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Your muscles

How muscles pull, why they work in pairs, and what a pulled muscle really is.

3 min read Last updated 3 Nov 2026
A bent arm with a rounded biceps and a curved arrow showing it flexing.

The short version

A muscle can only pull, never push. It is made of bundles of fibres that slide over each other and shorten when a nerve signal arrives. Because muscles can only pull, they work in pairs: one bends a joint and the other straightens it again.

I always assumed muscles pushed my arm straight as well as bending it.

They can't. They only ever shorten.

So straightening your arm needs a second muscle.

Muscles pull, tendons connect

The muscles you move on purpose, such as those in your arms and legs, are called skeletal muscles. Each is attached to bones by tough cords called tendons. When the muscle shortens, it pulls on the bone, and the joint bends.

The biceps, at the front of your upper arm, pulls your forearm up. To lower it again, the triceps at the back shortens and pulls it down. While one works, the other relaxes. This is called a working pair.

Three types of muscle

Skeletal

You control it

Moves your bones, and tires with use.

Cardiac

Heart only

Beats on its own without tiring, for life.

Smooth

Works on its own

Lines the gut and blood vessels, and works without you thinking.

How a fibre shortens

A muscle fibre with sliding strands, showing a relaxed state above and a shortened state below

Inside every muscle fibre are long strands of two proteins, actin and myosin. Tiny parts of the myosin grab the actin and pull it along, then let go and grab again, rather like a row of people pulling in a rope hand over hand. The strands slide past each other and the fibre gets shorter.

This needs energy, which the muscle gets from the food you eat and the oxygen your blood brings, and some of it is released as heat. That is why you warm up when you exercise.

A nerve signal triggers each contraction. For more about that, read how your brain sends signals.

About 600

The approximate number of skeletal muscles in the human body. Counts vary depending on how they are divided.

What a pulled muscle is

A pulled muscle, also called a strain, is a small tear in the fibres, or in the tendon, caused by stretching too far or too suddenly. The torn fibres bleed a little and the area becomes sore and swollen. Severity varies. The body repairs the damaged tissue over time; recovery can take days or weeks depending on the injury.

Lactic acid causes the soreness you feel a day or two after exercise.

Lactate levels fall relatively quickly after exercise. Soreness that develops a day or two later is linked to changes in muscle after unfamiliar or strenuous activity, not lactic acid left behind.

How muscles get bigger

When muscles are challenged repeatedly, training can stimulate the existing fibres to grow. Adaptation depends on the exercise, recovery and nutrition; muscle damage is not required for progress.

Muscle gets bigger because the existing fibres get thicker, not because you grow lots of new fibres. And if you stop using a muscle, the fibres shrink again.

Note: this explains how it works in general. It is not advice for your situation. For a decision about your health, money or legal position, check an official source or ask a qualified professional.

Sources: NHS: sprains and strains; Dudley Group NHS: sprains and strains. Muscle counts are approximate and vary by definition.

Key takeaways

  • Muscles can only pull, never push.
  • Tendons attach muscles to bones.
  • Muscles work in opposing pairs.
  • Fibres shorten as protein strands slide.
  • A pulled muscle is a small tear.

Now you know.

Muscles only pull.

So they work in pairs.

And next-day soreness is tiny tears, not lactic acid.

Last reviewed 3 Nov 2026

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