In ExPhysiology

October 01, 2007

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Continuing with my theme of muscle mechanics (following my article two months ago on the stretch-shortening cycle), this month I would like to explain the rationale behind the plethora of variable resistance machines and training concepts that are so common. It isn't that designers of exercise machines and fitness programs do not understand muscle mechanics (although some clearly don't), but that knowledge is often applied in ineffective and/or illogical ways.

Take the torque production from a muscle-joint complex for example. As your limbs rotate, the line of action of the muscle force changes, as does the force a muscle can exert at varying lengths, and this results in changes in torque production. Torque is a simple concept that everyone inherently understands. Nobody tries to get out of room by pushing close to the hinge of a door as we all realize that a smaller force applied farther from the axis of rotation will get the job done (over at the handle!). This is torque: mathematically it is force multiplied by the perpendicular distance to the axis of rotation. When working with free weights you learn this fast: keep the weight close to your body--i.e., as close as possible to both the joint's axis of rotation and to the body's center of gravity--and despite the obvious fact that the weight is the same, the torque will be lower and the load will feel more manageable.

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