WELL, SOMETHING LIKE THAT
One of the most basic principles and rules of nature, function, movement, and motion analysis, is that structure determines function. In other words, anatomy determines movement. Though I will focus on skeletal muscles and movement analysis, anatomy also determines physiology in many cases. Just saying...
Accordingly, the shape of a muscle alone, can reveal a lot of information, and serve us in the analysis of motion. The muscle's shape, gives away several aspects of it function and future influence on the joint/s it can create movement in. Thus, this post is dedicated to the analysis of the relationship between muscle shape, and muscle function.
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Fusiform Muscles - fusiform meaning "spindle like shape", are also known as belly like muscles. Muscles of fusiform shape are best suited for rapid movement and muscle shortening. They also allow a wide range of motion to be created. Their arrangement of muscle fibers, causes the force produced to run down the middle of the muscle fibers in a straight line between the origin and insertion point (point of muscle force application) of the muscle. Such skeletal muscles are often used in AFW levers built for speed (see post about levers). Smooth muscles are most often fusiform in shape.
Examples of fusiform skeletal muscles in the human body include the biceps brachii, tibialis anterior, triceps brachii, rectus femoris, sartorius (longest fusiform muscle in the human body), latissimus dorsi, semitendinosus, brachialis, brachioradialis, pronator teres, palmaris longus, and the psoas major.
Biceps Brachii Muscle
Circular Muscles - round in shape like a doughnut, they commonly function as sphincter muscles that surround an opening and regulate flow, entry, or exit through the opening. The human body includes an estimate one million circular muscles within its blood vessels, known as pre-capillary sphincters, that regulate blood flow.
Examples of circular muscles of the human body include the lower oesophageal sphincter (or cardiac sphincter), the sphincter pupillae (or pupillary sphincter), the orbicularis oculi muscle, the upper oesophageal sphincters, the pyloric sphincter, the ileocecal sphincter, the sphincter of oddi (or glisson's sphincter), the sphincter urethrae, the internal anal sphincter, the external anal sphincter, obicularis oris, and obicularis oculi.
Sphincter muscles at the entrance and exist of the stomach
Convergent Muscles - known also as "triangular muscles", these muscles' origin/s are very wide aspects of the body, much wider than where they insert onto the bone they influence. They muscle fibers go from wide to extremely narrow, literally converging into one spot. This arrangement allows for maximum force production while the muscle is greatly supported for stability via its wide area serving as its origin or origins. These muscles tend to be relatively big and cover a wide area. Usually, they converge into one tendon, causing a significant extent of force to be concentrated into the convergence point, via the tendon.
Examples of convergent muscles include the pectoralis, deltoid, latissimus dorsi, trapezius, and the temporalis muscle of the cranium.
Pectoralis Major muscle. Arrows so the points of convergence
Parallel muscles - also known as "strap muscles", these muscles have muscle fibers that are arranged parallel to each other in straight lines. Depending on who you ask, fusiform muscles can be included in the parallel muscle group. They tend to be of great length (elongated muscles), that are built for sustainable function (endurance) and greater ranges of motion. They are perfect for linear movement (in a straight line). They are also perfectly structured to be phasic muscles if they are shorter in length.Contrary to the longer strap muscles, shorter strap muscles are perfect for maximal power production, meaning maximal force production in minimal time.
Examples of parallel/strap muscles include the sternohyoid, sternothyroid, thyrohyoid and omohyoid (all four are known as the infrahyoid muscles), erector spinae, sartorius, sternocleidomastoid, digastric, stylohyoid, mylohyoid and geniohyoid (all four are known as the suprahyoid muscles).
Sternocleidomastoid Muscle
Pennate muscles - also known as a penniform muscle; these muscles take the shape of either a one-sided feather or a double-sided feather, which gives them their name pennate that means feather or bilateral in some cases. The muscle fibers attache diagonally to the tendon from the side, putting them is a slanting position. These muscles are characterized by an increased number of muscle fibers per unit, extensive force production, yet fatigue quickly. This family of muscles can further be divided into thre subgroups:
Uni-pennate muscles - these muscles have fibers connecting diagonally to the tendon from only one side of the tendon
Bi-pennate muscles - these muscles have fibers connecting diagonally to the tendon from both sides of the tendon
Multi-pennate - these muscles have a central tendon that branches out into to sub-tendons. Each sub-tendon, if analyzed separately takes the shape of a unipennate or bipennate, yet together the entire muscle looks like a fan
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Examples of uni-pennate muscles include the lumbricals (deep hand muscles), extensor digitorum communis (wrist and finger extensor), lexor pollicis longus, palmar interossei or volar, and vastus lateralis.
Vastus lateralis muscle
Examples of bi-ipennate muscles include the rectus femoris, biceps brachii, stapedius, the third and fourth medial lumbricals, dorsal interosse, gastrocnemius, and supraspinatus.
Gastrocnemius muscle
Examples of multi-pennate muscles include the deltoid, pectoralis major, trapezius, serratus anterior, soleus, masseter, and subscapolaris.
Deltoid muscle
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