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Week 11 · Chapter

Chapter 11 — The Muscular System

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 5 (explain skeletal-muscle structure from the whole muscle to the sarcomere, the sliding-filament model, and the ordered steps of excitation, contraction, and relaxation at the neuromuscular junction and inside the fiber, and describe the organization of the muscular system — origin and insertion, muscle teams, lever classes, and the major muscles by region and action), Objective 4 (describe bone as a living tissue — its functions, the gross and microscopic anatomy of a long bone, the four bone cells, ossification, growth, remodeling, and the calcium-homeostasis loop — and sort the skeleton into its axial and appendicular divisions by region while classifying joints by structure, function, and movement) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Name the Muscle, Name the Action lab all build on it.

In plain language

Last week you looked inside one muscle fiber; this week you step back and watch whole muscles move your bones. Every muscle hangs between two bones, and when it contracts it pulls one bone toward the other. Muscles work in teams (groups of muscles that share one movement, each with its own role), and they work across your joints as levers (rigid bars that turn about a fixed point). You will learn to read a muscle's name as a set of clues, to work out how hard a muscle must pull, and to name ten major muscles by what they do. By the end, a curl, a stair climb, and a step onto your toes will each read as a short, ordered story about pulling.

Why you already care about this

Picture three made-up scenes. Someone lifts a bag of groceries from the kitchen counter, and the elbow bends as the forearm rises. A hiker on a steep trail rises onto the toes to see over a rock. A parent lifts a toddler from the floor, holds the child still for a moment, then lowers the child slowly onto a couch. Each scene is a muscle pulling on a bone, and each one has a rule behind it.

Here is the first surprise. In all three scenes, not one muscle pushed. Muscles only pull, so every move you make back and forth needs a muscle on each side of the joint. The second surprise is stranger: the muscle that lifts your forearm is attached so close to your elbow that it must pull far harder than the weight in your hand. This chapter shows you why that design is worth its cost.

By the end of this chapter you will be able to:

name a muscle's origin, insertion, and belly, and say which attachment moves;
name the agonist, antagonist, synergist, and fixator of a described movement, and say why the roles can swap;
sort a contraction as concentric, eccentric, or isometric;
predict something about an unfamiliar muscle from its name;
classify a lever by what sits in the middle, and work the lever law with every step and unit shown;
match ten major muscles to their region and their main action.

Carry this line with you: a muscle pulls, never pushes — every joint needs a partner.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the anatomical position and the directional pairs. Week 5, Tissues, named skeletal muscle as striated, voluntary tissue attached to bone. Week 7, The Skeletal System: Bone Tissue & Structure, called bones levers and joints pivots. Week 8, the midterm review, added the three-step read: name the structure, state its function, name the clinical connection. Week 9, The Skeleton & Joints, named the bones, the joints, and the movement pairs such as flexion and extension. Week 10, Muscle Tissue & the Physiology of Contraction, showed how one fiber contracts as its filaments slide.
This week: origin and insertion, the pull-only rule, the muscle team, and the kinds of contraction; then the muscle-naming logic, the three lever classes, the forearm lever's trade-off, and ten major muscles.
How they connect: Week 10's sliding filaments make each fiber shorten; this week those fibers act together as whole muscles pulling on Week 9's bones across Week 9's joints.
Where we're going next: Week 12, Nervous Tissue, the Neuron & the Action Potential, opens the motor neuron that fires a motor unit and begins the nervous-system unit.

Here is what you lean on this week. In Week 9 you learned the movement pairs: flexion bends a joint and decreases the angle between the bones, extension straightens it, abduction moves a limb away from the midline, and adduction moves it back. You also learned that the radius sits on the lateral, thumb side of the forearm, read from the anatomical position. In Week 10 you learned that a contracting fiber shortens because its thin filaments slide past its thick filaments. A whole muscle is many of those fibers pulling at once, and this week you watch where that pull lands.

1. Find the anchor: origin, insertion, and belly

Set a bag of groceries on a counter and lift it with your elbow bent. Your upper arm hardly moves, but your forearm swings up. The muscle doing the work is fixed at one end and pulls the other end toward it. Anatomists give those two ends names, so they can say which bone moves.

The origin is the attachment of a muscle on the bone that stays fixed and stabilized while the muscle contracts.
The insertion is the attachment of a muscle on the bone that moves; a contracting muscle pulls its insertion toward its origin.
The belly is the thick middle part of a muscle, lying between its origin and its insertion.

This course uses one worked pair of attachments. The biceps brachii is the two-headed muscle of the arm that crosses the shoulder and the elbow, flexes the forearm, and helps supinate it; its origin is on the scapula and its insertion is on the radius. When it contracts, the radius is pulled toward the scapula, and the forearm rises.

Worked example — read the bag lift. You name the parts of the biceps brachii during the lift.

Which bone stays put? The scapula, so the attachment there is the origin.
Which bone moves? The radius, in the forearm, so the attachment there is the insertion.
Where is the bulge you can feel? In the middle, the belly.
In one line: the biceps pulls its insertion on the radius toward its origin on the scapula, and the forearm flexes.

Misconception check. Students often say "the origin is the end that moves," because "origin" sounds like the place a motion starts. The cure is the anchor picture. The origin is the anchor on the fixed bone; the insertion is pulled in toward it. If your sentence has the origin moving, the two words have traded places.

2. Pull, never push: why every joint needs two sides

Walk a dog on a leash. You can pull the dog back toward you, but you can never push the dog away with the leash; it goes slack. A muscle works the same way, because a contracting muscle shortens toward its own middle and can only pull.

The rule is short: a muscle only pulls. It contracts toward its own middle and cannot push, so every joint that moves two ways needs muscles on opposing sides. One muscle pulls the joint one way, and another pulls it back.

Worked example — the two-way elbow. You explain how the elbow bends and straightens without any push.

To bend the elbow, the biceps brachii pulls the forearm up. That is flexion.
The biceps cannot push the forearm back down, so a second muscle must pull it the other way.
The triceps brachii is the three-headed muscle of the arm that extends the forearm. It pulls the forearm straight. That is extension.
In one line: two pulls on opposite sides give one joint two directions.

Misconception check. Students often say the triceps "pushes" the arm straight, because straightening feels like pushing a door open. The cure is to look at what the muscle does, not what your hand does. Your hand pushes the door, but the triceps is pulling the forearm. Muscles only pull; the antagonist pulls the joint back the other way.

3. Build the team: agonist, antagonist, synergist, fixator

A kitchen at dinnertime runs on roles. One person cooks the main dish, another passes ingredients, and another holds the pan steady on the stove. Muscles share a movement the same way, and the roles have names.

The agonist is the principal muscle in an action, also called the prime mover; the role belongs to the movement, so it can change when the movement reverses.
The antagonist is a muscle that produces the action opposite to the agonist's and relaxes as the agonist contracts.
A synergist is a muscle that aids the prime mover in carrying out an action.
A fixator is a muscle that stabilizes the bone carrying the prime mover's origin so the pull goes into the moving bone.

Worked example — the team for a curl, then the reverse. You assign the roles twice.

The curl: the biceps brachii flexes the forearm, so it is the agonist. The triceps brachii does the opposite action, so it is the antagonist, relaxing as the biceps contracts.
The fixator: the biceps' origin is on the scapula, so any muscle that holds the scapula steady during the curl is acting as a fixator. Without it, the pull would tug the scapula instead of lifting the forearm.
The synergist: any muscle that helps the biceps flex the forearm is a synergist. This course names the role, not every muscle that fills it.
The reverse: when the triceps brachii straightens the arm, the triceps is the agonist and the biceps is the antagonist.

The roles swapped, but the muscles did not change. That is the point of the team.

Misconception check. Students often learn "the biceps is the agonist" as a permanent label, because the curl is the first example everyone meets. The cure is to ask "agonist of which movement?" The roles are relative to the movement and swap when it reverses: agonist in a curl, antagonist when the arm straightens.

Pause and think, then choose one. (a) Describe a team you belong to, at work, in a club, or at home, and match one person to each of the four muscle roles. (b) Pick one everyday movement of your own, such as opening a jar, and say which muscle you think is the agonist and how you would check that guess against this chapter.

4. Sort the contraction: isotonic versus isometric

Go back to the parent and the toddler from the opening scene. The parent lifts the child, holds the child still for a moment, then lowers the child slowly onto the couch. The same arm muscles worked the whole time, but they did three different things.

An isotonic contraction is a contraction in which the muscle changes length as it moves a load; it is concentric when the muscle shortens and eccentric when it lengthens.
A concentric contraction is an isotonic contraction in which the muscle shortens as it moves a load, as in lifting a bag.
An eccentric contraction is an isotonic contraction in which the muscle lengthens while still under tension, as in lowering a bag slowly.
An isometric contraction is a contraction in which the muscle develops tension without any change in the joint's angle, as in holding a bag still.
Muscle tone is the small, always-on contraction that a resting skeletal muscle keeps.

Worked example — the bag, three ways. You sort what the biceps brachii does in each phase of one bag lift.

Lifting the bag from the counter: the biceps shortens as it moves the load, so the contraction is concentric, a kind of isotonic contraction.
Holding the bag still at chest height: the biceps is tense, but the elbow's angle does not change, so the contraction is isometric.
Lowering the bag slowly back to the counter: the biceps lengthens while still under tension, so the contraction is eccentric, the other kind of isotonic contraction.
In one line: shortens, concentric; holds, isometric; lengthens under tension, eccentric.

Even while you sit and read this, your muscles are not fully off. Muscle tone keeps a small contraction running in a resting muscle.

Misconception check. Students often say "an isometric contraction does no work, so the muscle is resting." It is tempting because nothing moves. The cure is the definition: an isometric contraction develops tension without changing the joint's angle. Holding the bag still is real contraction; the angle stays put because the pull matches the load.

5. Decode the name: the muscle-naming logic

A street sign can tell you a lot before you arrive: "North Main Street" gives a direction and a place. Muscle names work the same way. Muscle-naming logic is the habit of reading a muscle's name for clues to its size, shape, fiber direction, location, number of origins, attachments, or action.

The seven kinds of clue, each with this course's examples, are in the table below.

The seven kinds of clue a skeletal muscle's name can carry, with this course's examples
Clue	What the name tells you	Examples
Size	how big the muscle is next to its neighbors	gluteus maximus, medius, minimus
Shape	the outline of the muscle	deltoid, orbicularis
Fiber direction	which way the fibers run	rectus (straight), oblique (slanted)
Location	the bone or region it sits on	frontalis, over the frontal bone; brachialis, named for the arm
Number of origins	how many heads it starts from	biceps (two), triceps (three), quadriceps (four)
Attachments	the places it connects	sternocleidomastoid, whose name starts with the sternum
Action	what it does	flexor, extensor, abductor, adductor

Worked example — three names you have never studied. You predict something about each muscle from its name alone.

Flexor digitorum: "flexor" is an action clue, so it flexes; "digitorum" points to the digits, so it flexes the digits.
Gluteus medius: "medius" is a size clue, the middle size of the three gluteal muscles, between the maximus and the minimus.
Rectus femoris: "rectus" is a fiber-direction clue, so its fibers run straight; "femoris" points to the femur, so it lies along the thigh. It is one of the four heads of the quadriceps femoris.
In one line: read the name, sort each part into one of the seven clues, and you can predict before you look.

How many skeletal muscles are there in all? The total depends on how muscles are counted, so this course teaches it in words as a figure that varies and never as a number to memorize.

Misconception check. Students often try to memorize muscle names as random sounds, because the Latin looks foreign. The cure is to treat every name as a short description. A name you can decode is a name you do not have to memorize from scratch.

Honesty note: the naming logic predicts a lot, but not everything. Some names, such as gastrocnemius, carry no clue this course decodes, and for those you learn the action directly.

6. Model the lever: three classes, named by the middle

Picture a playground seesaw. A bar rests on a pivot, someone pushes down on one end, and the other end rises. Your skeleton does the same thing every time a muscle pulls. Like every model in this course, the lever model is a simplification you can hold in one hand, not a full description of a real limb.

In the body's lever model:

A lever is a rigid bar that turns about a fixed point; in the body's lever model the lever is a bone turning about a joint.
The fulcrum is the fixed point a lever turns about; in the body's lever model it is a joint.
The effort is the force applied to a lever; in the body's lever model it is the muscle's pull at its insertion.
The load is the weight or resistance a lever moves; in the body's lever model it is the body part or object being moved.

Levers come in three classes, and each class is named by what sits in the middle.

A first-class lever is a lever with the fulcrum in the middle, between the effort and the load, as when the head nods on the neck.
A second-class lever is a lever with the load in the middle, between the fulcrum and the effort, as when a person rises onto the toes.
A third-class lever is a lever with the effort in the middle, between the fulcrum and the load, as when the biceps brachii flexes the forearm; it is the body's most common design.

Worked example — classify by the middle. You classify each of the three pinned body levers by asking one question: what sits in the middle?

The head nodding on the neck: the neck joint sits between the muscle's pull and the head's weight, so the fulcrum is in the middle. First class.
The hiker rising onto the toes: the body's weight sits between the toes, where the foot pivots, and the muscle's pull. The load is in the middle. Second class.
The bag lift: the biceps' pull at its insertion sits between the elbow and the bag. The effort is in the middle. Third class.
In one line: fulcrum in the middle, first; load in the middle, second; effort in the middle, third.

Misconception check. Students often guess that most of the body's levers are first or second class, because the seesaw is the lever everyone pictures. The cure is the body's own design. Most of the body's levers are third class, with the effort in the middle, like the biceps on the forearm.

7. Weigh the trade-off: force versus speed on the forearm lever

Hold a full water bottle in your hand with your elbow bent, then slide it out toward your fingertips. It feels heavier, even though it weighs the same. The lever law explains why, and it shows the price your arm pays for its speed.

The effort arm is the distance from the fulcrum to the point where the effort is applied, measured in centimeters in the forearm example. The load arm is the distance from the fulcrum to the point where the load sits, measured in centimeters in the forearm example. The lever law is the balance rule for a lever: effort times effort arm equals load times load arm. In symbols: effort × effort arm = load × load arm. Force is measured in newtons (N), length in centimeters (cm), and a force times its arm in newton-centimeters (N·cm).

Worked example — Dataset H, the forearm lever (illustrative: engineered by the course's discipline pack to land on clean numbers, not measured data). A 60 N load sits in the hand, 36 cm from the elbow. The biceps brachii inserts 4 cm from the elbow. How hard must the biceps pull to hold the load?

Name the parts: the forearm is the lever, the elbow is the fulcrum, the biceps brachii's pull at its insertion is the effort, and the 60 N weight in the hand is the load.
Classify: the effort, 4 cm from the elbow, sits between the fulcrum and the load, 36 cm from the elbow, so the effort is in the middle and the lever is third class.
Write the lever law: effort × effort arm = load × load arm, so effort × 4 cm = 60 N × 36 cm.
Multiply the load by its arm: 60 N × 36 cm = 2,160 N·cm.
Divide by the effort arm: 2,160 N·cm ÷ 4 cm = 540 N.
Compare the effort with the load: 540 N ÷ 60 N = 9, so the biceps pulls with nine times the force of the load.
Compare the arms: 36 cm ÷ 4 cm = 9, so the hand sits nine times as far from the fulcrum as the insertion, and a small shortening near the elbow swings the hand through a large, fast arc.
Check: 540 N × 4 cm = 2,160 N·cm, the same as the load side, so the lever balances.

Say it in words: the biceps must pull 540 N of effort versus a 60 N load, 9 times the load, because it inserts so close to the elbow. That is the force-versus-speed trade-off, the cost and payoff of a third-class lever: the muscle inserts close to the joint, so it must pull with more force than the load, but a small shortening swings the far end through a large, fast arc. Your arm gives up force and buys speed and range of motion at the hand.

Misconception check. Students often say "a third-class lever is strong," because the biceps is a famously strong muscle. The cure is the arithmetic. The biceps must pull nine times the load to hold it, so the lever puts the muscle at a force disadvantage. What the lever gives back is speed: the hand travels a large, fast arc while the insertion moves a little.

Caveat: Dataset H's numbers are engineered to land on clean whole numbers. A real forearm's distances and forces vary from person to person, and this course never treats them as a norm.

8. Name the movers: ten major muscles by region and action

Think of a stairwell. Each step up straightens your knee, swings your thigh back at the hip, and presses the sole of your foot down. Ten major muscles run the big movements of your limbs and trunk, and each one has a main action you can feel.

The deltoid is the shoulder muscle named for its shape; it is the main abductor of the arm.
The pectoralis major is a large muscle of the chest that draws the arm across the chest.
The rectus abdominis is the trunk muscle whose name gives its straight fiber direction and its place on the abdomen; it flexes the trunk.
The gluteus maximus is the largest of the three gluteal muscles; it extends the hip, swinging the thigh backward.
The quadriceps femoris is the four-headed muscle group of the thigh, the rectus femoris and the vastus lateralis, vastus medialis, and vastus intermedius, which extends the knee.
The hamstrings are the muscle group of the thigh that flexes the knee and extends the hip.
The gastrocnemius is the muscle of the leg that plantar flexes the foot, pressing the sole down.
The tibialis anterior is the muscle of the leg named for its anterior place by the tibia; it dorsiflexes the foot, lifting its front.

With the biceps brachii and the triceps brachii from Sections 1 and 2, that makes ten. The table gathers all ten.

Ten major skeletal muscles of this course, with each one's region and main action
Muscle	Region	Main action
Deltoid	upper limb (shoulder)	abducts the arm
Pectoralis major	upper limb (chest to arm)	draws the arm across the chest
Biceps brachii	upper limb (arm)	flexes the forearm and helps supinate it
Triceps brachii	upper limb (arm)	extends the forearm
Rectus abdominis	trunk	flexes the trunk
Gluteus maximus	lower limb (hip)	extends the hip
Quadriceps femoris	lower limb (thigh)	extends the knee
Hamstrings	lower limb (thigh)	flex the knee and extend the hip
Gastrocnemius	lower limb (leg)	plantar flexes the foot
Tibialis anterior	lower limb (leg)	dorsiflexes the foot

Worked example — the stairwell step. You name the muscle behind each part of one step up, using the table.

The knee straightens as you push up: extension of the knee, so the quadriceps femoris is the agonist.
The thigh swings back at the hip: extension of the hip, so the gluteus maximus works, with the hamstrings, which also extend the hip.
The sole presses down as your back foot pushes off: plantar flexion, so the gastrocnemius is the agonist.
In one line: knee, quadriceps; hip, gluteus maximus with the hamstrings; ankle, gastrocnemius.

A handy rule of thumb: a muscle that crosses the front of a hinge usually flexes it, and one that crosses the back usually extends it. The word "usually" matters. Use the rule as a first guess, then check the action in the table.

Misconception check. Students often say "the biceps extends the forearm," because the biceps is the most famous arm muscle and "extend" sounds like "reach out." The cure is the pair. The biceps brachii flexes the forearm; the triceps brachii extends it. Say it as one sentence: biceps bends, triceps straightens.

9. Read the pairs: structure, function, and the clinical connection

Stand up and lift the front of one foot, then press it down as if on a car's pedal. Two muscles took turns, the same way two arm muscles take turns when you lift and lower a bag of groceries. Each joint you move both ways has a pair like that, because a muscle only pulls.

Worked example — three opposing pairs. You read each pair as structure to function.

Elbow: the biceps brachii flexes the forearm and the triceps brachii extends it, so each is the other's antagonist.
Knee: the hamstrings flex the knee and the quadriceps femoris extends it, so they form the knee's opposing pair.
Ankle: the tibialis anterior dorsiflexes the foot and the gastrocnemius plantar flexes it, so they form the ankle's opposing pair.
In one line: one pull each way, three joints, three pairs.

Now the week's clinical connection, read with the three-step read. Structure: the neuromuscular junction, where a motor neuron meets a muscle fiber. Function: it passes the signal that starts the fiber's contraction. Clinical connection: a receptor blocker at the neuromuscular junction leaves a muscle unable to contract. Read with this week's rule, that muscle can no longer pull its insertion. You name the structure and the condition, and you stop there; this course gives no advice about any condition.

Misconception check. Students often think the antagonist "fights" the agonist and wastes effort. It is tempting because the two pull in opposite directions. The cure is the timing in the definition: the antagonist relaxes as the agonist contracts. The pair takes turns, and that turn-taking is what lets one joint move two ways.

Key terms

Origin — The attachment of a muscle on the bone that stays fixed and stabilized while the muscle contracts. (first used: 1. Find the anchor: origin, insertion, and belly)
Insertion — The attachment of a muscle on the bone that moves; a contracting muscle pulls its insertion toward its origin. (first used: 1. Find the anchor: origin, insertion, and belly)
Belly — The thick middle part of a muscle, lying between its origin and its insertion. (first used: 1. Find the anchor: origin, insertion, and belly)
Biceps brachii — The two-headed muscle of the arm that crosses the shoulder and the elbow, flexes the forearm, and helps supinate it; its origin is on the scapula and its insertion is on the radius. (first used: 1. Find the anchor: origin, insertion, and belly)
Triceps brachii — The three-headed muscle of the arm that extends the forearm. (first used: 2. Pull, never push: why every joint needs two sides)
Agonist — The principal muscle in an action, also called the prime mover; the role belongs to the movement, so it can change when the movement reverses. (first used: 3. Build the team: agonist, antagonist, synergist, fixator)
Antagonist — A muscle that produces the action opposite to the agonist's and relaxes as the agonist contracts. (first used: 3. Build the team: agonist, antagonist, synergist, fixator)
Synergist — A muscle that aids the prime mover in carrying out an action. (first used: 3. Build the team: agonist, antagonist, synergist, fixator)
Fixator — A muscle that stabilizes the bone carrying the prime mover's origin so the pull goes into the moving bone. (first used: 3. Build the team: agonist, antagonist, synergist, fixator)
Isotonic contraction — A contraction in which the muscle changes length as it moves a load; it is concentric when the muscle shortens and eccentric when it lengthens. (first used: 4. Sort the contraction: isotonic versus isometric)
Concentric contraction — An isotonic contraction in which the muscle shortens as it moves a load, as in lifting a bag. (first used: 4. Sort the contraction: isotonic versus isometric)
Eccentric contraction — An isotonic contraction in which the muscle lengthens while still under tension, as in lowering a bag slowly. (first used: 4. Sort the contraction: isotonic versus isometric)
Isometric contraction — A contraction in which the muscle develops tension without any change in the joint's angle, as in holding a bag still. (first used: 4. Sort the contraction: isotonic versus isometric)
Muscle tone — The small, always-on contraction that a resting skeletal muscle keeps. (first used: 4. Sort the contraction: isotonic versus isometric)
Muscle-naming logic — The habit of reading a muscle's name for clues to its size, shape, fiber direction, location, number of origins, attachments, or action. (first used: 5. Decode the name: the muscle-naming logic)
Lever — A rigid bar that turns about a fixed point; in the body's lever model the lever is a bone turning about a joint. (first used: 6. Model the lever: three classes, named by the middle)
Fulcrum — The fixed point a lever turns about; in the body's lever model it is a joint. (first used: 6. Model the lever: three classes, named by the middle)
Effort — The force applied to a lever; in the body's lever model it is the muscle's pull at its insertion. (first used: 6. Model the lever: three classes, named by the middle)
Load — The weight or resistance a lever moves; in the body's lever model it is the body part or object being moved. (first used: 6. Model the lever: three classes, named by the middle)
First-class lever — A lever with the fulcrum in the middle, between the effort and the load, as when the head nods on the neck. (first used: 6. Model the lever: three classes, named by the middle)
Second-class lever — A lever with the load in the middle, between the fulcrum and the effort, as when a person rises onto the toes. (first used: 6. Model the lever: three classes, named by the middle)
Third-class lever — A lever with the effort in the middle, between the fulcrum and the load, as when the biceps brachii flexes the forearm; it is the body's most common design. (first used: 6. Model the lever: three classes, named by the middle)
Effort arm — The distance from the fulcrum to the point where the effort is applied, measured in centimeters in the forearm example. (first used: 7. Weigh the trade-off: force versus speed on the forearm lever)
Load arm — The distance from the fulcrum to the point where the load sits, measured in centimeters in the forearm example. (first used: 7. Weigh the trade-off: force versus speed on the forearm lever)
Lever law — The balance rule for a lever: effort times effort arm equals load times load arm. (first used: 7. Weigh the trade-off: force versus speed on the forearm lever)
Force-versus-speed trade-off — The cost and payoff of a third-class lever: the muscle inserts close to the joint, so it must pull with more force than the load, but a small shortening swings the far end through a large, fast arc. (first used: 7. Weigh the trade-off: force versus speed on the forearm lever)
Deltoid — The shoulder muscle named for its shape; it is the main abductor of the arm. (first used: 8. Name the movers: ten major muscles by region and action)
Pectoralis major — A large muscle of the chest that draws the arm across the chest. (first used: 8. Name the movers: ten major muscles by region and action)
Rectus abdominis — The trunk muscle whose name gives its straight fiber direction and its place on the abdomen; it flexes the trunk. (first used: 8. Name the movers: ten major muscles by region and action)
Gluteus maximus — The largest of the three gluteal muscles; it extends the hip, swinging the thigh backward. (first used: 8. Name the movers: ten major muscles by region and action)
Quadriceps femoris — The four-headed muscle group of the thigh, the rectus femoris and the vastus lateralis, vastus medialis, and vastus intermedius, which extends the knee. (first used: 8. Name the movers: ten major muscles by region and action)
Hamstrings — The muscle group of the thigh that flexes the knee and extends the hip. (first used: 8. Name the movers: ten major muscles by region and action)
Gastrocnemius — The muscle of the leg that plantar flexes the foot, pressing the sole down. (first used: 8. Name the movers: ten major muscles by region and action)
Tibialis anterior — The muscle of the leg named for its anterior place by the tibia; it dorsiflexes the foot, lifting its front. (first used: 8. Name the movers: ten major muscles by region and action)

Summary

The origin is on the fixed bone; the insertion moves.
A muscle only pulls, so joints need opposing muscles.
Agonist, antagonist, synergist, and fixator are roles in a movement.
The roles swap when the movement reverses.
Concentric shortens, eccentric lengthens under tension, isometric holds the angle.
A muscle's name can encode size, shape, fiber direction, location, origins, attachments, or action.
Lever classes are named by what sits in the middle.
Third-class levers are the body's most common design.
On the forearm lever, the biceps pulls 540 N to hold 60 N.
A third-class lever trades force for speed.
The biceps brachii flexes the forearm; the triceps brachii extends it.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative forearm made up for this question, a watering can weighing 25 N is held 35 cm from the elbow, and the flexing muscle inserts 5 cm from the elbow. Classify the lever, find the effort, and say how many times the load it is.
You meet a muscle called the extensor digitorum for the first time. From its name alone, predict what it does and where it works, and name the two kinds of clue you used.
A made-up soccer player straightens the knee to kick a ball, then bends the knee back to recover. Name the agonist and the antagonist for each movement, and say what changed between them.
A made-up rock climber hangs from a ledge with the elbows bent and does not move for several seconds, then lowers slowly until the arms are straight. Name the kind of contraction in the arm flexors during each part.

Answers (worked reasoning — read after attempting)

The effort, 5 cm from the elbow, sits between the elbow and the can, 35 cm away, so the effort is in the middle and the lever is third class. Lever law: effort × 5 cm = 25 N × 35 cm = 875 N·cm. Effort: 875 N·cm ÷ 5 cm = 175 N. Compare: 175 N ÷ 25 N = 7, so the muscle pulls with seven times the load. As with the forearm lever, the muscle pays in force and the hand gains speed.
"Extensor" is an action clue, so the muscle extends; "digitorum" points to the digits, so it works on the digits. Prediction: it extends the digits. The two kinds of clue are action and location. This is the mirror image of the flexor digitorum from Section 5.
Straightening the knee is extension, so the quadriceps femoris is the agonist and the hamstrings are the antagonist. Bending the knee back is flexion, so the hamstrings are the agonist and the quadriceps femoris is the antagonist. The muscles stayed the same; the movement reversed, so the roles swapped.
Hanging still with the elbows bent: tension with no change in the joint's angle, so isometric. Lowering slowly until the arms are straight: the flexors lengthen while still under tension, so eccentric, a kind of isotonic contraction.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz. Next week the action potential, the signaling mechanism of the motor neuron that fires each motor unit, gets its own close-up.

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