There is an article on the Daily Telegraph online website about the degree of fitness that was required for a medieval jouster, and comparing it with that of modern athletes.
It can be read in two versions at Study reveals jousters are as fit as today's top athletes and at What it takes to be a jouster, the fittest sportsman of them all
It confirms what I have always suspected, that the men who competed in the lists, Kings and princes, aristocrats and knights, were physically very fit indeed. That it was in part training for warfare - just as modern equestrian sports derive from military training in more recent centuries for the cavalry - as indeed was hunting, is true, but, as with the chase, some practitioners were keener and more committed than others, and may well have spent a great deal of time training and keeping physically fit. It was not a hobby you could just indulge in without considerable, and expensive, preparation.
King Henry IV was a noted jouster in the 1390s and McNiven argues in a well-known article which seeks to give an analysis of the King's health, and its decline from fairly soon after his accession, that the King may have suffered from being unable to maintain his customary level of fitness but put on weight and that like some modern athletes suffered health problems as a consequence of his enforced idleness of retirement from physical competition.
It has also been suggested that King Henry VIII's last years were shaped in part by the consequences of injuries he had received in the lists or conditions which developed as a result of jousting. Robert Hutchinson has written about this in The Last Days of Henry VIII:Conspiracies, Treason, and Heresy at the Court of the Dying Tyrant
That said, of course, both these English Kings came off better that King Henri II of France who died as a result of a joust in 1559. No amount of fitness can prepare you for something like his fate.