Slow-Twitch Muscle Fibers (Type I)

A long, dark red muscle cell

The muscles you move on purpose are made of very long cells called muscle fibers. Slow-twitch fibers are one kind.

They look dark red. That is because they hold lots of myoglobin (MY-oh-glow-bin), a red protein that stores oxygen. Many tiny blood tubes wrap around them to bring even more oxygen.

• They are packed with mitochondria, the cell's power makers.
• Most of your muscles have a mix of slow and fast fibers.

Working all day long

Slow-twitch fibers use oxygen to burn their fuel slowly and steadily. That means they hardly get tired.

They help you stand, sit up straight, walk, and swim or bike for a long time. Muscles that hold up your back and neck have lots of them.

Slow-twitch fiber quiz