Anatomical Terms Of Muscle

Oh, muscles. Fascinating. Like complex clockwork, but messier. And far more prone to tearing. You want me to dissect this anatomical jargon? Very well. Just try not to bore me.

This article is part of a larger series dedicated to Anatomical terminology, a rather dry but necessary field for anyone who fancies themselves knowledgeable about the human form. Within this series, we delve into the specific language used to describe various aspects of our physical architecture. You'll find related articles on Bone, which forms our internal scaffolding; Location, detailing where things are in relation to everything else; Microanatomy, for the microscopic intricacies; Motion, explaining how we move; and Neuroanatomy, the wiring behind it all. And, of course, the subject at hand: Muscle.

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The lexicon of Anatomical terminology is crucial for precisely and unambiguously describing the myriad components of the human body. This includes the intricate details of skeletal muscle, the tireless engine of our voluntary movements; cardiac muscle, the relentless pump of the heart; and smooth muscle, the subtle, involuntary operator of internal systems. This terminology allows us to articulate everything from their specific actions and observable structures to their sheer size and precise locations.

Types

The human body harbors three distinct varieties of Muscle tissue, each with its own specialized role and characteristics: skeletal, smooth, and cardiac.

Skeletal muscle

Often referred to as "voluntary muscle" because we can consciously control its actions, Skeletal muscle is a type of striated muscle tissue. Its primary function is to attach to bone via tough cords known as tendons. Through its contractions, skeletal muscle is responsible for the movement of our limbs and torso, and crucially, for maintaining posture.[1] The most substantial part of a muscle, the portion that exerts the primary pull on the tendons, is colloquially known as the belly. It's the part you can feel when you flex.

Muscle slip

A muscle slip is a somewhat peculiar anatomical occurrence. It can manifest as an anatomical variant, meaning it's a deviation from the typical structure, or it can represent a branching of a muscle's fibers, as seen in certain rib connections of the serratus anterior muscle. Think of it as an extra tendril of muscle, sometimes present, sometimes not, sometimes doing something specific, sometimes just… there.

Smooth muscle

Unlike skeletal muscle, Smooth muscle operates entirely without our conscious command; it is involuntary. This type of muscle tissue is predominantly found in the internal organs, orchestrating essential bodily functions. In the digestive and urinary systems, it diligently propels contents forward – be it food, chyme, or feces in the former, and urine in the latter. Smooth muscle also plays vital roles elsewhere: in the uterus, its contractions are fundamental to facilitating birth, and within the eye, the pupillary sphincter muscle, a marvel of involuntary control, precisely regulates the size of the pupil.[3] It's the silent, tireless worker.

Cardiac muscle

Cardiac muscle holds a unique position, being exclusive to the heart. Like smooth muscle, its contractions are involuntary. What sets it apart further is its inherent self-excitatory nature; it contracts rhythmically and autonomously, without needing external stimulation.[4] It's the ultimate example of dedicated, relentless function.

Actions of skeletal muscle

Beyond the general anatomical terms of motion, which simply describe the movement a muscle produces, a more nuanced terminology is employed to define the roles muscles play in coordinated actions, particularly in sets.

Agonists and antagonists

This is where things get interesting, or at least, less straightforward.

• See also: Reciprocal innervation

Agonist muscles are the primary drivers of a movement, the ones doing the heavy lifting, so to speak. They are also known as prime movers because they generate the majority of the force and are essential for controlling the action.[6] When an agonist muscle activates, it directly causes a movement to occur.[7] Consider the upward phase of a push-up: your triceps brachii contracts, shortening in a concentric contraction, to extend the elbow. However, during the downward phase, that same triceps muscle is still the agonist, actively controlling the elbow flexion by resisting gravity. It's performing a lengthening (eccentric) contraction, but it remains the prime mover, the controller of that joint action.

A similar principle applies to a dumbbell curl. The elbow flexor group acts as the agonist during the lifting phase, shortening to achieve elbow flexion. In the lowering phase, these same flexor muscles lengthen eccentrically, still acting as agonists because they are controlling the descent of the weight. Throughout both phases, the "elbow extensor" muscles are the antagonists. They lengthen during the lifting phase and shorten during the lowering phase. It’s important to note that naming muscle groups by the action they perform during a shortening contraction is common practice, but it doesn’t limit their function solely to that. They can be agonists during lengthening contractions too. This concept is primarily applied to skeletal muscles.[8]

Antagonist muscles, on the other hand, are those that produce an opposing torque to the agonist.[9] This opposition isn't just about stopping movement; it can also help control it, especially in rapid, ballistic movements. Imagine throwing a dart: the triceps might contract forcefully in a brief burst to rapidly extend the elbow, and then the elbow flexors activate just as quickly to decelerate the movement, bringing it to a precise stop. It’s akin to a car’s accelerator and brake pedals. Antagonism isn't an inherent quality of a muscle; it’s a role a muscle plays relative to the current agonist. Even during slower movements involving gravity, both agonists and antagonists can lengthen and shorten. In the push-up example, the elbow flexors are antagonists throughout both the upward and downward phases. Similarly, during a dumbbell curl, the elbow extensors are antagonists during both phases.[10]

Antagonistic pairs

Agonist and antagonist muscles frequently work in tandem as antagonistic pairs. When one muscle contracts, its partner typically relaxes. The classic example is the biceps and triceps in the arm; to bend the elbow, the biceps contracts while the triceps relaxes. For movements that require reversing an action, antagonistic pairs are situated on opposite sides of a joint or bone. This includes pairs like abductor-adductor and flexor-extensor muscles. An extensor muscle increases the angle between bones, effectively "opening" a joint, while a flexor muscle decreases this angle, "closing" it.

However, the body isn't always so neatly divided. Sometimes, agonists and antagonists contract simultaneously to generate force – a phenomenon known as Lombard's paradox. Furthermore, it’s quite normal for an antagonist muscle to be slightly activated even when the agonist is controlling the primary action. This co-activation, when not excessive, actually helps to mechanically stiffen the joint, providing stability. It's only when this co-activation becomes problematic, disturbing the control of the joint action, that it's considered an issue.

Not all muscles fit neatly into these paired roles. The deltoid, for instance, is an example of a muscle that doesn't always adhere to strict antagonistic pairing.[11]

Synergists

Synergist muscles are the supporting cast, assisting the agonist muscle around a joint to enhance its action. They can also act to counter or neutralize unwanted forces generated by the agonist, in which case they are referred to as neutralizers.[12] Their role as neutralizers is to cancel out extraneous movements, ensuring that the force produced by the agonists is directed precisely along the desired plane of motion.

Consider the biceps flexing the lower arm. It doesn't do it alone. The brachioradialis in the forearm and the brachialis muscle, lying deep to the biceps in the upper arm, are both synergists that provide crucial assistance to this flexion.

It’s a known limitation that muscle fibers can only contract effectively up to about 40% of their fully stretched length.[ citation needed ] This is why muscles with shorter fibers, like pennate muscles, are better suited for generating power rather than achieving a wide range of motion. This limitation impacts all muscles. When a muscle spans multiple joints, it might be unable to shorten enough to produce the full range of motion at all of them simultaneously. This is termed active insufficiency. For example, you can't fully flex your fingers if your wrist is already fully flexed. Conversely, the opposing muscles might not be able to stretch sufficiently to allow such a movement – a situation known as passive insufficiency. Because of these limitations, synergists are often essential to stabilize certain joints, allowing others to move effectively. For instance, to achieve a full fist clench (maximum finger flexion), the wrist needs to be stabilized. Synergists are the muscles that perform this stabilization.

There's a subtle but important distinction between a "helping synergist" and a "true synergist." A true synergist's sole purpose is to neutralize an unwanted joint action. A helping synergist, however, not only neutralizes unwanted actions but also actively assists with the desired movement.[ citation needed ]

Neutralizer action

A muscle that stabilizes or holds a bone in place, thereby enabling the agonist to perform its intended movement without unintended deviations, is said to be performing a neutralizing action. The hamstrings provide a classic example. The semitendinosus and semimembranosus muscles are responsible for knee flexion and internal rotation of the knee. The biceps femoris, however, performs knee flexion and external rotation. For the knee to flex purely, without any rotation, all three of these muscles must contract to stabilize the joint, ensuring the movement occurs in the desired plane.

Composite muscle

A composite or hybrid muscle is characterized by having multiple sets of fibers that perform the same function, often with different sets being innervated by different nerves. The tongue serves as an excellent illustration; it's a composite muscle comprising longitudinal, transverse, and horizontal fibers, with different portions receiving their nerve supply from distinct sources.

Muscle naming

The nomenclature used to identify muscles is surprisingly systematic, drawing upon a variety of characteristics.

• Further information: Anatomical terms of motion and Anatomical terms of location

The seven general types of skeletal muscle are categorized by:

By size

Terms like brevis (short) and longus (long) describe length, while major (large) and maximus (largest) denote size, contrasting with minor (small) and minimus (smallest). These descriptors are typically appended to the muscle's name, as seen in the gluteus maximus and gluteus minimus.[13]

By shape

Shape is another common descriptor. Deltoid refers to a triangular shape, quadratus indicates four sides, rhomboideus describes a rhomboid form, teres signifies round or cylindrical, trapezius points to a trapezoid shape, and rectus means straight. Examples include the pronator teres, the pronator quadratus, and the rectus abdominis.[13]

By action

The function of a muscle is often reflected in its name. Abductor muscles move a limb away from the body's midline, while adductors move it towards it.[Abduction and adduction] Depressors move structures downwards, and elevators move them upwards.[Elevation and depression] Flexors decrease the angle at a joint, while extensors increase it, straightening the limb.[Flexion and extension] Pronators rotate the forearm to face downwards, and supinators rotate it to face upwards.[Anatomical terms of motion][13] Internal rotators rotate a limb towards the body's midline, and external rotators rotate it away.[Internal rotator][Anatomical terms of motion][External rotator]

Form

Rectus femoris (shown in red) originates from the anterior inferior iliac spine and a portion of the acetabulum. It inserts into the patellar tendon and is responsible for extending the knee and flexing the hip.

Insertion and origin

Every muscle is anchored at two points: its origin and its insertion. The connective tissue forming these attachments is known as an enthesis.

Origin

The origin of a muscle is typically the bone that is more proximal and possesses greater mass, making it the more stable point during a contraction.[14] For instance, in the latissimus dorsi muscle, the origin is the torso, and the insertion is the arm. Under normal circumstances, when this muscle contracts, the arm moves because it has less mass than the torso. This is what happens when you pull yourself up on a bar, as in a lat pull down machine. However, this can be reversed. In a chin up, for example, the torso moves upwards to meet the arm, effectively reversing the typical origin-insertion dynamic.

The caput musculi, or head of a muscle, is the part located at its origin, attaching to a fixed bone. Some muscles, like the biceps, have multiple heads.

Insertion

The insertion of a muscle is the structure it attaches to and, consequently, the structure that is moved by the muscle's contraction.[15] This structure can be a bone, a tendon, or even the subcutaneous dermal connective tissue. Most commonly, muscles insert onto bone via tendon.[16] The insertion is generally the more distal bone, possessing less mass and exhibiting greater motion compared to the origin during a contraction.

Intrinsic and extrinsic muscles

Intrinsic muscles originate within the body part they act upon and are contained entirely within it.[17] Conversely, extrinsic muscles originate outside the body part they influence.[18] The intrinsic and extrinsic muscles of the tongue and those found in the hand are prime examples.

Muscle fibres

• Main article: Muscle architecture

Different skeletal muscle types are also described by their fiber arrangement: A: fusiform. B: unipennate. C: bipennate. (PCS: physiological cross-section)

The direction in which muscle fibres are arranged within a muscle, known as its muscle architecture, provides another layer of classification.

Fusiform muscles feature fibers that run parallel to the muscle's length, giving them a spindle-shaped appearance.[19] The pronator teres muscle of the forearm is an example.
Unipennate muscles have fibers that extend along only one side of the muscle, resembling the vane of a quill pen. The fibularis muscles exhibit this arrangement.
Bipennate muscles are characterized by two rows of muscle fibers that angle obliquely in opposite directions, converging onto a central tendon. Bipennate muscles are generally stronger than unipennate and fusiform muscles due to a larger physiological cross-sectional area. While they shorten less than unipennate muscles, they generate greater tension, resulting in increased power but a reduced range of motion. Pennate muscles in general tend to fatigue more easily. Muscles such as the rectus femoris muscle of the thigh and the stapedius muscle in the middle ear are bipennate.

State

Hypertrophy and atrophy

• Main articles: Muscle hypertrophy and Muscle atrophy

Hypertrophy refers to an increase in muscle size, which occurs through the enlargement of individual muscle cells. This is typically the result of physical exercise.