This is a fun material you could use to detect if a band was stretched or not, silver coated elastic [0], near 0 ohm resistance when loose, resistance increases when stretched. I built a voltage divider with a patch of it when I was experimenting with fabric input devices, mostly just noise makers, but you can see how responsive it is [1]

[0] https://lessemf.com/product/stretch-conductive-fabric/

[1] https://youtu.be/Xjo4w4OiBS8