Sliding filaments and the cross-bridge cycle !
- Mechanism
- Exam priority: high
Muscle shortens because tiny myosin heads grab actin, pull, let go and grab again, burning one ATP per stroke.
Updated 2026-11-01
Also called: Sliding filament theory, Actin–myosin cycle
See it in 3D: Biceps (long head)
What it is
A sarcomere shortens when thin (actin) filaments slide past thick (myosin) filaments; neither filament changes length. Each myosin head binds actin, performs a power stroke, releases when ATP binds and is re-cocked by splitting that ATP. Many heads cycling out of step produce smooth force for as long as Ca²⁺ keeps the binding sites uncovered.
Key facts
- Sarcomere = Z disc to Z disc; it is the contractile unit of striated muscle.
- Thin filaments: actin, tropomyosin and troponin (C binds Ca²⁺, I inhibits, T binds tropomyosin).
- Thick filaments: myosin II, with two heads per molecule that carry ATPase activity.
How it works, step by step
- Rest: a myosin head holds ADP + Pi in a cocked, high-energy position; tropomyosin blocks actin.
- Ca²⁺ binds troponin C, tropomyosin rolls aside and the head binds actin (cross-bridge forms).
- Pi release triggers the power stroke: the head pivots and pulls the thin filament toward the M line.
- ADP leaves; the head stays tightly bound to actin (rigor state).
- A new ATP binds the head and myosin lets go of actin.
- ATP hydrolysis re-cocks the head, ready for the next cycle while Ca²⁺ remains high.
Sarcomere numbers (skeletal muscle)
| Parameter | Value | Note |
|---|---|---|
| Optimal sarcomere length | ≈ 2.0–2.2 µm | maximum filament overlap |
| Thick filament length | ≈ 1.6 µm | |
| Thin filament length (each side) | ≈ 1 µm |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Duchenne muscular dystrophy: absent dystrophin; contraction tears the fragile membrane, CK rises and fibers are replaced by fat and fibrous tissue (calf pseudohypertrophy, Gowers sign).
- Hypertrophic cardiomyopathy often comes from variants in sarcomere proteins such as β-myosin heavy chain or myosin-binding protein C.
Test yourself
Think of the answer, then tap the question to see it.
Which sarcomere band keeps its length during contraction, and what does that prove?
The A band (the length of the thick filaments) stays constant while the I band and H zone narrow. Filaments slide past each other rather than shortening.
Why does rigor mortis develop after death, and why does it eventually resolve?
Without ATP, myosin heads cannot detach from actin, so muscles lock. Rigor fades later as proteolytic enzymes break down the filaments.
Which step of the cross-bridge cycle is directly triggered by ATP binding, and which by Pi release?
ATP binding releases myosin from actin. Pi release triggers the power stroke that pulls the thin filament.
Why does serum CK rise early in Duchenne muscular dystrophy, even before obvious weakness?
Without dystrophin the membrane is torn by each contraction, so CK leaks out of damaged fibers. Levels are very high in early childhood and fall as muscle is lost.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Skeletal muscle excitation–contraction coupling
- Length–tension and force–velocity relationships
- Skeletal, cardiac and smooth muscle compared
- Smooth muscle contraction
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 10.3 Muscle Fiber Contraction and Relaxation (OpenStax (Rice University), 2022); 10.2 Skeletal Muscle (OpenStax (Rice University), 2022); Duchenne and Becker muscular dystrophy: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 6). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.