If this is too broad, let's say when plate armour was at the peak of it's usage (a quick wiki says Late Middle?). Would there be a focus on strength? Speed? Would nutrition at the time cause limitations?
It would be hard to give a solid answer. Modern Athlete's are surrounded by teams of people who monitor them and modify their routines based on their current needs and expenditures. It's actually hard for a n amateur athlete to compare with the physical fitness of a professional athlete.
I think the difficulty is the idea that a 'tournament knight' is that they are 'athletes' rather than warriors. /u/FuriousJester is close to pointing this out, but instead rather misses it by comparison to modern professionals and modern amateurs.
In the medieval context these certainly were professionals, not amateurs. They were professional warriors. This means, for reasons I set out here, it is almost impossible to discern between what was valued in the tournament and what was valued in warfare. There was overlap, true, but there were also significant divergences (especially in the later Middle Ages).
Having good 'breve' (wind or stamina) is an attribute cited as important to the medieval knight, although this is considered just as applicable in the twelfth- as fifteenth-century. A general sense of 'hardiness' underlay almost all chivalric ideals of fitness. Don't be fat, don't be gluttonous, be strong, well-winded, and skillful. I can't think of any of these attributes which, despite their generic nature, would be useless in either a tournament 'melee', joust, or battle. Does this mean that the 'tournament' knight had a different build to any other 'non-tournament' knight? It seems unlikely, but it is essentially impossible to prove either way.