A muscle gets shorter, but nothing inside it gets shorter. That sounds like nonsense until you see what is really going on: two sets of filaments slide past each other, like two hands with interlocking fingers being pushed together. Get that picture in your head and the whole topic falls into place.
📘 What you need to know
Muscle fibres contain myofibrils, which are made of thick filaments (myosin) and thin filaments (actin).
A sarcomere is the repeating unit between two Z lines. It is the bit that shortens.
Myosin heads form cross-bridges with the actin, then bend — the power stroke — dragging the thin filaments inwards.
ATP is needed to break the cross-bridge and to reset the head, not to make the stroke itself.
During contraction the I band and H band get shorter. The A band stays the same, because the filaments themselves do not change length.
Titin acts as a spring: it stops overstretching and gives a little energy back on the way in.
Starting big: what a muscle is made of
Work down through the layers and it stops feeling complicated:
A skeletal muscle is attached to bone and pulls on it.
The muscle is bundles of muscle fibres. These look stripy under a microscope, which is why skeletal muscle is called striated.
Each fibre is packed with myofibrils — the bundles of protein filaments that do the pulling.
A muscle fibre is an odd thing. It has many nuclei, which is why we usually say “fibre” rather than “cell”. Its cytoplasm has a special name, the sarcoplasm, and it is stuffed with mitochondria because contraction burns through ATP. Its endoplasmic reticulum also has a special name, the sarcoplasmic reticulum, and its job is to store calcium ions and release them everywhere at once when the signal arrives.
Every “sarco-” word just means “to do with muscle”. Sarcoplasm = muscle cytoplasm. Sarcoplasmic reticulum = muscle ER. Sarcomere = the muscle unit. One prefix, four marks.
The bands and lines of a sarcomere
The stripes come from how the two filaments are stacked. Learn these six names and the diagrams stop being scary.
Part
What is there
Z line
Where the thin (actin) filaments are anchored. It is really a disc, so you may see “Z disc”.
M line
Where the thick (myosin) filaments are anchored, right in the middle.
I band
Thin filaments only. This is the pale stripe.
A band
The full width of the thick filaments, including the parts where thin filaments overlap them.
H band
The zone in the middle with thick filaments only, no overlap.
Sarcomere
Everything between one Z line and the next.
Pale stripe, dark stripe. The I band is pale because only thin filaments sit there; the A band is dark because the thick filaments are in the way.
The two filaments up close
Thick filament: myosin
Myosin is a fibrous protein with a globular head.
The long fibrous tail anchors the molecule into the thick filament.
Many myosin molecules lie side by side, and all the heads point away from the M line — so both halves pull inwards, towards the centre.
Thin filament: actin
Actin is a globular protein. Many actin molecules join into a chain, and two chains twist together to make one thin filament.
Tropomyosin, a fibrous protein, is wound around the two actin chains.
Troponin is attached to the chains at regular intervals.
At rest, tropomyosin lies across the binding sites so myosin cannot grab them. Calcium ions change that.
Why the heads point outwards from the middle matters. If every head on both sides pulls its thin filament towards the M line, both Z lines are dragged inwards at once and the sarcomere shortens from both ends.
The sliding filament model
Here is the key idea, and it is worth writing out until it is automatic.
The sliding filament model
The filaments do not shorten — they slide past each other, so each sarcomere shortens and the Z lines are pulled closer together.
If an exam diagram shows the A band changing width, the diagram is wrong. That is a favourite “spot the error” question.
The cross-bridge cycle, step by step
🧩 What one myosin head does, over and over
Bind. Calcium ions released from the sarcoplasmic reticulum make tropomyosin shift, uncovering the binding sites on actin. The myosin head attaches, forming a cross-bridge.
Pull. The head changes angle and drags the thin filament towards the centre of the sarcomere. This is the power stroke.
Let go. An ATP molecule binds to the head, which makes it release the actin.
Reset. The ATP is hydrolysed, and the energy flips the head back to its starting angle. It now binds further along the actin, closer to the Z disc, and the cycle repeats.
Note where ATP is used: to let go and to reset. The power stroke itself runs on energy already stored in the head from the previous cycle.
Students often say “ATP powers the power stroke”. Examiners are fussy here. ATP binding breaks the cross-bridge, and ATP hydrolysis re-cocks the head. Say it that way and you cannot lose the mark.
Titin: the spring in the sarcomere
Titin is a huge protein joining the end of each thick filament to the Z line. Its many folds make it behave like a spring, and it does two useful jobs:
When the muscle is relaxed and stretched, titin is pulled out and stores chemical energy. It also stops the sarcomere overstretching, so the filaments never get pulled fully apart.
When the muscle contracts, titin recoils and releases that stored energy, which adds to the force of the contraction.
Worked examples
WORKED EXAMPLE
A sarcomere is 2.4 µm long when relaxed and 1.8 µm long when contracted. Calculate the percentage decrease in length. [2]
Step 1: find the change2.4 µm − 1.8 µm = 0.6 µmStep 2: divide by the original, then multiply by 100(0.6 ÷ 2.4) × 100 = 2525% decreasealways divide by the starting value, not the final one.
WORKED EXAMPLE
Explain what happens to the A band, I band and H band when a muscle contracts. [3]
Start from what does not changeA band: no change — it is set by the length of the thick filaments, and filaments do not shorten.Then the two that doI band: shorter, because the thin filaments slide in over the thick ones.H band: shorter, because the thin filaments now reach further into the middle.A band unchanged; I band and H band both narrowerthe reason is worth as much as the answer here — write both.
WORKED EXAMPLE
A muscle fibre runs out of ATP. Suggest why it stays stiff instead of relaxing. [2]
Ask what ATP was needed for
ATP has to bind to the myosin head before the head will let go of the actin.
So with no ATP…The cross-bridges cannot break, so the heads stay attached to the thin filaments.The filaments are locked together, so the muscle stays rigidthis is the biology behind rigor mortis — a nice line to add.
💡 Exam tip
Use the phrase “the filaments slide past each other, they do not shorten”. It is the single most rewarded sentence in this topic.
Give the order: calcium released, tropomyosin moves, binding sites exposed, cross-bridge forms, power stroke, ATP binds, head detaches, ATP hydrolysed, head resets.
Name the protein doing each job. “Something covers the site” scores nothing; “tropomyosin covers the site” scores.
For band questions, always justify with filament length — that is the reasoning the mark scheme wants.
Mention mitochondria and sarcoplasmic reticulum if a question asks how a fibre is adapted for contraction.
If a question mentions force of contraction, titin is a fair thing to bring in.
⚠ Common mix-up
Saying filaments shorten or contract. They slide. Nothing in the myofibril gets shorter except the sarcomere itself.
Swapping actin and myosin. Thin = actin. Thick = myosin. Use that spelling trick.
Saying the A band shortens. It never does. Common error, easy mark lost.
Mixing up tropomyosin and troponin. Tropomyosin is the long strand lying over the binding sites; troponin is the smaller protein attached at intervals.
Claiming ATP drives the power stroke. ATP binding detaches the head; hydrolysis resets it.
Calling a muscle fibre a cell. It has many nuclei, so we say fibre.
Forgetting calcium. Without calcium ions from the sarcoplasmic reticulum, nothing starts.
Up next: Skeletal Muscle — where the signal to contract comes from, and how the body controls how hard a muscle pulls.
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