What happens to the stitch when fabric is stretched

Short answer: The loop becomes long and narrow in the direction of the pull and draws yarn from its neighbours. When you let go, the yarn's own tension presses the loop back into its shape. That is why a knit can give so much without breaking.

The fabric is built from loops

A knitted fabric is made of rows of interlocked loops. Each loop is held by those around it, and they can move against each other. This structure makes jersey soft and elastic, and it sets it apart from a woven fabric, where the threads cross one another.

How the loop changes shape

When you pull the fabric, the loop becomes long and narrow along the pull. It borrows yarn from the neighbouring loops, which become slightly smaller. It looks as if the fabric grows longer, but in reality the yarn only moves around between the loops.

The yarn moves, it does not grow

The fibre itself stretches only a few percent. Most of the fabric stretch comes from the loops changing shape and borrowing yarn from each other. That is why a knit can give far more than the thread it is made from, and why it comes back afterwards.

Why the fabric goes back

When you let go, the yarn's own tension presses the loop back toward its resting shape. The loops pull the yarn back from each other, and the fabric lies as it did before. This happens by itself, as long as the fabric has not been stretched too far. See bamboo T-shirts for men.

Why the fabric gets thinner

When the loops are pulled long, there is less yarn per area and more light passes through. That is why a tight garment can look more see-through, especially where it fits closest. If unsure how close a piece should fit, look at bamboo jogging suits for men.

The limit of what comes back

If the fabric is stretched past the point where the loops are fully straight, the fibre itself is strained. Then it breaks, and the shape does not return. Many jersey fabrics stretch a good deal across, but the limit differs. See bamboo socks for men.

Why an old garment does not go back

Over time the elastane breaks down, and single fibres in the loops may have snapped. Then the fabric lacks what pulled the loops back, and it stays in its stretched shape. A garment that has gone slack has often been stretched too far or washed too hot.

Stretch is not harmful in itself

Stretch does no harm as long as it stays within what the fabric was made for. It is when it is repeated right to the limit that the fabric tires. Do not pull the garment to make it bigger, and put it on with calm movements, so the loops are strained least.

How to protect the shape of a knit

Wash gently and let the garment dry flat, so the weight does not pull the loops long while the fabric is wet. Do not hang heavy wet pieces on a hanger. Fold them and lay them flat instead. That way the fabric keeps its shape for longer.