Metal Gear Solid 4 did a somewhat similar technique by having 2 normal maps, a wrinkled one and a relaxed one, a tension map was used to helped control the blending, but they might of also been baked into the animations as well? I’m not completely sure. Here’s an example of a tension map Maya Spiral: SOUP / Tension Map Basic and where the technique was mentioned CGSociety
You could also use the same technique to blend between a relaxed and flexed version of a normal map for muscles, tendons, (particularly hands).
I haven’t messed with trying to add those effects to a character myself, but all these techniques work similarly and probably could be implemented at the same time.