I wrote this as a response to a comment by /u/purplelzrd which was deleted rather more quickly than I thought it would be. I hope it is of some use to purplelzrd should they ever return to find out why:

I think the mods will be forced to nuke this comment when it comes to their attention but as it's your first post I thought I'd break it down as a demonstrative example of 'how we do' (as your answer is generally correct but not properly sourced or formatted).

IMHO

Historians don't (usually) have humble opinions, I'm afraid. We might have informed ones, or strong ones, but historians must back them up with evidence lest they become the dreaded 'misinformed' or 'misleading' opinion. So you are correct in your assumption, there were no organised gyms with weights available.

Being fit was a product of being physically active. They would participate in various activities like jousting or other chivalrous forms of exercise.

Again correct. However, you need to prove this - even when it should be obvious.

So what sources might you draw on? Firstly you might want to set a loose time period (I will discuss the fifteenth-century in England drawing upon three texts printed in the 1480s) to frame your discussion (you have already done so implicitly by using the word 'chivalrous' which excludes the period prior to the development of 'chivalry' - c.1100-1170 - depending on whose argument you follow). If you wanted to address the medieval period, or simply not focus on 'chivalric' nations - England, France, Germany, etc. - then you might have preferred the term 'martial'.

At any rate, it is fairly easy to answer the question of how a nobleman might have stayed fit if he was consistently riding, hunting, and fighting. You have also given an answer as to why he might want to:

nobles were more likely to have to defend their territory

Now you could back this up with a discussion of the development of landholding and military service in the medieval period (although that might be going a bit far!) but you should probably cite an example or two in support of your (again accurate) contention that only 'some nobles were likely to have to defend their territory'.

(If you have read any primary texts, or written papers in the past, you might want to draw upon those for evidence to illustrate your point.)

Training is a key concern of authors of chivalric manuals, though [Ramon] Lull explores this in its most basic terms, for example the knight teaches his son to ride and care for a horse, much as a carpenter or tailor learns his trade, something [William] Caxton reiterates in his epilogue, ‘thexercyse of a knyghte’ is, ‘[knowing] his hors / & his hors hym (...) an hors that is accordyng and broken after his hand / his armures and harnoys mete and syttyng’. However, Lull’s most innovative idea, one that had not been implemented in Caxton’s time, was that of the ‘school’ of chivalry and teaching from the ‘scyence’ held within books. [Christine de] Pizan expands on this, citing Vegetius’s ‘propre scoles’ and details the Classical forms for educating children in warfare. While this idea may not have become popular until the sixteenth and seventeenth century when the other tenets had taken root, such as that a squire should, ‘ryde thurgh dyuerse coútrees with the knyƷtes’. In the [Le] Morte Darthur Tristram spends his youth hunting – another essential part of the knight’s existence that aids martial training – and travels to France to, ‘lerne the langage, and nurture, and dedes of armes’ for over seven years in the company of a ‘gentylman’ named Gouernayle.

There was also a very pragmatic association between natural build and a knight’s success. Lull is dismissive of those, ‘lame / or ouer grete or ouer fatte /. On numerous occasions [Sir Thomas] Malory makes reference to size or hardiness as a determining factor of prowess, as well as one’s stamina. From this we can draw recognition of physicality as playing a role in determining prowess.

You can then draw upon wider knowledge combined with your illustrative examples to argue that perhaps it made sense for nobles and kings to remain in good shape even if they need not fight themselves. You could, if you were inclined, argue that within a culture that praised martial prowess as central to its 'code' demonstrating laziness could be a very easy way to lose respect. Even the blind King of Bohemia rode into battle (and died) at Crécy, and recent evidence from the exhumation of Richard III suggests that he did suffer from physical disability.

So being physically fit wasn't a big thing for them. It was an occupational thing.

Now you might have noticed I am disagreeing with you on this point. While it might have emerged from occupational necessity physical fitness and beauty were important cultural and social tools - especially for monarchs.

In [Le] Morte Darthur beauty and noble birth go hand-in-hand: ‘fayre’ Tor, an illegitimate son of King Pellinore, who is of ‘vyle’ blood as well as ‘kynges blood’, is permitted into Arthur’s order before he is tested, as a result of Arthur’s act of largesse on his wedding day, and it falls to Merlin to reveal he will be a ‘good’ knight: ‘for he is comen of as good a man as ony is on lyue’. His lineage is evidence of his worth, something foreshadowed by the comparison with his ‘poure’ brothers, ‘al were shapen moche lyke the poure man, but Tor was not lyke none of hem (...) for he was moche more than ony of hem.’

A fuller answer might look into how ecclesiastical sources - whether vitae ('lives') or hagiographies of saintly kings; chronicles which detailed the deaths of kings in battle; sermons on sloth and gluttony - might stress different values to those aimed at a 'chivalric' audience, or examine in greater depth what kind of training a martially inclined nobleman might do.

Good luck with the future posting - you're on track but you need to remain within the rule of the sub.

Bibliography:

• Ramon Lull, The Book of the Ordre of Chyualry: translated and printed by William Caxton, from a French version of Ramón Lull’s”Le libre del orde de cauayleria”, together with Adam Loutfut’s Scottish transcript (Harleian MS. 6149), ed. and intro. A.T.P. Byles, Early English Text Society, Original Series No. 168, London, 1926.

• Sir Thomas Malory, Caxton’s Malory: A New Edition of Sir Thomas Malory’s Le Morte Darthur, based on the Pierpont Copy of William Caxton’s Edition of 1485, ed. and intro. J. Spisak, 2 vol., California, 1983.

• Christine de Pizan, The Book of Fayttes of Armes and of Chyvalre, trans. William Caxton, ed. and intro. A.T.P. Byles, Early English Text Society, Original Series No. 189, Oxford, 1937.

• Juliet Barker, The Tournament in England: 1100-1400, Woodbridge, 1986.

• Maurice Keen, Chivalry, London, 1984. | ‘The Medieval English King and the Tournament: with Juliet Barker’, in Nobles, knights, and men-at-arms in the Middle Ages, London, 1996, 83-99.

• Richard Kaeuper, Chivalry and Violence in Medieval Europe, Oxford, 1999. | ‘Literature as Essential Evidence for Understanding Chivalry’, Journal of Military History, ed. C.J. Rogers; K. DeVries; and J. France, v.5 (2007), 1-15. | ‘The societal role of chivalry in romance’, in The Cambridge Companion to Medieval Romance, ed. R.L. Krueger, Cambridge, 2000, 97-114.

Well, to answer this we would have to define what you mean by "nobleman". If you're refering to knights as depicted in literature and movies the answer is "it's part of their lifestyle".

The training of a knight began at an early age. Typically around the age of 7 a young boy would be sent off to another noble family to serve as a page. Before that he would recieve some education in things such as literacy and etiquette, either from relatives or a hired teacher.

Pages
The page is more or less a servant to a knight or lord. He runs errands, he cleans weapons and armor, he may even help his lord get dressed. Though this may sound like something that is "below" the son of a noble it was not considered as such at the time. Serving as a page was a form of training in exchange for labour. In exchange for these services the page would be trained in horsemanship, combat, hunting and other skills needed for a young noble or knight (anything from singing and playing instruments to further training with reading and writing).

After about seven years of this, the page "graduates" to become a squire.

Squires
A squire is more formally a knight in training, their responsibilites are increased. A squire (especially in the later years of their training) may even follow a knight into battle. Their responsibilities would include anything from saddling horses and caring for weapons and armor to ensuring that the knight recieved a proper burial should he be slain in battle and carrying his colours.

In the early medieval period a squire would be dubbed a knight after the completion of his training. This practice ended when knighthoods came to be something that could only be conferred by the king or queen. At this time the title of squire came to be recognized in its own right.

As you can see, the training of a knight is a very physical thing. Riding, fighting and hunting are no easy activities. This kind of upbringing would ensure that most young knights of the early medieval period would be relatively fit and strong. For knights, jousting, training and hunting were popular activities which probably would have ensured that most remained fit even after their training was completed.

For most of the history of medieval Europe, being a noble was, to greater or lesser extent, synonymous with having marital responsibilities. Especially if they were inheritors, nobles would train from a fairly young age in horseback riding, swordsmanship, and other martial skills. A defining characteristic of the feudal system was the linkage of land ownership with military responsibilities—vassals were granted titles and lands on the stipulation that they would maintain a certain degree of military readiness and manpower at the command of the king. Popular noble pastimes like hunting could also be quite physical.

Conclusively sourcing this super-broad-strokes answer would be quite hard given the sheer breadth of geography and time being considered, but for the Carolingian Era at least you can check out The Carolingian World, by Costambeys, Innes, and MacLean, which is the source I happened to have sitting around and has a nice chapter on aristocratic life. It's also kinda-sorta general-knowledgey enough that hopefully minimal sourcing can fly.