Time To Read
Date Last Modified
The Muscular System · Chapter 6 of 7
Between fevers the nurse watched her do twenty squats: no fatigability, full strength. This chapter builds what that test measured — a twitch, summation and tetanusIn this context, sustained muscle contractions due to calcium or electrolyte imbalances., recruitment and the size principle, tone, the length–tension curve of Amy’s dryer and ice cream cone, isotonicA solution with the same solute concentration as the inside of a cell, maintaining equilibrium. and isometric — and shows why a muscle can be charged for days and still grade a perfect pull.
The question you will answer
A muscle does not pull all-or-nothing; the body grades its force two ways, by how often each unit fires and by how many units it recruits, and the resting length sets the ceiling. What did twenty squats actually test — and what could they never have shown?
Opener · One Twitch · about 1¾ minutes
The opener is the same narrated animation you will watch in Scene 1.
Watch the opener first. It sets the question; the chapter answers it.
1
Read the chart
Patient chart · Stina
1988 · age 13 · the exam that found nothing
Strength
5/5 in every limb; twenty squats without fatigability; grip firm; reflexesAutomatic responses to stimuli. 2+ and symmetric.
Tone
Normal — no spasticity, no flaccidity; no twitching or fasciculation seen.
Posture
Rounded shoulders under the backpack; tender paraspinal bellies after a school day.
Not examined
An attack day. Every exam was on a quiet week.
Told
“Strong as an ox. Nothing wrong with her muscles.”
Chart clues this module: 1–5 · earlier chapters · 6 · this chapter
Before you go on · your prediction
Full strength and no fatigability on a quiet day, in a girl whose legs go heavy with every fever. The exam shows that…
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
Her muscles are normal and the heaviness is not a muscle problem
Ask what a strength test measures (recruitment, tetanus, length) and what it cannot (what the tissue was charged last week).
The exam was on the wrong day — a febrile muscle would have graded differently
Ask what would have changed on an attack day: the twitch, the summation, the recruitment, or the fuel behind them.
Her nervous system was compensating — recruiting more units to hide a weak muscle
Ask whether recruitment can hide a fuel shortage for twenty squats, and what the size principle says about which units go first.
From Stina’s diary
2
The story
Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.
I · Strong as an ox
On a quiet Tuesday the nurse had her do twenty squats and tested every limb: five out of five, grip firm, reflexes brisk and even, tone normal, no fatigability. Strong as an ox, she said, and in the sense the exam measures she was right. A strength test asks the nervous systemThe organ system that controls body functions using electrical and chemical signals. to recruit motor units and drive them into tetanus against a load, and Stina’s did, because nothing in her junctions, sarcomeres or calcium handling was broken. What the test does not ask is what the tissue was charged last week, whether the glycogenA storage form of glucose found in animals. store has been refilled, or what the thighs will feel like on Thursday when the thermometer climbs. Her exams were always on quiet days. This chapter builds what the exam measured — the twitch, the summation, the recruitment, the resting length — so you can say exactly what five out of five proves, and what it could never see.
See it move: Scene 1 · One Twitch · Scene 3 · Recruitment: Matilda
II · The twitch, the summation, the recruitment
A twitch is one motor unit’s answer to one action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. — how the lab studies muscle, not how the body moves. On a myogram it has three phases: a latent periodThe short delay between muscle stimulation and contraction., a few milliseconds of excitation–contraction coupling with no tension yet, the same length in every muscle; a contraction phaseThe period of increasing muscle tension during a twitch. as bridges cycle; a relaxation phase as calcium is pumped back and the fiber lengthens by recoil. Bigger muscles twitch longer — the eye fastest, calf and back slowest. Real control is graded two ways. Frequency: a second stimulus before full relaxation adds to the first because more calcium is in the sarcoplasm — wave summationThe increasing force of contraction with repeated stimulation.; repeated stimuliChanges in the environment that are detected by sensory receptors. give unfused tetanus, a staircase that never quite relaxes; very frequent stimuli give fused tetanusA sustained, maximal muscle contraction without relaxation., a smooth pull at three or four times a twitch, until fatigue drops it to zero. A cold muscle’s first pulls are weaker — treppe. Strength: recruitment, more units, the size principle adding small ones first and the brain cycling through them — Matilda the nine-pound bowling ball — so the tension holds. Tone is a few units cycling always, an isometric partial contraction kept by spinal reflexes; too little is flaccid, too much spastic. Tetany, the disease, is lockjaw: the masseterMasseter Elevator Jaw closer Runs along the jaw; raises the mandible to close your mouth. locked by a bacterial toxin — the booster every ten years.
See it move: Scene 1 · One Twitch · Scene 3 · Recruitment: Matilda
III · The dryer, the ice cream, and the ceiling
Four principles from the notes: what applies to one fiber applies to the organ; tension is the force a muscle exerts and load the weight that opposes it; a contraction is isotonic if length changes — concentric shortening to lift, eccentric lengthening to lower — and isometric if tension rises with no movementA fundamental property of life involving motion of the body or its parts., the handstand, the posture; and force and duration answer the stimulus, a slap versus a caress. The ceiling is resting length. Amy’s stackable dryer: crouched all the way down, she could not lift it, because her stretched sarcomeres had too little overlap for cross-bridges. Her ice cream cone: flex the elbow fully and you cannot flex it more, because the filaments are already fully overlapped. Between the two is the optimum — about eighty to a hundred and twenty percent of resting length, the top of the length–tension hill, where the most bridges form; the site says seventy to a hundred and thirty, the deck seventy-five. Elastic forces bring the arm back when the stimulus stops. Her exam sat on top of the hill with every unit available and every bridge fuelled for twenty squats. The right test for the wrong question.
See it move: Scene 4 · The Dryer and the Ice Cream · Scene 5 · Isotonic, Isometric, and Tone
3
Watch · explore · think · check
5 scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.
How to read the scenes
- Animation (dark teal) — a short animated explainer, one idea each
- Slide (dark teal) — a still to study: a micrograph, a diagram or a map
- Explore (purple) — something to push on while you watch
- Game (purple) — a quick challenge
- Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
- Check (gold) — a quick H5P that scores itself
Scene 1 of 5
One Twitch
Latent, contraction, relaxation — and why the latent period is the same for every muscle.
Animation · in production · 1 min 40 s
One Twitch
A stimulus arrow; the myogram rises after a pause — the latent period, E-C coupling, no tension; the contraction phase, bridges cycling; the relaxation phase, calcium reloaded, elastic recoil. Then three traces on one axisSecond cervical vertebra; has the odontoid process (dens) for pivoting head (“no” motion).: eye muscle peaking fast, calf and back later and longer — the same latent period on all three. The lecture’s correction: the gastrocnemiusPlantarflexor / Flexor Large calf muscle; pushes you forward when you walk or jump is mostly fast; the soleusPlantarflexor Under the gastrocnemius; helps stand or walk by pointing toes downward. is the slow one.
Prefer the full lecture? The Muscle Twitch (original video on the site)
Explore · in production
Read the myogram
A blank myogram and a stimulus button. Fire once and, before the trace draws, mark where tension should start and name what fills the gap; then shade the three phases yourself and say what the fiber is doing in each. Then pick a muscle — eye, calf, back — and predict the trace before it overlays. Last: drain the fiber’s ATPThe energy currency of cells used for muscle contraction. a little and predict which phase stretches.
Think · not graded
The latent period is excitation–contraction coupling and is the same in every muscle; the relaxation phase is calcium being pumped back, which costs ATP. On a fever day with the fuel account overdrawn, which phase would lengthen — and would a strength test on a quiet day ever see it?
The true/false spot-check for this prompt is in production.
Check · Multiple Choice
During the latent period of a twitch…
This self-scoring check is in production.
Evidence card 1
Add it to your evidence board
Three phases, one of them paid for in ATP — and a strength test never reads a single twitch.
Scene 2 of 5
Summation and Tetanus
Stimulate again before it relaxes and the tension climbs; often enough and it fuses.
Slide · in production
Wave Summation, Unfused and Fused Tetanus, Treppe
The Twitch lecture’s muscle at the top of the staircase: “Hey, I can’t create more tension. Can you please stop asking me to?”
Prefer the full lecture? The Muscle Twitch (original video on the site)
Explore · in production
Stack the twitches
A stimulator with a frequency dial. Fire at one a second and predict the trace; raise to five, ten, thirty, fifty a second and predict, before each trace, whether the twitches summate, stair-step, or fuse. Then hold fused tetanus and predict how long it lasts before the line drops. Then warm a cold muscle and predict the first five contractions — treppe. Last: name which setting a strength test uses, and what it is not measuring.
Think · not graded
Fused tetanus is what you hold a weight with; it needs calcium released faster than it is pumped back and ATP for every bridge. Marcus cannot hold it because his receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter drop out; a fever-day fiber could hold it but at a higher price. Which failure would twenty squats catch, and which would it miss?
The true/false spot-check for this prompt is in production.
Check · Flip Cards
Flip to check: twitch, wave summation, unfused tetanus, fused tetanus, treppe, tetany.
This self-scoring check is in production.
Evidence card 2
Add it to your evidence board
Tension is graded by frequency, and a held contraction is fused tetanus — which she produced on demand.
Scene 3 of 5
Recruitment: Matilda
More units for a heavier cat — the size principle and the brain cycling through units to hold her.
Animation · in production · 1 min 40 s
Recruitment
The ASPCA cage; a cat listed as “small”, two years ago; a nine-pound bowling ball. The brain assesses the load and adds motor units — one, two, three — the tension steps up (the notes’ Fig 9.16). The size principle: smallest units first, the big ones only when the load demands. Then the hold: units 1, 2, 3, 1, 2, 3 cycling so the tension stays constant and no unit tires. Muscle tone as a few units cycling forever; posture as isometric work.
Prefer the full lecture? Recruitment (original video on the site)
Game · in production
Lift the cat
A muscle with ten motor units of three sizes and a load dial. Predict how many units a pencil needs, then a backpack, then a nine-pound cat, then a dryer — and which sizes go first — before the units light. Then hold the cat for a minute and watch the brain cycle units; predict what happens if cycling stops. Last: set tone — pick the few units that keep a standing body from folding and predict whether anything moves.
Think · not graded
Recruitment adds units; the size principle adds small ones first; cycling keeps them fresh. Twenty squats recruit most of the thigh into fused tetanus twenty times. If every unit was available and every one of them fired, what does that prove about her junctions and sarcomeres — and what does it leave unmeasured?
The true/false spot-check for this prompt is in production.
Check · Image Hotspots
On the recruitment graph, mark the first stimulus, the step where a second unit joins, the plateau of constant tension, and the small units recruited first.
This self-scoring check is in production.
Evidence card 3
Add it to your evidence board
Twenty squats recruited every unit into fused tetanus on demand — the ignition and the sarcomeres passed; the fuel account was not on the test.
Scene 4 of 5
The Dryer and the Ice Cream
Too stretched, too shortened, and the top of the hill — the length–tension curve.
Slide · in production
Length–Tension
Amy and the stackable dryer: “I just created a tug of war that went nowhere.”
Prefer the full lecture? Length-Tension Relationship (original video on the site)
Explore · in production
Find the top of the hill
A sarcomere you can stretch with calipers and a tension meter. Before each stimulus, set the length — 60, 80, 100, 120, 150 percent — and predict the tension from the overlap you see; then fire and plot the point. Build the curve yourself, then mark the dryer and the ice cream on it. Last: a backpack day — predict where a muscle stretched over rounded shoulders for seven hours sits on the hill, and what it can and cannot do at the end of it.
Think · not graded
A muscle held stretched all day — under a twenty-pound backpack on rounded shoulders — spends the day off the top of its hill and in isometric work. What does that cost, what does it feel like by evening, and which of her complaints does it explain without any fever at all?
The true/false spot-check for this prompt is in production.
Check · Summary
Pick the true statement in each set about length and tension.
This self-scoring check is in production.
Evidence card 4
Add it to your evidence board
Rounded shoulders under a backpack all day is a muscle off its optimum doing isometric work — a real, ordinary ache that the fever ache was filed with.
Scene 5 of 5
Isotonic, Isometric, and Tone
Concentric, eccentric, isometric — and the low hum of tone that never moves anything.
Slide · in production
Types of Contraction
Lowering the backpack to the floor is eccentric work — the muscle lengthening while it pulls — which is the contraction that leaves you sore the next day.
Prefer the full lecture? Length-Tension Relationship (original video on the site)
Game · in production
Name the contraction
Eight short clips — lifting a bag, lowering it, holding it, standing still, a handstand, a slap, a caress, a shiver — and three bins. Sort each before the reveal and say whether length changed and which way. Then a day in her life: predict which type the backpack demanded most, which the fever-day stairs demanded, and which the twenty squats tested. Last: tone — predict what a limb does when its tone goes (flaccid) and when it climbs (spastic).
Think · not graded
Eccentric work — lowering a load — is the kind that damages sarcomeres and leaves ordinary soreness a day later; isometric work holds posture for hours. A thirteen-year-old with a heavy bag on concrete floors does both all day. Which of her aches is ordinary mechanics, which is the fever, and what one question separates them?
The true/false spot-check for this prompt is in production.
Check · Drag the Words
Match each description to its contraction type.
This self-scoring check is in production.
Evidence card 5
Add it to your evidence board
Every contraction type she made worked; the exam measured all of them — on the days nothing was being charged.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Normal machinery tested on a quiet day; the charge was elsewhere, Neither on its own, or A muscle that was weak, hidden by the nervous system. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.
Evidence board · drag and drop · in production
The sorting board for this chapter’s cards is being built. Until it arrives, sort your evidence cards on paper into the three columns above.
5
Your verdict
Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.
Look back at your prediction
Full strength and no fatigability on a quiet day in a girl whose legs go heavy with fevers — the exam shows that…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- Her muscles are normal and the heaviness is not a muscle problem
- The exam was on the wrong day — a febrile muscle would have graded differently
- Her nervous system was compensating — recruiting more units to hide a weak muscle
Build your verdict · no writing
Choose the statement that holds in each set. The ones you keep become your argument.
This activity is in production.
6
The finding
Finding · the right test for the wrong question
Strong as an Ox
1988: 5/5 strength, twenty squats without fatigability, normal tone, brisk even reflexes — every exam on a quiet week. Chart entry: a twitch has a latent, a contraction and a relaxation phase; force is graded by frequency (summation, unfused and fused tetanus, treppe) and by recruitment (the size principle, units cycling, tone); resting length sets the ceiling. She graded a perfect pull every time she was asked, which proves the machinery and says nothing about the account. The backpack gave her a mechanical ache — a muscle off its optimum doing isometric and eccentric work all day — and it was filed with the fever ache that was never mechanics at all.
Next · Chapter 7
Three Muscles, One Fire
Two muscles left: the heart, which beats on its own and was inflamed in its sac years later, and the smooth muscle that cramped her gut and wrapped every vessel. Chapter 7 builds both, then puts the muscular chart on one page — and reads it.
Reliable information on rare diseases
NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation
Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.
Hop to:
← Familial Mediterranean Fever — case home
All pages for Module 7 — The Muscular System
7.0 Module Overview
7.1 Muscles That Were Never Out of Shape
7.2 Built to Pull: Tendon to Sarcomere
7.3 The Signal: Nerve Meets Muscle
7.4 Calcium Turns the Key
7.5 The Fuel Bill
7.6 Grading the Pull
7.7 Three Muscles, One Fire
7.8 Conclusion and Assessment
7.9 The Evidence Behind the Case
All Modules
- Anatomical Language, Membranes & Homeostasis
- Just Enough Chemistry
- The Cell & Its Transport
- Making Cells & Proteins
- The Integumentary System
- The Skeletal System
- The Muscular System
- Nervous Tissue & the Senses
- The Spinal Cord
- The Brain & the Blood–Brain Barrier
- The Autonomic Nervous System
- Special Senses (in developmentThe process of growth and differentiation.)
- The Endocrine System
- Blood
- The Heart
- Blood Vessels
- The Digestive System
- The Respiratory System
- The Urinary System
- Fluids, Electrolytes & Acid–Base Balance
- The Reproductive System
- The Immune System
List of terms
- tetanus
- isotonic
- reflexes
- anatomy
- physiology
- nervous system
- glycogen
- action potential
- latent period
- contraction phase
- wave summation
- stimuli
- fused tetanus
- masseter
- movement
- axis
- gastrocnemius
- soleus
- ATP
- receptors
- development