Bones, Muscles, and Joints
The frame, the engine, and the hinges
Everything an athlete does starts here: a skeleton that holds the shape, muscles that pull on it, and joints that decide how far it can go.
The big idea
Muscles cannot push. They only pull. Every movement you have ever made was a muscle shortening and dragging a bone along for the ride.
The skeleton is scaffolding, not stone
An adult skeleton has 206 bones. A newborn starts with closer to 270 — extra pieces of cartilage and bone that fuse together through childhood. That is why the skull of a baby has soft spots and the skull of a teenager does not.
Bone is living tissue with a blood supply. It remodels itself in response to the loads you put on it, which is why weight-bearing sport builds denser bone and why a limb in a cast loses bone density in a matter of weeks.
Bones do four jobs at once: they give the body shape, they protect organs (the skull around the brain, the ribs around the heart and lungs), they act as levers for muscles to pull on, and their marrow manufactures blood cells.
- Long bones
- Longer than they are wide — the femur, humerus, and phalanges. Built for leverage and speed.
- Short bones
- Roughly cube-shaped, like the carpals of the wrist. Built for stability with small amounts of motion.
- Flat bones
- Thin and often curved: the sternum, ribs, scapula, and bones of the cranium. Built for protection and muscle attachment.
- Irregular bones
- Complicated shapes that fit nowhere else, such as the vertebrae and the bones of the face.
- Sesamoid bones
- Bones embedded inside a tendon. The patella (kneecap) is the largest one in the body.
Three kinds of muscle, one of which you control
Skeletal muscle is the striated, voluntary tissue attached to your bones — over 600 of them, and the only muscle type you consciously command. Cardiac muscle builds the heart wall and contracts on its own rhythm without a single instruction from you. Smooth muscle lines hollow organs and blood vessels, moving food through the gut and adjusting how much blood reaches your working legs.
Skeletal muscles work in pairs. When the biceps brachii shortens to bend the elbow, it is the agonist (the prime mover) and the triceps brachii is the antagonist, lengthening under control on the other side. Reverse the movement and the roles swap. Nothing in the body works alone.
A muscle attaches to bone through a tendon — tough, rope-like connective tissue built to transmit force. A ligament is a different structure entirely: it connects bone to bone and keeps a joint from traveling where it should not go. Students mix these two up constantly, and telling them apart is the single most useful thing on this page.
Joints decide what is possible
A joint is any place two bones meet. Some barely move at all: the fibrous sutures of the skull are locked together on purpose. Cartilaginous joints, like the discs between vertebrae, allow a small amount of give — stack thirty-three of them and a spine can curl.
Synovial joints are the athletic ones. They have a capsule, a slick layer of articular cartilage on the bone ends, and synovial fluid that lubricates the surfaces. Every joint you think of when you think of sport is synovial.
The shape of a synovial joint sets its rules. A hinge joint like the elbow flexes and extends in one plane. A ball-and-socket joint like the hip or shoulder moves in every direction — which is exactly why the shoulder is the most mobile joint in the body and also the most frequently dislocated. Mobility and stability trade against each other, always.
- Hinge
- Elbow, knee, ankle — motion in one plane, like a door.
- Ball-and-socket
- Shoulder and hip — motion in all three planes.
- Pivot
- The atlas and axis at the top of the neck, letting you shake your head "no."
- Saddle
- The base of the thumb, which is why your thumb can oppose your fingers and a dog's cannot.
- Condyloid
- The wrist — flexion, extension, and side-to-side, but no true rotation.
- Gliding
- Flat surfaces sliding past one another, as in the carpals and tarsals.
See it in the book
These pages of the Sports Medicine Coloring Book draw out what you just read. Tap any one to look closer.
Terms from this lesson
- Tendon
- Connects muscle to bone.
- Ligament
- Connects bone to bone.
- Origin
- The muscle attachment on the bone that stays relatively fixed.
- Insertion
- The muscle attachment on the bone that moves.
- Agonist
- The muscle primarily responsible for a movement.
- Antagonist
- The muscle on the opposite side that lengthens under control.
Check yourself
Answer out loud before you open the card. Recall is what makes it stick.
Classroom activity
Build a working elbow
Give each pair of students two paint stirrers (bones), a brad or paper fastener (the hinge joint), and two rubber bands (biceps and triceps). Attach the bands so that pulling one flexes the model and pulling the other extends it. Ask them to make the model hold a position halfway — they will discover that both bands have to be under tension at once, which is co-contraction, and it is exactly how you stabilize a joint in real life.