1 Answers
Architectural gear ratio, also called anatomical gear ratio is a feature of pennate muscle defined by the ratio between the longitudinal strain of the muscle and muscle fiber strain. It is sometimes also defined as the ratio between muscle-shortening velocity and fiber-shortening velocity.
AGR = εx/εf
where εx = longitudinal strain and εf is fiber strain In fusiform muscle, the fibers are longitudinal, so longitudinal strain is equal to fiber strain, and AGR is always 1.
As the pennate muscle is activated, the fibers rotate as they shorten and pull at an angle. In pennate muscles, fibers are oriented at an angle to the muscle's line of action and rotate as they shorten, becoming more oblique such that the fraction of force directed along the muscle's line of action decreases throughout a contraction. Force output is dependent upon the angle of fiber rotation, so changes in muscle thickness and the vector of change in thickness vary; based upon the force being produced. Due to the rotational motion; pennate muscles operate at low velocities. The shortening velocity of the pennate muscle as a whole is greater than that of the individual fibers. This gives rise to the property of AGR. Fiber rotation decreases a muscle's output force but increases output velocity by allowing the muscle to function at a higher gear ratio. Azizi and Brainerd demonstrated that the gear ratio of pennate muscle can vary; dependent on external load.