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There does not appear to be a definitive source counting all skeletal muscles. Different sources group muscles differently, regarding physical features as different parts of a single muscle or as several muscles. There are also vestigial muscles that are present in some people but absent in others, such as the palmaris longus muscle.
Agonist muscles are also called prime movers since they produce most of the force, and control of an action. [6] Agonists cause a movement to occur through their own activation. [ 7 ] For example, the triceps brachii contracts, producing a shortening (concentric) contraction , during the up phase of a push-up ( elbow extension ).
Muscles are often classed as groups of muscles that work together to carry out an action. In the torso there are several major muscle groups including the pectoral , and abdominal muscles ; intrinsic and extrinsic muscles are subdivisions of muscle groups in the hand , foot , tongue , and extraocular muscles of the eye .
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.
Only skeletal and smooth muscles are part of the musculoskeletal system and only the muscles can move the body. Cardiac muscles are found in the heart and are used only to circulate blood; like the smooth muscles, these muscles are not under conscious control. Skeletal muscles are attached to bones and arranged in opposing groups around joints. [8]
Each rectus abdominus is traversed by bands of connective tissue called the tendinous intersections, which interrupt it into distinct muscle bellies. In people with low body fat , these muscle bellies can be viewed externally in sets from as few as two to as many as twelve, although six is the most common.
In the eyes, the ciliary muscles, iris dilator muscle, and iris sphincter muscle are types of smooth muscles. The iris dilator and sphincter muscles are contained in the iris and contract in order to dilate or constrict the pupils. The ciliary muscles change the shape of the lens to focus on objects in accommodation.
The hypaxial muscles are located on the ventral side of the body, often below the horizontal septum in many species (primarily fish and amphibians). In all species, the hypaxial muscles are innervated by the ventral ramus (branch) of the spinal nerves, while the epaxial muscles are innervated by the dorsal ramus. [citation needed]