User:Bmeador14/sandbox

Architecture & fiber organization
Muscle architecture refers to the arrangement of muscle fibers relative to the axis of force generation of the muscle. This axis is a hypothetical line from the muscle's origin to insertion. For some longitudinal muscles, such as the biceps brachii, this is a relatively simple concept. For others, such as the rectus femoris or deltoid muscle, it becomes more complicated. While the muscle fibers of a fascicle lie parallel to one another, the fascicles themselves can vary in their relationship to one another and to their tendons. The different fiber arrangements produce three broad categories of skeletal muscle architectures: longitudinal, pennate or unipennate, and multipennate. Because of these different architectures, the tension a muscle can create between its tendons varies by more than simply its size and fiber-type makeup.


 * Longitudinal architecture

The fascicles of longitudinally arranged, parallel, or fusiform muscles run parallel to the axis of force generation, thus these muscles on a whole function similarly to a single, large muscle fiber. Variations exist, and the different terms are often used more specifically. For instance, fusiform refers to a longitudinal architecture with a widened muscle belly (biceps), while parallel may refer to a more ribon-shaped longitudinal architecture (rectus abdominis). A less common example would be a circular muscle such as the Orbicularis oris, in which the fibers are longitudinally arranged, but create a circle from origin to insertion.


 * Unipennate architecture

The fibers in unipennate muscles are all oriented at the same (but non-zero) angle relative to the axis of force generation. This angle reduces the effective force of any individual fiber, as it is effectively pulling off-axis. However, because of this angle, more fibers can be packed into the same muscle volume, increasing the Physiological cross-sectional area (PCSA). This effect is known as fiber packing, and--in terms of force generation--it more than overcomes the efficiency loss of the off-axis orientation. The trade-off comes in overall speed of muscle shortening and in total excursion. Overall muscle shortening speed is reduced compared to fiber shortening speed, as is the total distance of shortening. All off these effects scale with pennation angle; greater angles lead to greater force due to increased fiber packing and PCSA, but with greater losses in shortening speed and excursion. The Vastus lateralis is an example of unipennate architecture.

The fibers in multipennate muscles are arranged at multiple angles in relation to the axis of force generation, and are the most general and most common architecture. Several fiber orientations fall into this category; bipennate, convergent, and multipennate. While the determination of PCSA becomes more difficult in these muscle architectures, the same tradeoffs as listed above apply.
 * Multipennate architectures

Bipennate arrangements are essentially "V"s of fibers stacked on top of each other, such as in the rectus femoris.

Convergent arrangements are triangle or fan shaped, with wide origins and more narrow insertions. The wide variation of pennation angles in this architecture can actually allow for multiple functions. For instance, the trapezius, a prototypical convergent muscle, can aid in both shoulder elevation and depression.

Multipennate arrangements are not limited to a particular arrangement, but--when used specifically--commonly refer to what is essentially a combination of bipennate or unipennate arrangements with convergent arrangements. An example of this architecture would be the human deltoid muscle.