Mechanical Advantage of Muscle Pennation
The arrangement of fibers in a muscle dictates the classification of the muscles anatomical name (10). In many muscles the fibers are arranged in parallel, meaning the fibers contract in a direct line between the origin and insertion(4). In most other cases the fibers are arranged obliquely in aspect to the origin. This arrangement is classified as a pennated muscle and displays both mechanical advantages and disadvantages (4). The most recognized mechanical advantage of muscle pennation is the increased amount of force production in comparison to the volume it occupies in the body (9). Other factors that this advantage is dependent on are the angle of pull in respect to the axis of contraction in the muscle and rate of force production with each contraction. All advantages of the pennated muscle can best be observed in the comparison of other fiber arrangements.
Type of Fiber Arrangement
The mechanical advantage of the pennate muscle fiber arrangement is seen when compared to the parallel fiber type arrangement. The parallel fiber type arrangement is a direct line of muscle fibers from the origin to the insertion. It is a strap like muscle, example being the sartorius muscle of the thigh (2). The advantage to this fiber type is the rate of force production it creates. The parallel arrangement creates power better than the pennate arrangement. In comparison to the pennate muscle arrangement the fibers situated obliquely to the origin, an example of this arrangement it the rectus femoris in the tight (2). This means that more fibers can be fit in parallel to each other inside the body which creates a larger cross sectional area for work to occur (6). The body prefers the pennated muscle arrangement. The increased area of work allows the muscle to fit more fibers in the same volume of area compared to the parallel muscle (12). The greatest example of this is the hand. If all of the muscles in the hand were parallel the anatomical size of the hand would need to double due to the fact that the hand would have to house twice as many muscles (5). Pennated muscles allow work of the larger and stronger muscles of the hand to be located in the forearm. Final comparison of the fibers is the angle that fiber attaches to the tendon. The angle of pull the fibers are set at will influence the force that the pennate muscle can produce. It works in the same respect as a pulley. The pulley affect requires less force to move an object over a distance, but the rate of force is decreased (8).
Angle of Pull
The angle of pull created by the pennated muscle fiber type arrangement is not limited inherently to the muscle itself. The angle the tendon inserts to the bone also affects the amount of force production. When a pennated muscle is inserted further down the force arm a greater mechanical advantage is seen with a compromise of power (3). The angle of pennation in most muscle is greater than zero (4). When the cosign of the angle of pennation increases more muscle fibers act laterally when contracted to produce a force in the perpendicular direction (6). The ideal angle of pennation to produce a strong yet powerful contraction is 45° to the tendon (7). Muscle pennation is most important for its use in overall force production.
The angle of pull and amount of in parallel fibers in a given volume with muscle pennation creates a greater mechanical advantage for force production. Comparing the force production mathematical, ( = ) it shows that the force generated by the pennated arrangement is larger than the parallel arrangement for the same given volume (11). The drawback to this is that since the fibers are oriented laterally it will take longer for the fibers to transfer force in a vertical plane (1). Parallel fibers create a lower total force value, but since the angle of pennation is not zero the net force exerted by the fiber acts in the direction of the whole muscle force (5). Thus, the full force of the contraction is direct and requires less time to occur resulting in greater power output over the pennated muscle. Pennated muscles create a greater net force, but trade power output as a side effect (4).
The arrangement of the fiber, angle of pull on the tendon and bone, as well as the force production created are all determining factors of the mechanical advantage of muscle pennation. Angle of pulls pulley like effect on the bone and tendon create a mechanical situation that allows a greater amount of work to be done in comparison to a parallel fiber type arrangement (9). The fiber arrangement creates an environment in the body that allows a greater number of fibers to be placed in series as well as a greater number of total fibers in a given volume (4). These factors ultimately contribute to the overall net force production of a given pennated muscle. The trade off for these factors is the decreased rate of force production limiting the power output of a pennated muscle. But, ultimately the mechanical advantage of the pennated fiber is the net force production it creates compared to other fiber types in the same given area (5).
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