your pelvis so you can stand and balance with your standing leg vertical. Bring your leg forwards, you shift your center away from your pelvis, towards your head, and so your hips have to move back relative to your foot in order for you to stay balanced. Because your weight is further away from your pelvis your standing leg hip muscles (hams and glutes mainly) have to do more work to keep you stable because of the distance between your center and your hip. Again, mass times gravity times the distance from center to hip.

    In any seated forward bend, if you can bend forwards, you could rest your hands on the floor to support the weight of your upper body. Or you could lift your hands. Your hips have to work against the moment which is center times gravity times distance between center and hips. Reach your arms forwards and you shift your center further from your hips, increasing the moment and thus forcing your hips to do more work.

    In plow pose, if you have your feet on the floor your center is supported by your legs and your shoulders (assuming shoulder blades are pressing into the floor.

    Lift your legs an inch off of the floor. Your center of gravity is somewhere between your shoulders and the back of your head. Your hips have to work against the moment