Muscle architecture

Architecture type is determined by the direction in which the muscle fibers are oriented relative to the force-generating axis.

The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present.

[1] These muscles are often used for fast or extensive movements and can be measured by the anatomical cross-sectional area (ACSA).

[3] Parallel muscles can be further defined into three main categories: strap, fusiform, or fan-shaped.

Fusiform muscles are wider and cylindrically shaped in the center and taper off at the ends.

Due to the shape, the force produced by fusiform muscles is concentrated into a small area.

[3][6] Convergent muscles, such as the pectoralis major in humans, have a weaker pull on the attachment site compared to other parallel fibers due to their broad nature.

These muscles are considered versatile because of their ability to change the direction of pull depending on how the fibers are contracting.

[3] Typically, convergent muscles experience varying degrees of fiber strain.

This is largely due to the different lengths and varying insertion points of the muscle fibers.

It has been found that strain becomes uniform over the face of a convergent muscle with the presence of a twisted tendon.

[3][8] Because of this structure, fewer sarcomeres can be found in series, resulting in a shorter fiber length.

[1] The pennation angle in unipennate muscles has been measured at a variety of resting length and typically varies from 0° to 30°.

[1] The stapedius in the middle ear of humans, as well as the rectus femoris of the quadriceps are examples of bipennate muscles.

Muscular hydrostats are typically supported by a membrane of connective tissue which holds the volume constant.

Retaining a constant volume enables the fibers to stabilize the muscle's structure that would otherwise require skeletal support.

[10] A balance of synchronized, compressive and resistive forces along the three lines of action, enable the muscle to move in diverse and complex ways.

These fibers have fast contraction times and maintain some, though not a great amount of their force production with repeated activity due to being moderately fatigue resistant.

The degree of fiber rotation determines the cross-sectional area during the course of the movement which can result in increases of the thickness or width of the muscle.