In the category 'specific underwear' we find knee warmers. You wear them underneath your trousers, and their main function - obviously - is to keep your knees warm. And if you've ever been out on the road in cold conditions, or just travel a lot on a bike that doesn't have full fairing, you'll know why a knee warmer is useful.
Years of riding in winds and outdoor conditions, while your knees - joints after all - move little, it does leave its mark. Because prevention is better than cure.