Anatomy & Physiology · Chapter 8

The Appendicular Skeleton

If the axial skeleton is the body's central mast, the appendicular skeleton is everything that hangs off it and lets you act on the world — the arms that reach and grip, the legs that carry and spring, and the two bony girdles that bolt those limbs to the trunk. It is a set of 126 bones, and almost every one of them is a lever built for movement. In this chapter we work outward from each girdle to the fingertips and toes, and you will meet a recurring theme: the shoulder is engineered for freedom, the hip for strength, and the whole design is a series of trade-offs between the two. Learn the bones in this order and the hand and foot — 27 and 26 bones each — stop being a jumble and become a logical map.

By the end of this chapter you can

Define the appendicular skeleton and list its four divisions, accounting for all 126 bones.
Name the two bones of the pectoral girdle and explain why the upper limb's only bony link to the axial skeleton is the sternoclavicular joint.
Identify the bones of the upper limb — humerus, radius, ulna — and place the radius on the thumb side in anatomical position.
Sort the 27 bones of the hand and wrist into carpals, metacarpals, and phalanges, and name the eight carpals.
Describe how three bones fuse to form each hip bone and contrast the pelvic girdle's stability with the pectoral girdle's mobility.
Compare the male and female pelvis and explain the adaptations for childbirth.
Identify the bones of the lower limb and distinguish the weight-bearing tibia from the non-weight-bearing fibula.
Sort the 26 bones of the foot and ankle into tarsals, metatarsals, and phalanges, and explain how the foot's arches absorb shock.

Section 1The pectoral girdle

The pectoral girdle reference figure — **Reference.** The pectoral (shoulder) girdle — clavicle and scapula — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Two bones, one clever compromise

The pectoral girdle — the shoulder girdle — is the frame that connects each upper limb to the trunk, and it is built from only two bones on each side: the clavicle (collarbone) in front and the scapula (shoulder blade) behind. What is striking is how little the whole arm is bolted to the rest of the skeleton. The entire upper limb hangs from the axial skeleton at a single small joint — the sternoclavicular joint, where the medial end of the clavicle meets the manubrium of the sternum. The scapula, by contrast, is not directly joined to the ribs or spine at all; it floats on the back of the rib cage, held in place only by muscles. That loose arrangement is a deliberate compromise: the shoulder trades away stability to buy an enormous range of motion. You can circle your arm through nearly a full sphere — and the price is that the shoulder is one of the most easily dislocated joints in the body.

The clavicle: a slender strut

The clavicle is an S-shaped bone that acts as a brace, holding the shoulder out and away from the chest so the arm can swing freely. Its rounded sternal (medial) end articulates with the sternum; its flattened acromial (lateral) end meets the acromion of the scapula at the acromioclavicular (AC) joint. Because it is thin and sits right under the skin with nothing to cushion it, the clavicle is the most frequently fractured bone in the body — usually snapping in its middle third when someone falls onto an outstretched arm or the point of the shoulder.

The scapula: a triangle of landmarks

The scapula is a flat, triangular bone loaded with muscle-attachment features. Running across its back is the spine, which ends laterally in the acromion — the bony tip of your shoulder that you can feel. Curving forward like a finger is the coracoid process, an anchor for muscles and ligaments. Most important is the glenoid cavity (glenoid fossa), the shallow socket on the scapula's lateral angle that receives the head of the humerus to form the shoulder joint. That shallowness is the key to the whole design: a shallow socket lets the humerus swing widely, but it also lets the head slip out too easily.

Bone / feature	What it is	Why it matters
Clavicle	S-shaped collarbone, front of girdle	Braces the shoulder outward; most commonly fractured bone
Scapula	Triangular shoulder blade, behind	Held only by muscles; carries the socket for the arm
Glenoid cavity	Shallow socket on the scapula	Receives the humeral head — shallow = mobile but unstable
Acromion	Lateral tip of the scapular spine	Forms the point of the shoulder; joins the clavicle
Sternoclavicular joint	Clavicle meets sternum	The only bony joint linking the upper limb to the axial skeleton

The shoulder's freedom is its weakness. A "separated shoulder" is an injury to the acromioclavicular joint, while a "dislocated shoulder" is the humeral head popping out of the shallow glenoid cavity. Both are direct consequences of the same design choice — mobility purchased at the cost of stability.

Explore both bones and the landmarks that ride on them in the interactive below.

Interactive The shoulder girdle attaches each arm to the axial skeleton by just two bones — the clavicle and scapula. Open full ↑

Key points

The pectoral girdle is just two bones — the clavicle and scapula — on each side.
The upper limb's only bony attachment to the axial skeleton is the sternoclavicular joint; the scapula floats on muscle.
The glenoid cavity is shallow, trading stability for a huge range of motion.
The clavicle is the body's most commonly fractured bone.

Section 2Bones of the upper limb

Bones of the upper limb reference figure — **Reference.** Bones of the upper limb — humerus, radius, and ulna — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Three long bones between shoulder and wrist

Each upper limb contains 30 bones, but the three big long bones do the heavy structural work. The upper arm (the brachium) holds a single bone, the humerus. The forearm (the antebrachium) holds two bones side by side, the radius and the ulna. Learning which is which is easy if you anchor it to the anatomical position — palms facing forward: the radius is on the thumb (lateral) side, and the ulna is on the little-finger (medial) side. A phrase that sticks: the radius rotates around the ulna, and the radius is on the same side as the thumb.

The humerus and the elbow

The humerus is the long bone of the arm. Its rounded head fits into the glenoid cavity up at the shoulder. Just below the head is the surgical neck, a frequent fracture site that sits dangerously close to the axillary nerve. Down the shaft is the deltoid tuberosity, where the deltoid muscle attaches. At the elbow end, two rounded surfaces do the articulating: the capitulum meets the radius, and the pulley-shaped trochlea grips the ulna. Flanking them are the medial and lateral epicondyles — the ulnar nerve runs behind the medial one, in a shallow groove where it lies just under the skin with no muscle over it, so striking the epicondyle presses straight on the nerve and sends a jolt down the forearm (your "funny bone").

Radius and ulna: a rotating pair

The two forearm bones are joined along their length by a fibrous interosseous membrane and connect to each other at small joints top and bottom. This lets the forearm perform a beautiful trick: supination (palm up) and pronation (palm down). In pronation the radius pivots and crosses over the ulna. At the elbow, the ulna is the dominant partner — its olecranon forms the bony point of the elbow, and its trochlear notch wraps around the humeral trochlea like a wrench on a bolt. At the wrist, the radius takes over — it is the forearm bone that does most of the articulating with the carpal bones. Both bones end in a pointed styloid process.

Bone	Location	Key role
Humerus	Upper arm (brachium)	Head fits the glenoid; distal end forms the elbow
Radius	Forearm, thumb (lateral) side	Rotates in pronation/supination; forms the wrist
Ulna	Forearm, pinky (medial) side	Olecranon forms the elbow point; stabilizes the forearm

Thumb side = radius. Whenever you lose track, look at your palm-forward hand: the bone lined up under your thumb is the radius, and it is the one that "radiates" out to spin the wrist. The ulna stays put at the elbow — think "elbow-u-lna."

Tap each of the three bones and rotate the forearm from supination to pronation in the interactive.

Interactive Three long bones — one in the upper arm, two in the forearm; the radius sits on the thumb side, the ulna on the pinky side. Open full ↑

Key points

The upper limb has three long bones: the humerus (arm) and the radius and ulna (forearm).
In anatomical position the radius is lateral (thumb side), the ulna is medial (pinky side).
The ulna dominates the elbow (olecranon); the radius dominates the wrist and does the rotating.

Section 3The hand & wrist

The hand and wrist reference figure — **Reference.** Bones of the hand and wrist — carpals, metacarpals, and phalanges — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Twenty-seven bones in three groups

The hand is the most dexterous structure in the body, and its dexterity comes from packing 27 bones — and all the joints between them — into a small space. They sort neatly into three groups working from the wrist outward: 8 carpals in the wrist, 5 metacarpals in the palm, and 14 phalanges in the fingers. Get those three numbers (8, 5, 14) and their order and you have the whole hand.

The carpals: eight little stones

The carpals are eight small bones arranged in two rows of four, bound tightly by ligaments so the wrist bends as a unit. From the thumb side across the proximal row, then the distal row, they are: scaphoid, lunate, triquetrum, pisiform, then trapezium, trapezoid, capitate, hamate. A classic mnemonic keeps the order: "Sally Left The Party To Take Cathy Home." The scaphoid deserves special attention — it is the most commonly fractured carpal, usually from a fall on an outstretched hand, and because its blood supply enters from one end, a fracture can starve part of the bone and cause it to die.

Metacarpals and phalanges

The metacarpals are the five bones of the palm, numbered 1 to 5 starting at the thumb. Their rounded distal heads are your knuckles. The thumb's metacarpal meets the trapezium at a saddle joint — the special joint that lets the thumb swing across the palm to touch the other fingers, the opposable grip that makes the human hand so capable. The phalanges are the finger bones: each finger has three (proximal, middle, distal), but the thumb (pollex) has only two. That is 14 per hand. (A single one is a phalanx.)

Group	Count	Where / notes
Carpals	8	Wrist; two rows of four (scaphoid is the one most often broken)
Metacarpals	5	Palm; numbered 1–5 from the thumb; heads = knuckles
Phalanges	14	Fingers have 3 each; the thumb has only 2

Wrist pain after a fall on the hand? Think scaphoid. Tenderness in the "anatomical snuffbox" at the base of the thumb suggests a scaphoid fracture — and because these fractures can be invisible on an early X-ray yet risk avascular necrosis, they are splinted and re-imaged rather than dismissed.

Tap each of the three groups — wrist, palm, and fingers — in the interactive below.

Interactive Each hand packs 27 bones into three groups — the wrist, the palm, and the fingers. Open full ↑

Key points

The hand has 27 bones: 8 carpals + 5 metacarpals + 14 phalanges.
The eight carpals run scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate.
Each finger has 3 phalanges; the thumb has 2, and its saddle joint gives opposition.

Section 4The pelvic girdle

The pelvic girdle reference figure — **Reference.** The pelvic girdle and the male vs female pelvis — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Built for strength, not freedom

Where the shoulder was engineered for mobility, the pelvic girdle is engineered for the opposite — strength and weight-bearing. It has to transmit the entire weight of the upper body down into the legs and absorb the pounding of every step. So instead of a bone floating on muscle, the pelvic girdle attaches firmly to the axial skeleton at the strong, nearly immovable sacroiliac joints. The girdle itself is the pair of hip bones (also called coxal bones or os coxae); together with the sacrum and coccyx they form the bony pelvis.

Three bones fused into one

Each hip bone begins life as three separate bones that fuse during adolescence into a single unit, meeting at a deep, cup-shaped socket called the acetabulum. The ilium is the large, fan-shaped upper part — its rim, the iliac crest, is the "hip" you rest your hands on. The ischium is the posterior-inferior part, and its rough ischial tuberosity is what you sit on. The pubis is the anterior part; the two pubic bones meet in front at the pubic symphysis, a joint of fibrocartilage. Notice the contrast with the shoulder: the acetabulum is a deep socket that grips the head of the femur firmly, so the hip almost never dislocates — the mirror image of the shallow, mobile glenoid.

Male and female pelvis: shaped by childbirth

The pelvis is the one part of the skeleton that differs clearly between the sexes, because the female pelvis is adapted to let a baby pass through. Compared with the male, the female pelvis is wider and shallower, with a larger, more rounded pelvic inlet, and a broader subpubic angle (the angle of the pubic arch) that is greater than 90°. The male pelvis is narrower and heavier, with a heart-shaped inlet and a subpubic angle less than 90°. These differences are reliable enough that anatomists and forensic scientists use the pelvis to determine sex from a skeleton.

Feature	Female pelvis	Male pelvis
Overall shape	Wider, shallower, lighter	Narrower, deeper, heavier
Pelvic inlet	Rounded, oval, larger	Heart-shaped, smaller
Subpubic angle	Wide — greater than 90°	Narrow — less than 90°
Purpose of the difference	Roomy birth canal for childbirth	—

Deep socket, stable joint. The acetabulum grips the femoral head far more deeply than the glenoid grips the humerus. That is the whole story of girdle design in one line: the shoulder gives up stability for motion, the hip gives up motion for stability — each shaped by its job.

Tap the ilium, ischium, and pubis, then toggle between the male and female pelvis in the interactive.

Interactive The pelvic girdle is built for strength: each hip bone fuses three bones into one — and its shape differs in males and females. Open full ↑

Key points

The pelvic girdle is two hip bones, each a fusion of ilium + ischium + pubis meeting at the acetabulum.
It attaches firmly to the axial skeleton at the sacroiliac joints — built for weight-bearing, not mobility.
The female pelvis is wider with a subpubic angle greater than 90°, adapted for childbirth.

Section 5Bones of the lower limb

Bones of the lower limb reference figure — **Reference.** Bones of the lower limb — femur, patella, tibia, and fibula — OpenStax, CC BY 3.0 (via Wikimedia Commons)

The strongest bones in the body

The lower limb carries your entire body weight and drives every step, so its bones are the largest and strongest you own. Like the arm, each limb has 30 bones, and four of them are the big structural players. The thigh holds a single bone, the femur. The knee is guarded by the patella (kneecap). The lower leg holds two bones, the tibia and fibula — the counterparts to the forearm's radius and ulna, but with a crucial difference we will get to.

Femur and patella

The femur is the longest, heaviest, and strongest bone in the body. Its ball-shaped head sits deep in the acetabulum; below the head, the angled neck is the classic site of a "hip fracture" in older adults with thin bone. Bumps called the greater and lesser trochanters anchor powerful hip muscles, and a ridge on the back, the linea aspera, gives more muscle attachment. At the knee, the femur's rounded condyles roll on the top of the tibia. The patella is the body's largest sesamoid bone — a bone that forms inside a tendon. Sitting in the quadriceps tendon, it acts like a pulley that improves the leverage of the thigh muscles when you straighten the knee, and it shields the joint from a direct blow.

Tibia and fibula: one carries, one steadies

Here is the difference from the forearm. In the leg the two bones do not share the load. The tibia (shinbone) is the thick, medial bone, and it bears essentially all the weight — its broad top (the tibial plateau) supports the femur, and its lower end forms the medial malleolus, the inner ankle bump. The fibula is the slender, lateral bone. It carries almost no weight; its jobs are to stabilize the ankle and give muscles a place to attach. Its lower end forms the lateral malleolus, the outer ankle bump. Notably, the fibula does not even reach the knee — only the tibia meets the femur there.

Bone	Location	Role
Femur	Thigh	Longest, strongest bone; head fits the acetabulum
Patella	Front of knee	Largest sesamoid bone; improves quadriceps leverage
Tibia	Leg, medial	Weight-bearing shinbone; forms the medial malleolus
Fibula	Leg, lateral	Non-weight-bearing; stabilizes ankle, forms lateral malleolus

Tibia takes the load, fibula takes the side. The tibia is medial and weight-bearing; the fibula is lateral and mostly for stability and muscle attachment. Because the fibula does so little load-bearing, surgeons can harvest a piece of it to graft elsewhere in the body with little effect on walking.

Tap each of the four bones and find the two malleoli at the ankle in the interactive.

Interactive The leg carries your whole weight, so its bones are the largest and strongest — the thigh, the kneecap, and two in the lower leg. Open full ↑

Key points

The lower limb's four big bones are the femur, patella, tibia, and fibula.
The femur is the body's longest and strongest bone; the patella is its largest sesamoid bone.
The tibia (medial) bears the weight and forms the knee; the fibula (lateral) only stabilizes the ankle.

Section 6The foot & ankle

The foot and ankle reference figure — **Reference.** Bones of the foot and ankle — tarsals, metatarsals, and phalanges — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Twenty-six bones, built to bear weight

The foot echoes the hand — three groups of bones — but it is tuned for a completely different job: instead of grasping, it must bear weight and spring you forward. It holds 26 bones: 7 tarsals in the ankle and heel, 5 metatarsals in the sole, and 14 phalanges in the toes. It is the same three-group plan as the hand, with slightly different counts to remember (7, 5, 14).

The tarsals and the weight it takes

The tarsals are seven bones forming the ankle and back of the foot. Two of them do the serious work. The talus sits at the top and is the bone the tibia and fibula grip — it receives the body's weight from the leg and passes it down. The calcaneus (heel bone) is the largest tarsal; it forms the heel and takes the weight when your foot strikes the ground. The other five — the navicular, the cuboid, and three cuneiforms — bridge the ankle to the sole. The metatarsals (numbered 1–5 from the big toe) form the sole and the ball of the foot, and the phalanges are the toe bones: three in each toe, but only two in the big toe (the hallux) — exactly the pattern of the hand.

The arches: the foot's suspension

What truly separates the foot from the hand is its arches. The tarsals and metatarsals are held by ligaments and tendons into three curves — a medial longitudinal arch (the high one on the inside), a lateral longitudinal arch, and a transverse arch across the foot. These arches turn the foot into a natural spring and shock absorber: they flatten slightly to soak up the impact of each step, then recoil to push you off. When the arches drop, the result is flat feet (pes planus), which can lead to foot and leg fatigue because the shock absorber is gone.

Group	Count	Where / notes
Tarsals	7	Ankle & heel; talus takes weight from the leg, calcaneus is the heel
Metatarsals	5	Sole; numbered 1–5 from the big toe; heads = ball of the foot
Phalanges	14	Toes have 3 each; the big toe (hallux) has 2

Weight lands on a tripod. Standing weight is shared among the heel (calcaneus) and the heads of the first and fifth metatarsals — a stable three-point base. The arches spanning between them are why a healthy foot can absorb tons of cumulative force over a day without pain.

Tap each of the three groups — ankle, sole, and toes — and see the arches in the interactive below.

Interactive Each foot has 26 bones in three groups — built to bear weight, with bony arches that act as shock absorbers. Open full ↑

Key points

The foot has 26 bones: 7 tarsals + 5 metatarsals + 14 phalanges.
The talus receives weight from the leg; the calcaneus is the heel and largest tarsal.
Three arches make the foot a shock absorber and spring; their collapse is flat feet.

The chapter in one breath

The appendicular skeleton is 126 bones — the two limb girdles plus the arms and legs that hang from them.
The pectoral girdle (clavicle + scapula) links the arm to the trunk by a single bony joint (sternoclavicular), buying mobility at the cost of stability.
The upper limb has three long bones — humerus, plus radius (thumb side) and ulna (pinky side); the ulna rules the elbow, the radius rules the wrist.
The hand is 27 bones: 8 carpals, 5 metacarpals, 14 phalanges, with an opposable thumb for dexterity.
The pelvic girdle is two hip bones (ilium + ischium + pubis fused at the acetabulum), built for strength and weight-bearing; the female pelvis is adapted for childbirth.
The lower limb has the body's strongest bones — femur, patella, and the weight-bearing tibia with the stabilizing fibula.
The foot is 26 bones (7 tarsals, 5 metatarsals, 14 phalanges) whose arches make it a spring and shock absorber.
Throughout, the design is a trade-off: the shoulder chooses freedom, the hip chooses strength.

ReferenceWords worth knowing

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Acetabulum: The deep, cup-shaped socket on the hip bone that receives the head of the femur to form the hip joint.
Appendicular skeleton: The 126 bones of the limbs and the two girdles (pectoral and pelvic) that attach the limbs to the axial skeleton.
Calcaneus: The heel bone; the largest tarsal, which bears weight when the foot strikes the ground.
Carpals: The eight small bones of the wrist, arranged in two rows of four.
Clavicle: The S-shaped collarbone that braces the shoulder; the most commonly fractured bone in the body.
Femur: The thigh bone; the longest, heaviest, and strongest bone in the body.
Fibula: The slender lateral bone of the leg; non-weight-bearing, it stabilizes the ankle and forms the lateral malleolus.
Glenoid cavity: The shallow socket of the scapula that receives the head of the humerus at the shoulder joint.
Hip bone (os coxae): The coxal bone; each one is a fusion of the ilium, ischium, and pubis, and the pair forms the pelvic girdle.
Humerus: The single long bone of the upper arm, running from the shoulder to the elbow.
Metacarpals: The five bones of the palm, numbered 1 to 5 from the thumb; their heads form the knuckles.
Metatarsals: The five bones of the sole of the foot, numbered 1 to 5 from the big toe.
Patella: The kneecap; the largest sesamoid bone, formed within the quadriceps tendon to improve leverage.
Pectoral girdle: The shoulder girdle — clavicle and scapula — that attaches each upper limb to the axial skeleton.
Pelvic girdle: The two hip bones that attach the lower limbs to the axial skeleton and bear the body's weight.
Phalanges: The bones of the fingers and toes; three per digit except the thumb and big toe, which have two (singular: phalanx).
Radius: The lateral (thumb-side) bone of the forearm; it rotates in pronation and forms most of the wrist joint.
Scapula: The triangular shoulder blade, held to the trunk only by muscles and carrying the glenoid cavity.
Talus: The tarsal bone that the tibia and fibula grip; it receives the body's weight from the leg.
Tarsals: The seven bones of the ankle and posterior foot, including the talus and calcaneus.
Tibia: The medial shinbone; the weight-bearing bone of the leg that forms the knee and the medial malleolus.
Ulna: The medial (pinky-side) bone of the forearm; its olecranon forms the point of the elbow.

Check yourself

1. The appendicular skeleton totals 126 bones. Name its four divisions.

The pectoral girdles, the upper limbs, the pelvic girdle, and the lower limbs. These are the "appendages" and the two girdles that bolt them to the axial skeleton; together they make up 126 of the skeleton's 206 bones.

2. Which two bones make up the pectoral girdle, and what is the only bony joint connecting the entire upper limb to the axial skeleton?

The clavicle and scapula; the sternoclavicular joint is the only bony link. The scapula is held to the trunk by muscles alone, which is what gives the shoulder its enormous range of motion.

3. Why is the shoulder easy to dislocate while the hip almost never dislocates?

The glenoid cavity is a shallow socket; the acetabulum is a deep one. The shoulder trades stability for mobility, so the humeral head slips out easily; the hip trades mobility for stability, so the femoral head stays put.

4. In anatomical position, which forearm bone is on the thumb side, and which dominates the elbow?

The radius is on the thumb (lateral) side; the ulna dominates the elbow. The ulna's olecranon forms the point of the elbow, while the radius does most of the articulating at the wrist and rotates during pronation and supination.

5. How many bones are in the hand, and how do they divide into three groups?

27 bones: 8 carpals (wrist) + 5 metacarpals (palm) + 14 phalanges (fingers). Each finger has three phalanges; the thumb has only two, which still totals 14 per hand.

6. A patient falls on an outstretched hand and has tenderness at the base of the thumb. Which carpal bone is most likely fractured, and why is it worrying?

The scaphoid. It is the most commonly fractured carpal, and because its blood supply enters from one end, a fracture can cut off blood to part of the bone and cause it to die (avascular necrosis).

7. Each hip bone is formed by the fusion of three bones meeting at one landmark. Name the three bones and the landmark.

The ilium, ischium, and pubis, fused at the acetabulum. The ilium is the broad upper part (its crest is the "hip"), the ischium is what you sit on, and the pubis meets its partner at the pubic symphysis in front.

8. Give two ways the female pelvis differs from the male pelvis, and explain why.

The female pelvis is wider and shallower with a larger, rounded inlet, and its subpubic angle is greater than 90°. These changes widen the birth canal for childbirth; the male pelvis is narrower with a heart-shaped inlet and a subpubic angle under 90°.

9. Which bone of the leg bears the body's weight, and which mostly provides stability? Which one forms the knee?

The tibia bears the weight and forms the knee; the fibula mainly stabilizes the ankle. The fibula does not even reach the knee joint — only the tibia articulates with the femur there. The two form the medial and lateral malleoli at the ankle.

10. What kind of bone is the patella, and what does it do for the knee?

It is the body's largest sesamoid bone — a bone that forms inside a tendon. Sitting in the quadriceps tendon, it acts as a pulley that improves the leverage of the thigh muscles when straightening the knee, and it shields the joint from blows.

11. Name the two tarsal bones that do the main weight-bearing work of the foot and the job of each.

The talus receives weight from the leg; the calcaneus (heel bone) takes weight at foot strike. The talus is the bone the tibia and fibula grip, and the calcaneus is the largest tarsal, forming the heel.

12. What structural feature makes the foot a shock absorber, and what is the condition called when it fails?

Its arches — the medial and lateral longitudinal arches and the transverse arch — and the failure is flat feet (pes planus). The arches flatten to absorb each step's impact and recoil to push off; when they collapse, the foot loses its spring and can cause fatigue and pain.

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