A muscle can do exactly one thing: pull. It cannot push, and it cannot lengthen itself. That single limit explains almost everything on this page — why you need a hard skeleton, why joints exist, and why muscles always come in pairs that work against each other.
📘 What you need to know
Movement needs muscle plus an incompressible skeleton for the muscle to pull against.
Bones and exoskeletons give anchorage for muscles and act as levers.
Endoskeletons are inside (vertebrates). Exoskeletons are outside (arthropods) and are made of chitin.
Every lever has an effort, a load and a fulcrum (the pivot). In the body the joint is the fulcrum.
Synovial joints have a cavity of synovial fluid made by the synovial membrane, which cuts friction.
Muscles only pull, so they work in antagonistic pairs — one contracts while the other relaxes.
The internal and external intercostal muscles are the pair you should be able to describe in detail.
Why a skeleton is needed at all
Imagine a muscle attached to a rubber tube. The muscle contracts, the tube squashes, and nothing moves anywhere. The muscle has wasted its pull.
Now attach it to something incompressible — something that will not squash or shorten. Every bit of the pull is turned into movement of that structure. That is the job of a skeleton:
It gives muscles a firm anchorage, so their pull goes somewhere useful.
It acts as a system of levers, changing the size and the direction of the force.
It supports the body, and in arthropods it also protects the soft tissue inside.
Endoskeleton and exoskeleton
Vertebrates have an endoskeleton: internal bones with the tissues wrapped around the outside.
Many other animals, especially arthropods, have an exoskeleton on the outside of the body. It is made of a polysaccharide called chitin.
Arthropods with exoskeletons include crustaceans, insects, arachnids, and centipedes and millipedes.
The leverage works the same way in both. Arthropods have jointed legs and jointed body parts, with muscles attached in antagonistic pairs — just anchored to the inside surface of the exoskeleton instead of the outside of a bone.
Easy way to keep them straight: endo = inside, like endoplasmic reticulum. Exo = outside, like exocytosis. Same prefixes you already know from cells.
Skeletons as levers
A lever is a rigid bar that turns around a pivot. Three things matter:
Effort (force) — where the push or pull is applied. In the body, this is where a muscle attaches by its tendon.
Load — what is being moved. Usually the weight of a body part plus anything it is carrying.
Fulcrum — the pivot point. In the body, this is the joint.
Which class of lever you have depends only on the order of those three along the bar.
A third class lever trades force for speed and range: the muscle pulls hard over a short distance, and the hand travels a long way.
Synovial joints
Synovial joints are the most common joints in the human body, and they are the ones built for movement.
There is a joint cavity filled with synovial fluid, which reduces friction between the moving surfaces.
The fluid is produced by the synovial membrane, which lines the joint.
Cartilage covers the ends of both bones, giving a smooth surface so bone never grinds on bone.
Ligaments hold bone to bone and stop the joint moving too far. They are tough connective tissue.
Tendons hold muscle to bone. They are flexible but do not stretch, so all the pull is passed straight to the bone.
The synovial membrane lines the inside of the capsule and makes the fluid that fills the cavity.
The movements a joint allows
Which movements are possible depends on the shape of the joint and on the ligaments holding it.
Flexion — bending, closing the angle at the joint.
Extension — straightening, opening the angle.
Rotation — turning around the long axis of the limb.
Add and abduct are the pair students always swap. Adduction adds the limb back to the body. Abduction takes it away, like an abduction.
The hip joint in detail
It is a ball and socket synovial joint. The head of the femur is the ball; the pelvis provides the socket.
Cartilage covers both bone surfaces to stop them rubbing.
Synovial fluid is sealed inside by a membrane and lubricates the joint.
Ligaments encircle the whole joint and hold the bones in place.
Skeletal muscles move the femur in the socket, and they are attached to each bone by tendons.
You do not need to name the individual muscles and ligaments of the hip. You do need to name the femur and the pelvis, and to say what cartilage, synovial fluid, ligaments and tendons each do.
Antagonistic muscles
There are over 600 skeletal muscles in the human body, and every one of them has the same limitation: it can contract or relax, but it cannot push. A relaxed muscle does not stretch itself back out.
So muscles are arranged in antagonistic pairs. They sit across the same joint and pull in opposite directions. When one contracts, it moves the bone and pulls its partner back out to full length, ready for next time.
Neither muscle can push the arm anywhere. Each one only undoes what the other did.
Both at once? Yes. If the two muscles of a pair contract together, the joint is held at a fixed angle and nothing moves. That is an isometric contraction — a contraction without motion, and it is how you hold posture.
The intercostal muscles: the pair to learn
The rib cage is moved by two sets of intercostal muscles working antagonistically. Their fibres run in different directions in the two layers, which is why they pull the ribs opposite ways.
🧩 Breathing in and out
Inhalation. The external intercostal muscles contract and pull the ribs up and out. This increases the volume of the thorax, so the pressure inside the lungs falls below the pressure outside, and air is drawn in.
Exhalation. The external intercostals relax and the ribs drop down and in. Volume decreases, so pressure inside rises above the outside, and air is forced out. Contraction of the internal intercostals pulls the rib cage down for a more forceful breath out.
There is a neat link back to the sarcomere here. As the external intercostals contract and expand the rib cage, the internal intercostals are stretched. That stretching stores potential energy in the titin proteins of their sarcomeres, which is released when they contract in turn.
The diaphragm also contracts and relaxes during breathing, but it is not part of an antagonistic pair — nothing pulls it back the other way. Examiners have asked exactly this.
Worked examples
WORKED EXAMPLE
Explain why skeletal muscles must be arranged in antagonistic pairs. [3]
Start with the limitationA muscle can only contract and pull. It cannot push, and it cannot lengthen itself.So one muscle alone would be stuck
It could bend the joint, but nothing would straighten it again.
Say what the partner doesThe second muscle pulls the bone back and stretches the first one out again“they work in opposite directions at the same joint” is the phrase to include.
WORKED EXAMPLE
Describe how contraction of the external intercostal muscles leads to air entering the lungs. [4]
Step 1: what the muscles do to the ribs
The external intercostals contract and pull the rib cage up and out.
Step 2: what that does to the space insideThe volume of the thorax increases.Step 3: what happens to pressurePressure in the lungs falls below the pressure of the air outside.Step 4: the resultAir flows down the pressure gradient into the lungsvolume up, pressure down, air in — never skip the middle step.
WORKED EXAMPLE
A person holds a heavy bag with the elbow bent at 90°. Identify the effort, the load and the fulcrum. [3]
Find the pivot first — it is always the jointFulcrum: the elbow joint.Then the muscle doing the pullingEffort: the force from the biceps, applied through its tendon to the forearm bone.Then what is being moved or heldLoad: the weight of the bag plus the forearm and hand.Effort between the fulcrum and the load, so this is a third class leverthe bag alone is not the whole load — the limb counts too.
💡 Exam tip
Use the word incompressible when you explain why a skeleton is needed. It is the word in the syllabus.
Ligament = bone to bone. Tendon = muscle to bone. Write it on your hand if you have to.
For breathing questions, always give the full chain: muscle → rib movement → volume → pressure → air flow.
Name the joint as the fulcrum in every lever question. It is a free mark.
Say “one contracts while the other relaxes” for antagonistic pairs — both halves are needed.
If a question mentions holding a position, the word you want is isometric.
⚠ Common mix-up
Saying a muscle pushes. It never does. This single error costs marks all over this topic.
Swapping ligaments and tendons. The most common slip in the whole unit.
Confusing abduction and adduction. Adduction adds the limb back to the body.
Calling the diaphragm part of an antagonistic pair. It is not.
Thinking the exoskeleton works differently. Same levers, same antagonistic pairs, just anchored from the outside in.
Forgetting that cartilage and synovial fluid do different jobs. Cartilage is the smooth surface; the fluid is the lubricant.
Saying pressure change causes the rib movement. It is the other way round — muscles move ribs, which changes volume, which changes pressure.
Up next: Movement of Joints — the practical skill of measuring how far a joint can actually move.
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