Fast and Steady Muscle

Muscles contain fibres that fire fast but tire quickly, and fibres that fire slowly but last a long time. The kind of training you repeat changes which type your muscles rely on more.

This picture is part of the full library

Every lesson is open to everyone for 7 days after it is published. This one came out on September 12, 2026: its text stays readable here, and its picture opens with a subscription.

The app is free. A subscription opens every lesson, in every school year.

Age 13: open this week

In the picture

Badge 1 sits on the sprinter’s leading thigh.
Badge 2 sits on a pale elongated fibre in the left circular inset.
Badge 3 sits on the distance runner’s rear calf.
Badge 4 sits on a dark elongated fibre in the right circular inset.

Examples

Sprinting 100 metres uses quick fibres that tire fast.
Jogging for 30 minutes uses slow fibres that keep going.

Key takeaways

Repeated training shifts the balance toward the fibres you use most.
Fast fibres burst and tire quickly; slow fibres endure.

Carry on in the app

Three questions on this lesson. It has left the free week, so its quiz is for subscribers.

At least one new illustrated lesson every weekday for your child’s year, with its quiz and a reminder at 6:15 pm.

Free app, optional subscription
App Store Google Play