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The parallel muscle architecture is found in muscles where the fibers are parallel to the force-generating axis. [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.
The muscle cross-sectional area (blue line in figure 1, also known as anatomical cross-section area, or ACSA) does not accurately represent the number of muscle fibers in the muscle. A better estimate is provided by the total area of the cross-sections perpendicular to the muscle fibers (green lines in figure 1).
A less common example of a parallel muscle is a circular muscle such as the orbicularis oculi, in which the fibers are longitudinally arranged, but create a circle from origin to insertion. [29] These different architectures, can cause variations in the tension that a muscle can create between its tendons.
Such complementary muscle groups are termed antagonistic pairs. The muscle fibers in a muscular hydrostat are oriented in three directions: parallel to the long axis, perpendicular to the long axis, and wrapped obliquely around the long axis. [1] [2] The muscles parallel to the long axis are arranged in longitudinal bundles.
Hill's elastic muscle model. F: Force; CE: Contractile Element; SE: Series Element; PE: Parallel Element. The three-element Hill muscle model is a representation of the muscle mechanical response. The model is constituted by a contractile element (CE) and two non-linear spring elements, one in series (SE) and another in parallel (PE).
Fusiform muscles have fibres that run parallel to the length of the muscle, and are spindle-shaped. [19] For example, the pronator teres muscle of the forearm . Unipennate muscles have fibres that run the entire length of only one side of a muscle, like a quill pen .
A pennate or pinnate muscle (also called a penniform muscle) is a type of skeletal muscle with fascicles that attach obliquely (in a slanting position) to its tendon. This type of muscle generally allows higher force production but a smaller range of motion. [1] [2] When a muscle contracts and shortens, the pennation angle increases. [3]
Structure of a skeletal muscle. A key component in lateral force transmission in skeletal muscle is the extracellular matrix (ECM). Skeletal muscle is a complex biological material that is composed of muscle fibers and an ECM consisting of the epimysium, perimysium, and endomysium. It can be described as a collagen fiber-reinforced composite.