A female flexes her arms during a workout session, facing away from the camera

How do muscles contract?

Minuscule motors are behind muscle contractions

Muscle fibres contain protein molecular motors. These are nanoscale machines (around one-millionth of a millimetre in size) that perform several tasks inside all cells, such as transporting material or moving chromosomes during cell division. All work on the same basic principle: a protein molecule binds to and hydrolyses the energy-rich chemical adenosine triphosphate (ATP). The energy released is used to change the protein’s shape. Repeat the cycle and you have a regular movement.

A muscle cell (fibre) is made up of myofibrils, bundles of protein that include actin and myosin. Muscle movement is powered by a change in shape of the myosin. The myosin head binds to actin and breaks down ATP. This releases energy that pulls the actin filaments along. Myosin then disconnects. The muscle contracts, not because the filaments shorten, but as actin and myosin filaments slide past one another.

In skeletal muscle, the contraction is started by a rise in the concentration of calcium ions, triggered by the arrival of a neurotransmitter (a chemical signal) from the nerve ending that attaches to each muscle cell at the neuromuscular junction. The calcium ions, in turn, interact with two other proteins, troponin and tropomyosin. These two proteins change shape to allow myosin to bind to actin. The myosin heads tilt, which pulls up the actin and causes the muscle to contract. Our sliding filament theory animation explains this in further detail.

Lead image:

coba/Flickr CC BY NC ND


About this resource

This resource was first published in ‘Exercise, Energy and Movement’ in January 2012 and reviewed and updated in August 2016.

Cell biology, Physiology
Exercise, Energy and Movement
Education levels:
14–16, 16–19, Continuing professional development