100 Fun Facts About Muscular System

Which body part, often wrongly called the body's strongest muscle, is really eight muscles woven together?

The tongue is eight muscles woven together with no bone inside, which is why it can bend and twist like an octopus arm or an elephant's trunk.

Roughly how many skeletal muscles does an adult human body contain?

About 600 is the low end: counts run past 800, because a complex muscle such as the biceps can be tallied as one muscle or two.

Which muscle shows up as a 'six-pack' on a lean, fit torso?

Rectus abdominis is a single pair of long muscles, not six: fibrous bands cross each one, and the bulges between the bands make the 'six-pack'.

Which muscle type can keep contracting on its own, with no signal from the nervous system?

Cardiac muscle starts every beat itself, in built-in pacemaker cells; nerves only speed the rhythm up or slow it down.

Which muscle type is non-striated?

Smooth muscle can lock its filaments together and hold a squeeze for long periods on very little energy, which is how artery walls and other organs keep their tone.

What are the basic contractile units of striated muscle, repeated end to end along each myofibril?

Sarcomeres line up in register across the fiber, so their alternating dark and light bands show as the stripes that give striated muscle its name.

Which theory best explains how muscle fibers contract?

Sliding filament model: the filaments themselves never shorten, they just slide past each other, as two separate teams, each including a scientist named Huxley, showed in 1954.

Which substance was long blamed for next-day muscle soreness, a link researchers have since rejected?

Lactic acid levels after exercise show little link to the ache; the soreness that peaks a day or two later is thought to come from small-scale damage to muscle fibers and the repair that follows.

Which molecule powers the swivelling of the myosin heads during contraction?

ATP also does the letting go: each myosin head needs a fresh molecule before it can release, which is why muscles lock stiff after death once the supply runs out.

Which ion is released from the sarcoplasmic reticulum to trigger contraction?

Calcium uncovers the binding sites on the thin filaments, and the muscle can relax only once pumps have hauled the ion back into storage.

Which stored chemical gives a working muscle its fastest energy top-up, lasting only the first seconds of all-out effort?

Creatine phosphate hands its energy over in a single quick reaction, and topping up that reserve is the idea behind the creatine supplements that sprinters and weightlifters take.

What neurotransmitter is released at the neuromuscular junction to fire a skeletal muscle?

Acetylcholine is the signal that curare, the South American arrow poison, blocks: with the muscle's receptors jammed, the nerve fires but the muscle stays limp.

Which disease is nicknamed lockjaw for the rigid muscle spasms it causes?

Tetanus toxin jams the nerves that control muscles, and the spasms it sets off, which often begin in the jaw, can be strong enough to break bones.

A nerve cell plus all the muscle fibers it controls is called what?

A motor unit that steers the eye holds only a handful of fibers for precision, while one in the thigh can hold a thousand or more for power.

Which part of the brain fine-tunes muscle commands so movements come out fluid and coordinated?

The cerebellum takes up only about a tenth of the brain's volume, yet it holds more than half of all the brain's neurons.

What connects a muscle to a bone?

A tendon lets a muscle work at a distance: the main muscles that bend and straighten your fingers sit up in the forearm and pull on them through long tendons.

What is the wasting of muscle from disuse, a hazard for astronauts in weightlessness, called?

Atrophy sets in quickly when muscles no longer work against gravity, which is why space station crews exercise for about two hours every day.

The Latin word for muscle, musculus, literally means a little version of which animal?

A mouse: the shape and movement of a flexing muscle, notably the biceps, were thought to resemble one, and the same Latin word also gave English the shellfish name 'mussel'.

What is the thick, muscular middle layer of the heart wall called?

Myocardium is what a heart attack damages: the medical name, myocardial infarction, means the death of a patch of this muscle after its blood supply is cut off.

Heart muscle cells connect at branching junctions known as what?

Intercalated discs both anchor the cells to each other and pass the electrical signal straight from cell to cell, so heart muscle contracts in one coordinated wave.

Which tiny skin muscles make body hair stand up?

Arrector pili fluff up a furry animal's coat to trap warm air or make it look bigger; on thinly haired humans the same reflex leaves only goosebumps.

Slow-twitch muscle fibers dominate the leg muscles of which elite athletes?

Marathon runners lean on slow-twitch fibers because these burn fuel with oxygen and tire slowly, while the fast-twitch fibers a sprinter needs give quick bursts of power and fatigue fast.

Anabolic steroids build muscle by mimicking which hormone?

Testosterone is what these synthetic drugs copy, and the body answers the surplus by shutting down its own supply, which is why long-term misuse can shrink the testicles.

Which protein makes a chicken's leg meat darker than its breast meat?

Myoglobin, not the blood's hemoglobin, stores oxygen inside muscle cells, so muscles a bird stands and walks on all day hold more of it than ones saved for quick bursts.

Pound for pound, which body tissue takes up more room than muscle because it is less dense?

Fat is the lighter tissue at about 0.9 kg per liter, less dense than water, which is why weighing a person underwater can show how much of the body is fat.

Where is the soleus, a muscle that works all day to hold you upright?

In the calf, the soleus is mostly slow-twitch fiber that tires slowly, and its constant pull at the ankle is what stops a standing body from toppling forward.

Which ape's legendary 'super strength' shrank when researchers found its muscle only about a third more powerful than human muscle?

Chimpanzee muscle holds about twice the share of fast-twitch fibers that human muscle does, which buys short bursts of power, while human muscle is tuned for low-cost endurance.

Ring-shaped muscles that open and close body passages are called what?

Sphincters mostly work unnoticed: when the one at the top of the stomach weakens or relaxes at the wrong moment, stomach contents escape upward and you feel heartburn.

What is the cell membrane of a muscle fiber called?

Sarcolemma does more than wrap the fiber: it carries the electrical signal along the surface and dips inward as tiny tunnels, so the signal reaches the fiber's core.

Which dormant population sits on the surface of each muscle fiber until an injury calls it into action?

Satellite cells stay dormant until injury or hard exercise wakes them, then they multiply and fuse with the damaged fiber to rebuild it.

Why does weight training thicken the muscle fibers you have instead of adding new ones?

They cannot divide once mature, so a lifter's muscles grow by packing more contractile strands into each existing fiber, not by adding fibers.

What does the word deltoid mean in a muscle name?

Triangular, because the shoulder muscle is shaped like the Greek capital letter delta, the same letter that names the fan of sediment at a river's mouth.

A muscle with three heads is called what?

Triceps is Latin for 'three-headed', and the familiar one runs along the back of the upper arm, where it straightens the elbow.

What does an adductor muscle do?

Moves a limb toward the midline: the name comes from Latin for 'to lead to', while an abductor leads a limb away, the same root as a kidnapper's 'abduct'.

In muscle naming, what does the word latissimus mean?

Widest suits the latissimus dorsi, the 'lats' that fan across the middle and lower back and haul the body upward in a chin-up.

Which stretch receptors sit in the fleshy belly, not at the junction with bone?

Muscle spindles set off the knee-jerk: a tap below the kneecap stretches the thigh muscle, the spindles report the stretch, and the muscle answers by contracting.

What name is given to the connective tissue wrapping each individual muscle fiber?

Endomysium is more than wrapping: it carries the capillaries and nerves that feed and fire each fiber.

Which involuntary muscle action does a cold body switch on purely to generate heat?

Shivering is rapid muscle contraction that moves nothing, so the energy it burns ends up as warmth, the same by-product muscles give off whenever they work.

Signalling molecules secreted by muscle after contraction are called what?

Myokines are why researchers now count skeletal muscle as a secretory organ: released into the blood, they carry signals to the liver, the immune system and other tissues.

Which regulatory protein changes shape when it binds the trigger ion, letting contraction start?

Troponin has a second life in the emergency room: damaged heart muscle leaks its own version into the blood, so a troponin test is a routine way to confirm a heart attack.

What is the smallest skeletal muscle in the human body?

Stapedius sits in the middle ear, where it tugs on the stirrup, the body's smallest bone, to damp the vibrations of loud sounds before they reach the inner ear.

Which muscle is the largest in the human body?

Gluteus maximus is so large and powerful because it has the job of holding the trunk erect, and it is the chief antigravity muscle when you climb stairs or straighten up from a stoop.

Which thigh muscle, once called the tailor's muscle, is the longest in the body?

Sartorius comes from the Latin for tailor, probably because the muscle helps fold the legs into the cross-legged position in which tailors once sat to work.

Which chewing muscle is the most superficial and one of the strongest?

Masseter is a contender for the body's strongest muscle because it works on a very short lever, the jawbone, which turns its pull into a powerful bite.

Which sheet of skeletal muscle is the most important muscle of breathing?

Diaphragm spasms are what hiccups are: the muscle jerks without warning and the vocal cords snap shut a moment later, which makes the 'hic'.

Why is the right half of the breathing muscle slightly higher than the left?

The diaphragm is an upward-curved, C-shaped dome of muscle and fibrous tissue.

Besides bending the elbow, what does the biceps do to the forearm?

Supinates it, meaning it turns the palm upward: the biceps is a powerful twister of the forearm as well as an elbow bender, and it bends the elbow best with the palm facing up.

Which tendon is the thickest and strongest in the human body?

Achilles tendon is named for the Greek hero whose one vulnerable spot was his heel, an odd namesake for the thickest and most powerful cord of its kind in the body.

Muscular dystrophy, a group of progressive muscle-wasting disorders, is caused by what?

Genetic mutations are behind every form; in Duchenne, the most common childhood type, the faulty gene sits on the X chromosome, which is why it mostly affects boys.

Myasthenia gravis, which leaves muscles weak and quick to tire, is what kind of disease?

Autoimmune: the body's own antibodies block the nerve's signal from reaching the muscle, so the weakness grows with activity and eases with rest.

Which body parts hold none of the muscles that move them, and are worked remotely through long cords?

Fingers are worked like puppets by muscles in the palm and forearm, whose long cords can be seen moving across the back of the hand. No muscle that bends a finger joint lies in the finger itself.

Which Italian scientist found that the leg muscles of a dead frog twitch when struck by an electric spark?

Luigi Galvani believed he had found 'animal electricity' flowing in the muscle itself. His friendly rival Alessandro Volta disagreed, yet coined the word galvanism in his honor.

Which trio of muscles behind the thigh is named from an Old English word for the bend of the knee?

Hamstrings joins 'ham', an Old English word for the hollow behind the knee, to 'string', for the cords felt on either side of it. The trio is the semimembranosus, semitendinosus and biceps femoris.

Which wrinkle treatment works by weakening small muscles of the face with a heavily diluted bacterial nerve poison?

Botox is a brand of botulinum toxin, counted among the deadliest natural substances known. A treated muscle takes several months to recover, which is why the injections are repeated.

Which stiffening of the limbs begins a few hours after death, once muscle fibers can no longer relax?

Rigor mortis locks the limbs because muscle runs out of the fuel that lets its fibers release. It peaks about half a day after death, then eases as the tissue breaks down.

In which stage of sleep are the body's voluntary muscles almost completely paralyzed, a state called atonia?

REM sleep switches off nearly every voluntary muscle, which stops sleepers from moving with their dreams. People who lose this paralysis physically act out what they are dreaming.

Which Yankees first baseman's name became a byword for ALS, the disease that kills the nerve cells driving muscles?

Lou Gehrig played a then-record 2,130 consecutive games for the Yankees before ALS ended his career in 1939. He died two years later.

Which reptile has a crushing bite but jaw-opening muscles so weak that tape can hold its mouth shut?

Crocodile handlers rely on the mismatch: the muscles that open the jaws are so small that tape, or a band cut from a car's inner tube, keeps them closed for study or transport.

Which job is done by a group of head muscles that, unusually, insert into the skin instead of into bone?

Facial expression depends on muscles lying just under the surface: when they shorten, the skin moves with them, producing a grin, a scowl or a raised eyebrow.

A doctor's hammer tap below the kneecap makes which muscle group contract, kicking the lower leg forward?

Quadriceps fibers fire within a few hundredths of a second of the tap, because the signal loops through the spinal cord without waiting for the brain. Doctors use the kick to test the lower spine's wiring.

Which thick, fan-shaped muscle does the bench press mainly build?

Pectoralis major makes up the bulk of the chest, and its name comes from pectus, the Latin for 'breast'. Lifters shorten it to 'pecs'.

Which large muscle of the upper back and neck do weightlifters build with shoulder shrugs?

Trapezius gets its name from its trapezium outline, which runs from the base of the skull down the spine and out to each shoulder blade. Its upper part carries the weight of the arms.

Which neck muscle, named for its anchors on the breastbone, collarbone and skull, turns the head sideways?

Sternocleidomastoid packs its three anchor points into one word: sternum, clavicle (cleido-) and the mastoid bump of the skull. Working as a pair, the two also bend the neck forward.

Which two-headed muscle forms the rounded bulge below the back of the knee?

Gastrocnemius takes its name from the Greek for 'stomach of the leg', after the bulge it forms below the knee. It does its main work in running and jumping, and less in walking or standing.

Which cheek muscle lifts the corners of the mouth into a smile and, when forked, is thought to cause dimples?

Zygomaticus major runs from the cheekbone, or zygomatic arch, down to the corner of the mouth. The same muscle helps curl the upper lip in a look of contempt.

Which muscle running from the lower spine toward the thigh is the cut that butchers sell as beef tenderloin?

Psoas major does very little work in cattle, which is why the cut is the tenderest on the animal. In people it teams up with the iliacus to form the iliopsoas.

Kegel exercises, named after an American gynecologist, strengthen which group of muscles?

Pelvic floor muscles are squeezed and released in a Kegel. Arnold Kegel described the routine in 1948, though Margaret Morris had published one in 1936 and received little credit.

The muscle with the longest name of all, levator labii superioris alaeque nasi, lets a person do what?

Snarl is what the Latin spells out: the name means 'lifter of both the upper lip and of the wing of the nose', so the muscle raises the lip and flares a nostril at once.

A sprain is an overstretched or torn ligament, but which term names the equivalent injury to a muscle?

Strain means that some of the muscle's fibers have torn. Doctors grade the damage by degree, from a little tearing with mild tenderness up to a complete rupture.

Which group of four muscles holds the head of the upper-arm bone in its small, shallow shoulder socket?

Rotator cuff muscles go by the mnemonic SITS: supraspinatus, infraspinatus, teres minor and subscapularis. Athletes who throw over and over, such as baseball pitchers, frequently injure them.

Which muscle over the ribs is nicknamed the 'boxer's muscle' because it drives the shoulder blade forward in a punch?

Serratus anterior takes its name from the Latin for 'to saw', after the jagged row of slips by which it grips the upper ribs.

Which fan-shaped chewing muscle can be felt bulging on the side of the head when the jaw clenches?

Temporalis fills the shallow hollow on each side of the skull and closes the mouth by lifting the lower jaw. Grinding the teeth at night can overwork it and leave it aching.

Which muscle deep in the buttock is pierced by the sciatic nerve in a minority of people?

Piriformis is simply Latin for 'pear-shaped'. In most people the sciatic nerve slips beneath the muscle, and when the muscle irritates the nerve the result is soreness in the buttock that spreads along the nerve's path.

Which muscle hidden beneath the biceps is the prime mover of elbow bending?

Brachialis shares the work with the biceps and the brachioradialis, yet it is the most powerful of the three at bending the elbow. It lies hidden under its famous neighbor, forming part of the floor of the elbow pit.

Which slender wrist muscle is missing in 10 to 20 percent of people, with no effect on grip strength?

Palmaris longus is so dispensable that surgeons harvest its long cord as a spare part when another one ruptures in the wrist. Removing it causes no loss of function.

Which ring of contractile tissue inside the eye changes the shape of the lens to focus on nearby objects?

Ciliary muscle tightens to let the lens bulge into a rounder shape for close work such as reading. It has no say over the size of the pupil, which other muscles set.

Which muscles lift the ribs up and outward to widen the chest each time a person breathes in?

Intercostals run in layers between neighboring ribs. The outer layer lifts the ribs to draw air in, while an inner layer pulls them back down during a forced breath out.

Which flat abdominal muscle turns the trunk the opposite way, so the right one fires when twisting left?

External oblique is the largest and outermost of the three flat muscles at each side of the abdomen. The layer beneath it works the other way round, turning the trunk toward its own side.

The myometrium, a middle layer made mostly of involuntary muscle, forms the wall of which organ?

Uterus walls owe their strength to the myometrium, whose main job is to produce contractions. After a birth it keeps squeezing to expel the placenta and clamp the blood vessels shut.

Which protein forms the thin strands inside a muscle fiber that the thicker myosin strands grab and pull?

Actin is no muscle specialist: it also builds the microfilaments that give almost every cell its internal scaffolding and let it change shape.

Which muscle protein has a full chemical name of 189,819 letters, sometimes called the longest word in English?

Titin, named after the Titans of Greek myth, is the largest protein in the human body. It works as a molecular spring, giving a stretched muscle its passive elasticity.

Which many-branched carbohydrate do muscles stockpile as a ready reserve of glucose for exercise?

Glycogen is the body's main way of storing glucose. A 70 kg adult packs roughly 400 grams of it into skeletal muscle, over three times the amount held in the body's other big store.

Which name is given to a bundle of skeletal muscle fibers wrapped in its own sheath of connective tissue?

Fascicle bundles are in turn bound together by an outer wrapping to make a whole muscle, so a muscle is built rather like a cable made of smaller cables.

Skeletal muscle fibers form when many precursor cells fuse, leaving each long fiber with many of which structure?

Nuclei line the inside edge of the fiber's membrane, each one donated by a precursor cell called a myoblast during the fusion.

Which term describes a muscle growing larger because its existing cells increase in size, the goal of bodybuilders?

Hypertrophy enlarges the cells a muscle already has. Left to nature it normally stops in the late teens, once a person is fully grown, which is where strength training takes over.

Which type of muscle contraction, used to hold a plank, builds tension without changing the muscle's length?

Isometric joins the Greek words for 'equal' and 'measuring', because neither the length of the muscle nor the angle of the joint changes while it works.

During a dumbbell curl, which term names the role played by the elbow extensors?

Antagonist muscles produce a turning force opposite to that of the agonist, the term for the muscle that drives the movement. In a push-up it is the elbow flexors that play the part.

Which wave of muscle contraction pushes each swallowed mouthful along the digestive tract?

Peristalsis runs without any conscious effort: a nerve network in the gut wall, reaching from the esophagus to the rectum, is responsible for each wave.

Which sense, fed by stretch sensors inside the muscles, tells the brain where the limbs are without looking?

Proprioception is the sense of self-movement, force and body position. Doctors check it by asking a patient to stand with feet together and eyes closed, then watching for a loss of balance.

Which sudden, involuntary and painful muscle contraction tends to strike at rest, most often in the legs at night?

Cramp usually lets go when the muscle is stretched. For one in the back of the lower leg, pulling the toes up toward the shin brings almost immediate relief.

Which energy-supplying structures do well-exercised muscles build more of, along with a denser web of capillaries?

Mitochondria multiply in muscles that are worked regularly, so training adds stamina as well as size. Every fiber carries many of them to meet its energy needs.

Which branch of anatomy is devoted to the muscular system, covering the structure, function and diseases of muscle?

Myology covers how muscles are built, how they work and what goes wrong with them. Its sister fields are osteology, for the bones, and arthrology, for the joints.

Which South American arrow poison paralyzes the skeletal muscles and was later used to relax them in surgery?

Curare does nothing when swallowed: it becomes active only once it gets into a wound or the bloodstream. Hunters boil it down from bark into a dark paste for their arrows and darts.

Which test records the electrical activity that skeletal muscles produce?

Electromyography can be done with fine needles placed in the muscle, a method neurologists use to track down trapped nerves in complaints such as sciatica and carpal tunnel syndrome.

Which term names the gradual loss of muscle mass and strength that comes with aging?

Sarcopenia can go unnoticed until it is severe, yet it raises the risk of falls and fractures and is counted as part of frailty syndrome.

In which condition does damaged skeletal muscle break down so rapidly that it can turn urine the color of tea?

Rhabdomyolysis floods the blood with the contents of ruptured muscle cells, some of which can injure the kidneys. Crush injuries and strenuous exercise are among the usual triggers.

Which protein, missing in people with Duchenne, ties a muscle fiber's inner scaffolding to its surroundings?

Dystrophin is a rod-shaped protein that anchors the fiber's internal skeleton, through the cell membrane, to the material outside. Without it that membrane deforms too easily.

Which anatomist based De humani corporis fabrica, whose second book covers the muscles, on his lectures at Padua?

Andreas Vesalius broke with custom by dissecting a corpse himself to illustrate what he was teaching. His book was a major advance on the long-dominant work of Galen.

Which field is the scientific study of how the human body moves?

Kinesiology draws on anatomy, physiology and biomechanics alike. In North America a student can take a full Bachelor of Kinesiology degree in the subject.

Which small muscle draws the eyebrows down and together, creasing vertical frown lines just above the nose?

Corrugator supercilii is known as the 'frowning' muscle, and it may be regarded as the principal muscle a face uses to show suffering.