You have those muscles after a long or hard ride that are beginning to tie up. Bruises sometimes but those muscles are seizing. By getting out for an easy ride for around 5 to 10 miles at a lower speed- lower gear and higher cadence- you get those muscles moving again and just the using of those muscles will ease the stiffness out of them.

Gives you time to reflect on the ride that has caused the problems and initially the thought will be never again but by the time the legs and back have got some life back into them you realise you have achieved something and these long rides are fun.

Incidentally it is not the same if it is the butt that has caused the problem. 10 miles standing up all the way will confirm the original thought