Step by Step
A
The A band — stays the same length
The A band represents the full length of the myosin filament, and it does not change length during contraction — this is the key exception to remember, since almost everything else in the sarcomere does change.
I
The I band — shortens
The I band is the actin-only zone between A bands, and it shortens as the Z lines move closer together during contraction.
H
The H zone — disappears at full contraction
The H zone is the myosin-only region at the center of the A band (no actin present there at rest). As contraction proceeds, actin filaments slide inward and eventually fill this zone completely, causing it to disappear at full contraction.
Z
Z lines — move closer together
The Z lines, which mark the boundaries of each sarcomere, move closer together as the muscle contracts — this is what actually defines the sarcomere shortening overall, even though neither the actin nor myosin filaments themselves change length.
During full muscle contraction, the H zone (the myosin-only region) completely disappears as actin filaments slide inward and fill that space — while the A band itself, representing the full length of the myosin filament, remains exactly the same length throughout the entire contraction process.
Applied Walkthrough
1
A student is confused about how a sarcomere can shorten if neither the actin nor the myosin filaments themselves actually get shorter.
2
Ask: if the filaments don't shrink, what's actually happening? The filaments slide relative to each other — actin filaments slide further into the space between myosin filaments, without either filament type changing its own individual length at all.
3
This sliding action is exactly why the A band (representing myosin's fixed length) doesn't change during contraction, while the I band (pure actin territory) shrinks as more of the actin slides inward, and the H zone (myosin territory without actin) shrinks and eventually disappears as actin fills that space.
4
Understanding that contraction is fundamentally about filaments sliding past each other, not filaments physically shrinking, is the core insight of the sliding filament theory — and it's exactly why this concept is named the way it is.
Exam Application
Exams test which sarcomere zones change during contraction and how: A band (unchanged), I band (shortens), H zone (disappears at full contraction), and Z lines (move closer together) — along with the underlying principle that filaments slide past each other rather than physically shortening.
⚠ Common Trap
The most common trap is assuming the actin or myosin filaments themselves get shorter during contraction. They don't — the sarcomere shortens because the filaments slide relative to one another, which is precisely why it's called the sliding filament theory rather than a 'shrinking filament' model.
✓ Quick Self-Check
1. What happens to the A band during muscle contraction?
Nothing — it stays the same length, since it represents the fixed length of the myosin filament.
Tap to reveal / hide
2. What happens to the I band during contraction, and why?
It shortens, because it's the actin-only zone between A bands, and the Z lines move closer together.
Tap to reveal / hide
3. What happens to the H zone during full contraction, and why?
It disappears, because actin filaments slide inward and fill this previously myosin-only zone.
Tap to reveal / hide
4. Do the actin and myosin filaments themselves get shorter during contraction?
No — they slide relative to each other; neither filament actually changes its own length.
Tap to reveal / hide
5. What happens to the Z lines during contraction?
They move closer together, which is what defines the sarcomere shortening overall.
Tap to reveal / hide