They did have chairs and they did sit back in the middle ages. Knights would ride places on horseback rather than in a car, but still, seated.
Not to pick, but there's a marked difference between riding horseback and riding in a car...last time I rode I was sweating rather profusely- a combination of a fairly warm day (80s temp) and the fact that it is a bit of a workout, especially in the core (unless you ride like a sack of potatoes lol) and the inner thighs.
That's why they had to tie down anything that couldn't keep up on the horse- sacks of grain and prisoners included. 
Oh, absolutely, I agree with you. I was going towards most of the surviving cased greaves we have, being for people with very slim calves. We're not sure if that was because those muscles were under-developed from riding so much (not walking), or if they were somehow strong but slim. The cardio exercise you'd get from walking would be more than riding. Not to say they had it easy; just that it was easier riding than walking and that their muscles wouldn't be as developed as they would be if they walked everywhere. Like we joked at VARF - it's not so hard to be in full harness when the horse is moving the weight around. 
You've a point there...because fighting on foot wasn't something they would've practiced or indulged in had they had a choice. What about the upper body, have you noticed if they were especially large up top, Sir James?
