Advice on a "stretch" sensor

There is such a thing as a string potentiometer (a rotary potentiometer, turned by an unwinding string, and the string under tension from a rewind spring).

But, in this case, the range is so small that I think you could use a linear potentiometer with a spring preload. You might be able to adapt a slide pot used in audio balance controls.

The precision ones made for long term motion are kind of expensive:

Maybe you can come up with something based on variable overlap of two insulated plates to form a capacitor that varies as the wearer breaths.

Another idea:

Use black antistatic foam, the kind chips are pushed into to keep the legs at the same potential.

Put a few such foam pads between two metal plates, the plates can be unetched clean copper pcb material.

When this sensor is pressed together the resistance through the foam will decrease.

Tie a rope around the chest of the person under test, put the sensor between the rope and the chest.

When air is drawn in the chest is getting bigger, pressing the sensor harder together, the resistance between the plates is decreasing.

Such foam can be used for many other sensor tasks, like put it under a mat to indicate when people walk on it.

movements)

"Rope around their chest". I thought the Spanish Inquisition had ended?

Anyway... my idea is a strain guage.

Hi Luca Stretch sensors are sold by this company - (It is in the UK)