Movement: skeleton and muscles · System
Antagonistic muscle pairs
Every muscle you own can do exactly one thing. So how does an arm come back?
Start here
A digger can push its arm out. You cannot.
A hydraulic ram does both jobs: pump oil in and it extends, let oil out and it retracts. A muscle has one move. It shortens, and it pulls. There is nothing it can do to push.
You push a door open, straightening your arm. What straightens it?
A second muscle, on the other side of the same bone, pulling the opposite way. The biceps cannot push your forearm out any more than a rope can push a bucket up a well. Everything you do twice — up and down, open and shut — needs two muscles.
Muscles are attached to bones by tendons, across a joint. When a muscle contracts it gets shorter and fatter, and the bone it is attached to has no choice but to move.
At the bench · drive the arm, then break it
Two muscles, one elbow
0 of 4 settings tried
Contract one, contract the other, contract both, contract neither. Then switch a muscle off and find out exactly what is lost.
Commit first. If both muscles contract hard at the same time, what does the arm do?
Contract
Switch a muscle off
Nothing is pulling. The arm is falling under its own weight.
Elbow angle
10°
Biceps · in front
relaxed
Triceps · behind
relaxed
What this tells you: Neither is pulling, so the arm falls under its own weight. Gravity will straighten a hanging arm for free — which is one reason the triceps does less work than you would expect.
Key fact
A muscle can only pull. Moving a bone back again needs a second muscle pulling the other way — an antagonistic pair.
Your turn · four movements
Which muscle is doing the work?
0 of 4 decided
In every pair, one muscle contracts and its partner relaxes. Decide which one is contracting. The last one is not what most people say.
Someone stands up from a chair. The knee straightens.
The quadriceps contract. They pull the lower leg straight while the hamstrings relax. Gravity is pulling the other way here, which is why standing up is work and sitting down is not.
Someone pulls their heel up towards their bottom, bending the knee.
The hamstrings contract. The same joint, the opposite muscle. Every pair is one muscle for each direction, and neither can do the other one’s job.
Someone rises onto their tiptoes.
The calf muscle contracts. It pulls upwards on the heel bone through the Achilles tendon, and the whole body pivots over the toes. The muscle at the front relaxes; its job is lifting the toes, which is the opposite movement.
Someone lowers a heavy box slowly to the floor, straightening their arms as it goes down.
The biceps — still contracting. This is the one most people get wrong. Gravity is doing the moving; the biceps is holding on and letting the box down a little at a time, getting longer while it pulls. Let go and the box drops. The triceps has nothing to do here at all.
Six words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“To straighten your arm, the triceps contracts and the biceps stretches itself back out.”
Half of that sentence is right. Commit to which half before you read on.
The triceps contracting is right. The biceps stretching itself is not: a muscle has no way of making itself longer. It can shorten, or it can stop shortening. What lengthens it is being pulled — by its partner, or by the weight of whatever it is holding.
This is why the word is relaxes, not stretches. Relaxing is a muscle letting go so that something else can lengthen it. If both muscles refuse to let go, the joint locks — you felt that on the bench.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which pair of muscles works the knee?
Rung 2 · The one that catches people
The biceps and the triceps both contract hard at the same time. What is the main effect?
Rung 3 · Explain
Explain why one muscle at the elbow could never be enough, however strong it was.
Rung 4 · Take it somewhere new
A nerve injury leaves someone with a working biceps and a triceps that will not contract at all. Describe what they can and cannot do with that arm, and how they could still get it straight.
Key note
Muscles pull and never push. They work in antagonistic pairs on either side of a joint: as one contracts, the other relaxes and is lengthened. Biceps and triceps at the elbow; quadriceps and hamstrings at the knee.
Going further
Contracting both muscles of a pair at once is not a mistake — it is how you hold something steady. A surgeon's hand, a gymnast on the rings and a hand steadying a phone camera are all doing it. The joint stiffens because both sides are pulling, so a small knock cannot move it. It costs energy and it tires you quickly, which is why you cannot hold a heavy tray at arm's length for long even though the tray is not going anywhere.
Before this lesson
Next in this unit
At GCSE this becomes
- Muscle fibres and how a contraction actually happens, and respiration supplying the energy for it.
Where to next
Ask Mr Badmus AI
Not sure why a muscle cannot push?
Lesson content © MrBadmusAI.