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Abduction of the eye moves the visual axis away from the nose. The lateral rectus muscle is the main muscle for this horizontal action. It differs from torsional rotation of the eyeball around the visual axis.

Acetylcholinesterase is an enzyme that hydrolyses the neurotransmitter acetylcholine to choline and acetate. Its activity helps terminate signalling at cholinergic synapses, including the neuromuscular junction. Chemically, it is a catalyst: the reaction consumes acetylcholine and water, while the enzyme can be reused.^[ref]

The distinction between removal of a messenger and inhibition of its receptor matters. Reducing acetylcholinesterase activity leaves more acetylcholine available to activate receptors; it does not directly substitute for the messenger or block every receptor. Thus the duration of a chemical signal depends both on its release and on its clearance. Measuring reaction-product accumulation in an enzyme assay tests the clearance machinery separately from a responding muscle or neuron.

Adduction of the eye moves the visual axis toward the nose. The medial rectus muscle is the main muscle for this horizontal action. Position changes the pulling geometry of other extraocular muscles, making muscle actions depend partly on initial gaze direction.

The adductor brevis is a short medial thigh muscle deep to adductor longus. It contributes to hip adduction and helps stabilize the femur relative to the pelvis. Its deep location is an anatomical distinction rather than evidence of a wholly separate movement type.

The adductor longus is a medial thigh muscle extending from the pubic region to the femur. It contributes to hip adduction and has position-dependent effects on flexion. Its relationship to nearby muscles defines an important part of the medial thigh architecture.

The adductor magnus is a large medial thigh muscle with regions differing in attachment, action and innervation. It contributes to hip adduction, while its posterior component also supports extension. Regional anatomy explains why one named muscle need not behave as a single uniform actuator.

An adductor muscle moves a body part toward a defined reference axis or plane. Hip adductors move the thigh toward the body midline. The movement name must be interpreted relative to the specified joint and reference, rather than inferred from muscle position alone.

The anconeus is a small muscle on the posterolateral elbow. It assists elbow extension and stabilization and is supplied by the radial nerve. Its action complements the much larger triceps rather than producing an unrelated elbow movement.

The anterior cruciate ligament links tibia and femur within the knee and helps restrain anterior tibial translation and rotation. Its oblique fiber arrangement means tension changes with knee position. It acts together with other ligaments, menisci and muscles.

The anterior horn is the ventral gray-matter region of the spinal cord, containing somatic motor neurons and associated circuitry. Its organization links spinal segments to muscle groups. It is distinct from the roots through which motor axons leave the cord.

Impaired planning or programming of the coordinated movements needed for speech, distinguished from a primary muscle weakness or a deficit in grammatical knowledge. It can arise in developmental or acquired conditions.

Physiologist who investigated heat production in muscle. Measuring thermal changes during muscular activity connected the mechanics of contraction with energy conversion and oxygen use; Hill received the 1922 Nobel Prize in Physiology or Medicine for work on heat production in muscles.^[ref]

The biceps brachii crosses the shoulder and elbow and inserts toward the radius. It contributes to elbow flexion and forearm supination. Its effective action depends on joint position and load, connecting a familiar muscle name to a multi-joint mechanical problem.

Biomechanics applies mechanics to living organisms, from the forces exerted by a muscle to the deformation of a biological cell. It connects geometry, motion, material response and biological activity. A bone supporting a load, blood moving through a vessel, and a muscle producing work are all mechanical systems, but their living components can remodel, consume chemical energy and change their properties.

A simple torque balance explains why an internal muscle force can greatly exceed an external load. In an idealized stationary forearm, suppose a downward load $W=20\mathrm{N}$ acts with perpendicular moment arm $r_W=0.30\mathrm{m}$ about the elbow and the balancing tendon force has muscle moment arm $r_m=0.030\mathrm{m}$. Neglect the forearm’s own gravitational weight and other muscles. Zero net torque requiresThese are perpendicular moment arms, not necessarily distances measured along the bone. Force balance also requires a joint reaction; the torque equation alone does not determine every internal force.

Geometry must also be separated from material behavior. A tendon with sectional area $20{\mathrm{mm}}^2$ carrying that illustrative $200\mathrm{N}$ has mean axial stress $10\mathrm{MPa}$. Its extension depends on its length, constitutive equation and loading history, not just the applied force. Measurements of stress against strain therefore answer a different question from measurements of force against extension. For a living specimen, hydration, temperature and the rate of deformation can all be controlled experimental variables.

An influential motivating experiment is Archibald Vivian Hill’s measurement of heat and work during muscle contraction. In his 1938 primary paper, a thermopile detected tiny temperature changes; an optically amplified galvanometer made the response fast enough to study shortening. Comparing contractions under different loads connected shortening speed, mechanical work and liberated heat. Such measurements constrain an active muscle model in a way that simply stretching a passive spring cannot.^[ref] The example captures a central aim of biomechanics: use measurable forces, motion and energy transfers to test how a biological mechanism works.

Bipes is a taxonomic genus of burrowing amphisbaenians distinguished by retaining external forelimbs despite an elongated body and absence of external hindlimbs. It is an informative comparison with other amphisbaenians, which lack external limbs: external appearance alone does not reveal how much internal anatomy remains.^[ref]

In a 2019 anatomical study, investigators used iodine-enhanced computed tomography to distinguish soft tissues and reconstruct shoulder muscles as well as bones. Bipes retained an almost complete shoulder girdle, while other sampled amphisbaenians had much more reduced bony structures. Nevertheless, all sampled amphisbaenian forms retained the full set of 17 shoulder muscles, sometimes fused or rearranged. Muscle attachment could shift from bone to surrounding soft tissue.^[ref]

The result challenges a tempting shortcut: losing a visible limb does not necessarily delete all its associated muscles at once. Bipes supplies the limbed comparison that helps reveal which internal components have persisted or changed during the evolution of the burrowing body form.

Bladder trabeculation is a ridged appearance of the inner bladder wall associated with prominent muscle bundles and intervening recesses. It can reflect chronic remodeling under altered loading. The appearance is a structural observation rather than a complete diagnosis of the cause of urinary dysfunction.

The brachialis spans the anterior humerus to the ulna and flexes the elbow. Because its insertion is on the ulna, its leverage is less directly changed by forearm rotation than that of biceps brachii. This is an example of attachment geometry constraining muscle function.

The buccinator muscle forms an important muscular part of the cheek. It keeps food positioned against the teeth during mastication and helps control oral pressure. Its role differs from the jaw-closing muscles even though they cooperate during eating.

The bulbospongiosus is a superficial perineal muscle associated with the bulb and nearby erectile structures, with sex-specific arrangements. Its contractions contribute to local support and expulsion of fluid in relevant anatomy. It is distinct from the deeper urethral sphincter complex.

Cardiac muscle is the specialized striated muscle of the heart. Its contractile cells are mechanically connected and electrically coupled at intercalated discs. Mechanical connections transmit force, while gap junctions help electrical excitation spread between cells.^[ref]

Excitation and contraction are related but distinct stages. Changes in membrane potential regulate calcium handling, and calcium enables the actin-myosin machinery to generate force. A sinoatrial node normally sets the pace for the coordinated sequence, rather than an independent motor neuron commanding each beat.

This separation explains why an electrical recording and a measurement of pumping answer different questions. An electrocardiogram reports summed electrical activity; contraction also depends on cellular mechanics, filling and the loads against which the heart pumps.