Startup Organic Robotics Corporation is using fibre optics, which could be woven into a garment, to measure muscle fatigue in athletes. This appears early work and it is not entirely clear what physical phenomena is being used for measurement, but it could be something like reduced light intensity as the fibre is stretched or bent. THe technology has previously be used to give robot some sense of touch.
Fibre optics have previously been used in garments for aesthetic purposes, allowing strips of light that can change colour through a source LED (see Met gala dress and TwentyFour 15 collection ). The light output is generally low, making it an effect that is practically only usable in the near dark. In this case the aesthetic is not the issue, rather it is a sensor for movement and pressure.
On their website there are a couple of introductory videos
Source: Organic Robotics Corporation