Yes, it’s a pain like no other. You take the bike out for a spin for the first time in years and yowtch!!! Keep riding a few times per week though and in short order, there’s no more bike-ass problem.
What’s the underlying mechanism? Brusing makes little sense as there’s nothing to see and tolerance develops easily. Muscle aching doesn’t fit either as the pain begins to shortly after the ride and the aching area isn’t seeminly strained by sitting.