Muscles have gears.  A muscles shape changes during a contraction  and it varies with the force of the contraction. Gearing influences muscle force and speed. 

A muscle fascicle is a bundle of skeletal muscle fibers, how they are arranged determines what type of movement a muscle can make. Pennate muscles are muscles whose fascicles are in a slanting position to its tendon. The fibers are classified by how they are arranged unipennate, bipennate and multipennate. Pennate muscles are known in general for high force production and a smaller range of motion. Most muscles in our structure have some degree of pennation.

When a muscle belly contracts pennation angle changes, the fascicles both shorten and rotate to greater angles. Muscle-shape change acts like an automatic transmission allowing a muscle to shift from a high gear during fast contractions to a low gear during slow, high force contractions.

Fiber rotation decreases a muscle's output force and increases output velocity, a muscle resists fiber rotation at high forces. The muscle variable gear ratio, high or low,  is a relationship of ‘muscle velocity/fiber velocity’ and relative to the magnitude of the fiber rotation as the muscle shortens or lengthens.  

The architecture of a muscle changes during a contraction and is variable and affects force and speed. Transformation in pennation angle can actually result in muscle speed exceeding fiber shortening speed.  When a muscle is contracted it alters its thickness, width or both and there is not a relationship of the length changes of the fiber relative to the muscle, other than pennation angle.

What does all this mean to a coach and athlete?  It accentuates the importance of acquiring skill.  The muscles line of pull and the shape the muscle achieves is variable and is relative to numerous factors; nervous system input, joint synchrony, external factors to the body such as outside resistive actions, the surfaces participating on, implements being used and more and more. Each factor affects the force and velocity output and ultimately what ‘gear’ a muscle is in. Which leads to what skill acquisition is about - trying to control the unlimited changeableness of muscle action.  

Whether you are strength training in the weight room, conditioning or practicing your skill, each phase of development requires perfect practice to reduce the variability and maximize the results....... ‘Perfect Practice makes Perfect’ - not just Practice.

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