---
title: "The Sliding Filament Model of Muscle Contraction"
description: "Calcium released inside a fibre uncovers binding sites, and myosin heads pull actin filaments past each other in a repeating cycle that spends ATP. The filaments do not shorten; their overlap grows."
canonical: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8
source: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# The Sliding Filament Model of Muscle Contraction

Calcium released inside a fibre uncovers binding sites, and myosin heads pull actin filaments past each other in a repeating cycle that spends ATP. The filaments do not shorten; their overlap grows.

Subject: Science · Area: The Human Body · Ages 17 to 18
Page: https://lightmysky.com/learn/science/the-sliding-filament-model-of-muscle-contraction-mt_BsB4s4HfD8

## Ready when they can

- Explains what happens to each band of the sarcomere during contraction.
- Orders the steps of the cross-bridge cycle and says where ATP is used.
- Links calcium release to the uncovering of the myosin binding sites.

## Lesson: Ropes That Slide, Not Shrink

Thin actin slides past thick myosin, so overlap grows while neither filament shortens. Under the microscope the I bands and H zone shrink as the Z lines close in. The A band keeps its length because the myosin itself never shortens.

At rest, tropomyosin strands block the binding sites on actin. Calcium from the sarcoplasmic reticulum binds troponin, troponin tugs tropomyosin aside, and the sites open. The order runs from nerve signal to calcium flood through T tubules.

**Example.** Myosin heads then ratchet along: grab actin, pivot in the power stroke, detach, re-cock, and grab further on. ATP powers each reset and each detachment, and it also pumps calcium back so the fibre can relax.

**Tip.** No ATP means heads stay locked on, as in rigor mortis. That is why ATP is needed even to relax, not only to pull.

**Recap.** Calcium uncovers the sites, myosin heads ratchet actin inward, and the sarcomere shortens while the A band stays put.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Synaptic Transmission, Summation and Inhibition](https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B)
- [Protein Structure from Primary to Quaternary](https://lightmysky.com/learn/science/protein-structure-from-primary-to-quaternary-mt_bMdQS2lqSI)

## Opens up

- [Motor Control: From Intention to Muscle](https://lightmysky.com/learn/science/motor-control-from-intention-to-muscle-mt_hrWOuRtPDl)
- [Motor Proteins and Directed Movement Along Tracks](https://lightmysky.com/learn/science/motor-proteins-and-directed-movement-along-tracks-mt_TUTmILr7y0)