The anatomical arrangement of skeletal, or striated muscle fascicles creates four types of skeletal muscles classified on the basis of fiber arrangement: parallel, convergent, pennate, and sphincter. Skeletal muscles can also be classified based on their predominant mechanism of energy production, with Type I fibers favoring oxidative metabolism and Type II fibers being primarily glycolytic.
Parallel muscles are characterized by fascicles that run parallel to one another, and contraction of these muscle groups acts as an extension of the contraction of a single muscle fiber. Most skeletal muscles in the body are parallel muscles; although they can be seen in a variety of shapes such as flat bands, spindle shaped, and some can have large protrusions in their middle known as the belly of the muscle.
Convergent muscles have a common point of attachment, from which the muscle fascicles extend outward, not necessarily in a specific spatial pattern, allowing the muscle to cover a broad surface. These muscles do not tend to exert as much force on their tendons. Muscle fibers can often exert opposing effects during contraction, such as not pulling in the same direction depending on the location of the muscle fiber. Covering a broad surface these fibers allow for more versatile types of movement. Because the fascicles pull on the tendons at an angle, they do not move the tendon as far as their parallel muscle counterparts. Despite this they generate greater tension because they possess a greater amount of muscle fibers than similarly sized parallel muscles.
In Pennate muscles , the tendon runs through the length of the muscle. Fascicles pull on the tendon at an angle, thus not moving as far at the parallel muscles during a contraction. However, these muscles tend to have relatively more muscle fibers than similarly sized parallel muscles, and thus carry more tension.
The fibers of the sphincter or circular muscles are arranged concentrically around an opening or recess. As the muscle contracts, the opening it circumvents gets smaller. For this reason, these muscles are often found at the entrances and exits of external and internal passage ways.
In addition to structural classifications, skeletal muscle can be classified on the basis of mechanism of energy production determined by the type of myosin (fast or slow) and the degree of oxidative phosphorylation that the fiber undergoes. Type I fibers appear red due to the presence of the oxygen binding protein myoglobin. These fibers are suited for endurance and are slow to fatigue because they use oxidative metabolism to generate ATP. Type II fibers are white due to the absence of myoglobin and a reliance on glycolytic enzymes. These fibers are efficient for short bursts of speed and power and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type. These fibers are quicker to fatigue.