Anatomical terminology is used to uniquely describe aspects of

skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

, cardiac muscle, and

smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

such as their actions, structure, size, and location.

Types

There are three types of muscle tissue in the body: skeletal, smooth, and cardiac.

Skeletal muscle

Skeletal muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

, or "voluntary muscle", is a

striated muscle tissue Striations means a series of ridges, furrows or linear marks, and is used in several ways: * Glacial striation * Striation (fatigue), in material * Striation (geology), a ''striation'' as a result of a geological fault * Striation Valley, in Anta ...

that primarily joins to bone with tendons. Skeletal muscle enables movement of bones, and maintains posture. The widest part of a muscle that pulls on the tendons is known as the belly.

Muscle slip

A muscle slip is a slip of muscle that can either be an

anatomical variant An anatomical variation, anatomical variant, or anatomical variability is a presentation of body structure with morphological features different from those that are typically described in the majority of individuals. Anatomical variations are categ ...

, or a branching of a muscle as in

rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...

connections of the serratus anterior muscle.

Smooth muscle

Smooth muscle Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...

is involuntary and found in parts of the body where it conveys action without conscious intent. The majority of this type of muscle tissue is found in the digestive and urinary systems where it acts by propelling forward food,

chyme Chyme or chymus (; from Greek χυμός ''khymos'', "juice") is the semi-fluid mass of partly digested food that is expelled by a person's stomach, through the pyloric valve, into the duodenum feces Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

in the former and urine in the latter. Other places smooth muscle can be found are within the uterus, where it helps facilitate birth, and the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

, where the

pupillary sphincter The iris sphincter muscle (pupillary sphincter, pupillary constrictor, circular muscle of iris, circular fibers) is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor ...

controls pupil size.

Cardiac muscle

Cardiac muscle is specific to the heart. It is also involuntary in its movement, and is additionally self-excitatory, contracting without outside stimuli.

Actions of skeletal muscle

As well as anatomical terms of motion, which describe the motion made by a muscle, unique terminology is used to describe the action of a set of muscles.

Agonists and antagonists

Agonist muscles and antagonist muscles are muscles that cause or inhibit a movement. Agonist muscles are also called prime movers since they produce most of the force, and control of an action. Agonists cause a movement to occur through their own activation. For example, the triceps brachii contracts, producing a shortening (concentric) contraction, during the up phase of a push-up (

elbow extension The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the media ...

). During the down phase of a push-up, the same triceps brachii actively controls elbow flexion while producing a lengthening (eccentric) contraction. It is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action. Another example is the dumb-bell curl at the elbow. The elbow flexor group is the agonist, shortening during the lifting phase (

elbow flexion The elbow is the region between the arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and the media ...

). During the lowering phase the elbow flexor muscles lengthen, remaining the agonists because they are controlling the load and the movement (elbow extension). For both the lifting and lowering phase, the "elbow extensor" muscles are the antagonists (see below). They lengthen during the dumbbell lifting phase and shorten during the dumbbell lowering phase. Here it is important to understand that it is common practice to give a name to a muscle group (e.g. elbow flexors) based on the joint action they produce during a shortening contraction. However, this naming convention does not mean they are only agonists during shortening. This term typically describes the function of

s. Antagonist muscles are simply the muscles that produce an opposing joint torque to the agonist muscles. This torque can aid in controlling a motion. The opposing torque can slow movement down - especially in the case of a ballistic movement. For example, during a very rapid (ballistic) discrete movement of the elbow, such as throwing a dart, the triceps muscles will be activated very briefly and strongly (in a "burst") to rapidly accelerate the extension movement at the elbow, followed almost immediately by a "burst" of activation to the elbow flexor muscles that decelerates the elbow movement to arrive at a quick stop. To use an automotive analogy, this would be similar to pressing the accelerator pedal rapidly and then immediately pressing the brake. Antagonism is not an intrinsic property of a particular muscle or muscle group; it is a role that a muscle plays depending on which muscle is currently the agonist. During slower joint actions that involve gravity, just as with the agonist muscle, the antagonist muscle can shorten and lengthen. Using the example of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists at the elbow during both the up phase and down phase of the movement. During the dumbbell curl, the elbow extensors are the antagonists for both the lifting and lowering phases.

Antagonistic pairs

Antagonist and agonist muscles often occur in pairs, called antagonistic pairs. As one muscle contracts, the other relaxes. An example of an antagonistic pair is the

biceps The biceps or biceps brachii ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join ...

and triceps; to contract, the triceps relaxes while the biceps contracts to lift the arm. "Reverse motions" need antagonistic pairs located in opposite sides of a joint or bone, including abductor-adductor pairs and flexor-extensor pairs. These consist of an

extensor muscle In anatomy, extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extendin ...

, which "opens" the joint (by increasing the angle between the two bones) and a

flexor muscle Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

, which does the opposite by decreasing the angle between two bones. However, muscles don't always work this way; sometimes agonists and antagonists contract at the same time to produce force, as per

Lombard's paradox Lombard's paradox describes a paradoxical muscular contraction in humans. When rising to stand from a sitting or squatting position, both the hamstrings and quadriceps contract at the same time, despite them being antagonists to each other. The r ...

. Also, sometimes during a joint action controlled by an agonist muscle, the antagonist will be slightly activated, naturally. This occurs normally and is not considered to be a problem unless it is excessive or uncontrolled and disturbs the control of the joint action. This is called agonist/antagonist co-activation and serves to mechanically stiffen the joint. Not all muscles are paired in this way. An example of an exception is the

deltoid Deltoid (delta-shaped) can refer to: * The deltoid muscle, a muscle in the shoulder * Kite (geometry), also known as a deltoid, a type of quadrilateral * A deltoid curve, a three-cusped hypocycloid * A leaf shape * The deltoid tuberosity, a part of ...

Synergists

Synergist muscles also called ''fixators'', act around a joint to help the action of an

agonist muscle Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location. Types There are three types of muscle tissue in the body: skeletal, smooth, a ...

. Synergist muscles can also act to counter or neutralize the force of an agonist and are also known as neutralizers when they do this. As neutralizers they help to cancel out or neutralize extra motion produced from the agonists to ensure that the force generated works within the desired plane of motion. Muscle fibers can only contract up to 40% of their fully stretched length. Thus the short fibers of pennate muscles are more suitable where power rather than range of contraction is required. This limitation in the range of contraction affects all muscles, and those that act over several joints may be unable to shorten sufficiently to produce the full range of movement at all of them simultaneously (active insufficiency, e.g., the fingers cannot be fully flexed when the wrist is also flexed). Likewise, the opposing muscles may be unable to stretch sufficiently to allow such movement to take place (passive insufficiency). For both these reasons, it is often essential to use other synergists, in this type of action to fix certain of the joints so that others can be moved effectively, e.g., fixation of the wrist during full flexion of the fingers in clenching the fist. Synergists are muscles that facilitate the fixation action. There is an important difference between a ''helping synergist'' muscle and a ''true synergist'' muscle. A true synergist muscle is one that only neutralizes an undesired joint action, whereas a helping synergist is one that neutralizes an undesired action but also assists with the desired action.

Neutralizer action

A muscle that fixes or holds a bone so that the agonist can carry out the intended movement is said to have a neutralizing action. A good famous example of this are the

hamstrings In human anatomy, a hamstring () is any one of the three posterior thigh muscles in between the hip and the knee (from medial to lateral: semimembranosus, semitendinosus and biceps femoris). The hamstrings are susceptible to injury. In quadrupe ...

; the semitendinosus and

semimembranosus muscle The semimembranosus muscle () is the most medial of the three hamstring muscles in the thigh. It is so named because it has a flat tendon of origin. It lies posteromedially in the thigh, deep to the semitendinosus muscle. It extends the hip joint ...

s perform knee flexion and knee internal rotation whereas the

biceps femoris The biceps femoris () is a muscle of the thigh located to the posterior, or back. As its name implies, it has two parts, one of which (the long head) forms part of the hamstrings muscle group. Structure It has two heads of origin: *the ''long h ...

carries out knee flexion and knee external rotation. For the knee to flex while not rotating in either direction, all three muscles contract to stabilize the knee while it moves in the desired way.

Composite muscle

'' Composite'' or ''hybrid'' muscles have more than one set of fibers that perform the same function, and are usually supplied by different nerves for different set of fibers. For example, the tongue itself is a composite muscle made up of various components like longitudinal, transverse, horizontal muscles with different parts innervated having different nerve supply.

Muscle naming

There are a number of terms used in the naming of muscles including those relating to size, shape, action, location, their orientation, and their number of heads. ;By size: ''brevis'' means short; ''longus'' means long; ''major'' means large; ''maximus'' means largest; ''minor'' means small, and ''minimus'' smallest. These terms are often used after the particular muscle such as

gluteus maximus The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. It is the single largest muscle in the human ...

, and

gluteus minimus The gluteus minimus, or glutæus minimus, the smallest of the three gluteal muscles, is situated immediately beneath the gluteus medius. Structure It is fan-shaped, arising from the outer surface of the ilium, between the anterior and infer ...

. ;By shape: ''deltoid'' means triangular; ''quadratus'' means having four sides; ''rhomboideus'' means having a rhomboid shape; ''teres'' means round or cylindrical, ''trapezius'' means having a trapezoid shape, ''rectus'' means straight. Examples are the

pronator teres The pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two attachments, to t ...

, the

pronator quadratus Pronator quadratus is a square-shaped muscle on the distal forearm that acts to pronate (turn so the palm faces downwards) the hand. Structure Its fibres run perpendicular to the direction of the arm, running from the most distal quarter of the ...

and the rectus abdominis. ;By action: '' abductor'' moving away from the midline; '' adductor'' moving towards the midline; '' depressor'' moving downwards; '' elevator'' moving upwards; '' flexor'' moving that decreases an angle; '' extensor'' moving that increase an angle or straightens; ''

pronator Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

'' moving to face down; ''

supinator In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm. Structure Supinator consists of two planes of fibers, between whi ...

'' moving to face upwards; ''

Internal rotator Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

'' rotating towards the body; ''

external rotator Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

'' rotating away from the body.

Form

Insertion and origin

The insertion and origin of a muscle are the two places where it is anchored, one at each end. The connective tissue of the attachment is called an enthesis.

Origin

The origin of a muscle is the bone, typically proximal, which has greater mass and is more stable during a

contraction Contraction may refer to: Linguistics * Contraction (grammar), a shortened word * Poetic contraction, omission of letters for poetic reasons * Elision, omission of sounds ** Syncope (phonology), omission of sounds in a word * Synalepha, merged ...

than a muscle's insertion. For example, with the latissimus dorsi muscle, the origin site is the torso, and the insertion is the arm. When this muscle contracts, normally the arm moves due to having less mass than the torso. This is the case when grabbing objects lighter than the body, as in the typical use of a

lat pull down The pull-down exercise is a strength training exercise designed to develop the latissimus dorsi muscle. It performs the functions of downward rotation and depression of the scapulae combined with adduction and extension of the shoulder joint. The ...

machine. This can be reversed however, such as in a chin up where the torso moves up to meet the arm. The head of a muscle, also called ''caput musculi'' is the part at the end of a muscle at its origin, where it attaches to a fixed bone. Some muscles such as the

have more than one head.

Insertion

The insertion of a muscle is the structure that it attaches to and tends to be moved by the

of the muscle. This may be a bone, a tendon or the subcutaneous dermal

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

. Insertions are usually connections of muscle via tendon to bone. The insertion is a bone that tends to be distal, have less mass, and greater motion than the origin during a contraction.

Intrinsic and extrinsic muscles

Intrinsic muscles have their origin in the part of the body that they act on, and are contained within that part. Extrinsic muscles have their origin outside of the part of the body that they act on. Examples are the intrinsic and extrinsic muscles of the tongue, and those of the hand.

Muscle fibers

Muscles may also be described by the direction that the

muscle fibers A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...

run, in their muscle architecture. * Fusiform muscles have fibers that run parallel to the length of the muscle, and are

spindle-shaped Fusiform means having a spindle-like shape that is wide in the middle and tapers at both ends. It is similar to the lemon-shape, but often implies a focal broadening of a structure that continues from one or both ends, such as an aneurysm on a ...

. For example, the pronator teres muscle of the forearm. * Unipennate muscles have fibers that run the entire length of only one side of a muscle, like a quill pen. For example, the

fibularis muscles The fibularis muscles (also called peroneus muscles or peroneals) are a group of muscles in the lower leg. Description The muscle group is normally composed of three muscles: fibularis longus, fibularis brevis, and fibularis tertius. The fib ...

. * Bipennate muscles consist of two rows of oblique muscle fibres, facing in opposite diagonal directions, converging on a central tendon. Bipennate muscle is stronger than both unipennate muscle and fusiform muscle, due to a larger

physiological cross-sectional area In muscle physiology, physiological cross-sectional area (PCSA) is the area of the cross section of a muscle perpendicular to its fibers, generally at its largest point. It is typically used to describe the contraction properties of pennate muscl ...

. Bipennate muscle shortens less than unipennate muscle but develops greater tension when it does, translated into greater power but less range of motion. Pennate muscles generally also tire easily. Examples of bipennate muscles are the rectus femoris muscle of the thigh, and the stapedius muscle of the middle ear.

State

Hypertrophy and atrophy

Hypertrophy is increase in muscle size from an increase in size of individual muscle cells. This usually occurs as a result of exercise.

References

;Books * * {{Portal bar, Anatomy Human anatomy Muscular system Muscle terminology Anatomical terminology