Physiology
Muscle contraction: the sliding filament theory
Muscles don't shorten because the filaments shorten — they shorten because the filaments slide past one another. That distinction is the whole theory.
Sarcomere anatomy
The sarcomere is the functional unit of a muscle fibre, running from one Z-line to the next.
| Region | Contains | During contraction |
|---|---|---|
| Z-line | Anchors thin filaments | Z-lines move closer together |
| I band | Thin (actin) only | Shortens |
| A band | Full thick (myosin) length | Stays the same |
| H zone | Thick only, no overlap | Shortens |
The classic exam question
Which band does not change length during contraction? The A band. It corresponds to the full length of the myosin filament, and myosin does not shorten — the actin simply slides further over it.
The cross-bridge cycle
- A nerve impulse triggers calcium release from the sarcoplasmic reticulum.
- Ca2+ binds troponin, which shifts tropomyosin off the myosin-binding sites on actin.
- The myosin head binds actin, forming a cross-bridge.
- The power stroke: the head pivots, pulling actin inward, and ADP + Pi are released.
- A new ATP binds myosin, causing it to detach from actin.
- ATP is hydrolysed, re-cocking the head, and the cycle repeats while calcium remains elevated.
ATP has two separate jobs here
ATP binding causes detachment; ATP hydrolysis re-cocks the head. This is why rigor mortis occurs — with no ATP after death, myosin heads cannot detach from actin, and the muscle locks in place.
Fibre types
Which fibres a muscle recruits shapes its fuel demands — slow-twitch fibres rely heavily on oxidative metabolism, fast-twitch fibres on glycolysis and phosphocreatine. See muscle fibre types and nutrition and the three energy systems.
Frequently asked questions
Which sarcomere band stays the same length during contraction?
The A band. It corresponds to the full length of the myosin filament, which does not shorten. The I band and H zone both shorten as actin slides over myosin.
What is the role of calcium in muscle contraction?
Calcium binds troponin, which moves tropomyosin away from the myosin-binding sites on actin, allowing cross-bridges to form. Without calcium, those binding sites remain blocked.
Why does rigor mortis occur?
ATP is required for myosin heads to detach from actin. After death, ATP production stops, so cross-bridges cannot release and the muscle remains locked in a contracted state.
Keep reading: Muscle fibre types · The three energy systems · What is ATP?