Muscles pull on tendons and bones to make us move. As in any system of levers, the transformation of muscle-fiber motion into limb motion can be described by a mechanical advantage, or ratio of an output force to an input force. Muscle systems typically have mechanical advantages less than 1—they reduce forces and amplify speeds so researchers in the field of muscle mechanics work with the reciprocal quantity, the ratio of an output speed to an input speed. They call that number the gear ratio, in analogy to the gears of cars, bicycles, and other machines.
Just as a cyclist can speed up or slow down without shifting gears, a muscle with a fixed gear ratio can produce motion with a range of speeds because the muscle fibers themselves don’t always contract at the same speed. But you can achieve greater maximum speed on a bicycle by shifting to a...
