Sliding filament model of muscle contraction

Organisms (Form and function) · Muscle and motility · note 2 of 6

Spec B3.3.2

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Sliding filament model of muscle contractionSpec B3.3.2

In short

The sliding filament model explains muscle contraction: myosin heads bind to actin filaments, forming cross-bridges, and pull the actin towards the centre of the sarcomere using energy from ATP. The actin and myosin filaments slide past each other without changing length, so the sarcomere, and the whole muscle, shortens.

A skeletal muscle fibre contains many myofibrils. Each myofibril is a chain of repeating units called sarcomeres, separated by Z lines.

Thin actin filaments are attached to the Z lines and point towards the centre of the sarcomere.
Thick myosin filaments lie in the centre, overlapping the actin. Each has many myosin heads.
The dark band (A band) is the length of the myosin; the light bands (I bands) contain only actin; the H zone in the centre contains only myosin.

A nerve impulse causes calcium ions to be released from the sarcoplasmic reticulum. They bind to regulatory proteins on the actin, exposing binding sites for myosin.
Myosin heads, already energised by hydrolysing ATP, bind to the binding sites on actin, forming cross-bridges.
The heads swivel (the power stroke), pulling the actin filaments towards the centre of the sarcomere. ADP and phosphate are released.
A new ATP binds to each myosin head, which then detaches from the actin.
Hydrolysis of ATP re-cocks the head, ready to bind further along the actin. The cycle repeats while calcium is present.

The filaments slide past each other; neither changes length. During contraction the Z lines move closer together, the I band and H zone shorten, and the A band stays the same length.

Two sarcomeres, relaxed and contracted, showing Z lines, thin actin filaments attached to the Z lines and thick myosin filaments with heads, with the A band, I bands, H zone and sarcomere length marked; in the contracted sarcomere the Z lines are closer and the I bands and H zone are shorter while the A band is unchanged; three panels below show a myosin head binding to actin, swivelling in the power stroke and detaching when ATP binds. — Sliding filaments: the sarcomere shortens but the A band stays the same length, because actin slides past myosin.

Common mistake:

The filaments do not shorten or contract. They slide past each other, so only the sarcomere shortens.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

How does a sarcomere contract?

A sarcomere contracts when myosin heads bind to actin filaments, swivel and pull the actin towards the centre of the sarcomere, using energy from ATP. Repeated cycles of binding, pulling and detaching make the filaments slide past each other, so the Z lines move closer together and the sarcomere shortens.

Why do muscles work in antagonistic pairs?

Muscles work in antagonistic pairs because muscle tissue can only exert force when it contracts: it can pull but not push. One muscle moves a body part one way and the other moves it back, stretching the first. For example, the external and internal intercostal muscles move the ribcage in opposite directions.

What is the role of titin in muscle?

Titin is an immense, spring-like protein in each sarcomere. When the sarcomere is stretched, titin stores potential energy and then recoils, helping the sarcomere return to its resting length. It also prevents overstretching, which would pull the actin and myosin filaments apart.

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