I just came across this;

http://www.cantitoeroad.com/docs/con...ns_Stretch.pdf

I can't see whether the stretch was plastic or elastic. The length
doesn't appear to have been measured again with a 10kg load.

If it's plastic ... ouch! I hope it's not. I don't think it can be.

0.5% under 150 kg load. 39 tooth CR has a radius of about 79mm, and
175mm cranks gives a force multiplier of 2.2. A 75kg dude could easily
stretch a stretchy chain 0.5% just by standing on the pedals.

--
JS.