You might get some newer answers, but in the mean time you might like to read my old answer to a similar question.

The answer, in short, is that muscularity tends to be emphasized in art by the 16th century, but before that, different aspects of the body were the focus. Particularly, 15th century knights were often shown with very small waists and were generally quite thin. However, we should be aware that illustrations were not photographs, and art has connections to other art as much as it has a connection to reality, and so the depiction of knights as thin to the point of emaciation is likely a reflection of artistic shorthand - connecting knights to Christ's suffering, for instance, and emphasizing a virtue in simplicity and asceticism, even if that wasn't really a major thing for the knights in question - rather than a depiction of any particular knight's actual body.

All that said, knights were the product of a medieval training culture that valued fitness and dynamic athleticism, and young men were encouraged from an early age to dance, wrestle, fence, swim, ride, and climb, and to combine several of those skills in tourneys and hunting. My answer linked above also talks about this training culture, but I also go into more detail here

So, in short, the answer is likely quite fit, though that might not mean rippling muscles and visible abs like it tends to today. I'm happy to address any followup questions.

I know this is a history sub rather than a archeological one, but I do find it odd that that you don’t mention the osteopathic research in the matter. There are a lot of instances where research into medieval bones has shown that a lot of men-at-arms at the time was very robustly built, especially in the upper body (at least from the 14th century an onwards). I’m curious about your opinion in the matter?

There have been some great insights in other replies and to these we can also add some of the discussions of physical training and hardships found in some chivalric sources (here meaning sources written by or about knights). These two examples will come from a the 15th century, but earlier ones are available. A discussion of the physical descriptions of knights found in romance could also be considered for another perspective.

It has been briefly mentioned a few times in other replies, but the biography of Marshal Boucicaut (written c. 1409) provides a description of how he prepared himself for the 'pursuit of arms'. (Following extracts from The Chivalric Biography of Boucicaut, Jean II le Meingre, translated by Craig Taylor and Jane H.M. Taylor (Boydell, 2016), pp.30-31)

'So instead, he would train himself to leap fully armed onto his horse’s back, or on other occasions he would go for long runs on foot, to increase his strength and resistance, or he would train for hours with a battle-axe or a hammer to harden himself to armour and to exercise his arms and hands, so that he could easily raise his arms when fully armed.'

We get a little more later on: 'when he was at home, he would never tire of competing with the other squires in throwing a lance and other warlike exercises'

However, the text is more focused on what he was capable of doing rather the aesthetic results of such training. According to his biographer, Boucicaut was quite the athlete:

'he could do a somersault fully armed but for his bascinet'

'he could dance equipped in a coat of mail' (Whether he did full routines and chorus scenes, alas, the biographer does not say)

'he could leap fully armed onto his courser, without putting his foot in a stirrup'

'he could leap up from the ground onto the shoulders of a tall man mountedon a large horse, simply by grabbing the man’s sleeve in one hand'

'by placing one hand on the saddlebow of a great courser and the other between its ears, he could vault between his arms over the horse, holding its mane'

'if two plaster walls, the height of a tower, stood an arm’s width apart, hecould climb up them using just feet and hands, no other aid, and without falling'

'fully armed in a coat of mail, he could climb right to the top of the undersideof a scaling ladder leaning against a wall, simply swinging from rung to rung by histwo hands– or without the coat of mail, by one hand only'

There are no mean feats, so the marshal must have had some decent muscle beneath his mail. Of course we do have to remain a critical of our source. Boucicaut's biography was written as a 'highly detailed defence of the marshal's career, and in particular the decisions that he had made during his service as governor of Genoa' (Introduction, in Boucicaut trans. Taylor and Taylor, p.11). The author was unashamedly trying to portray Boucicaut as an ideal knight, potentially at his own behest, and outlining his physical capabilities only furthers this end. It seems likely that our author was guilty of some exaggeration, but there's almost definitely some truth to his descriptions.

As to his 'workout routine', his biography seems to suggest two things. Firstly, that he would train himself to do things which his profession (war) would require him to do. Secondly, that he did some training in armour, both to replicate a real fight and (intentionally or not) increasing the physical benefits of his exercise. The exact weight he was wearing, but a sleeved mail hauberk was about 12kg so this ought to be the minimum. Fully equipped, it might've been over 25kg.

As to his physique, a lot of damage has been done by modern media representations of what the 'peak male body' should look like. We should definitely be looking more towards professional athletes rather than bodybuilders. He must have been well muscled according to what he needed his body to do, but the reduced energy levels from having low percentages of body fat would be impractical for a man whose life was physically strenuous and made even more so by his own personal standards.

There's also been some talk of diet in this thread, and I'd like to briefly expand on this with another chivalric source: Le Jouvencel by Jean de Bueil and his accomplices (Le Jouvencel, translated by Craig Taylor and Jane H.M. Taylor, (Woodbridge, 2020). Le Jouvencel is a semi-fictional blend of romance and military treatise that was composed in the 1460s by one of the most veteran military men of the final stages Hundred Years War. It follows the career of a young man, Jouvencel, who climbs the military hierarchy because of his quality. Partially based on Bueil's own experiences over a 30+ year career of war, the text is especially valuable because it frequently gives a unique perspective on 'what it was really like' to be a soldier'. It is also an interesting read in its own right, so I highly recommend checking it out if you can. Le Jouvencel also comes with its own wagon train of complications, but it does give us another perspective on what the life of a man-at-arms, or knight, might have looked like.

For the purposes of this question, we get some particular insights on the various hardships of war.

Very early in the text, we are told that 'any man who wishes to win honour and glory via the pursuit of war must be patient in enduring the hardships and sufferings that are inescapable in war' (p.37). These might constitute: 'obliged to wear armour night and day, to eat poorly most of the time, and to exchange blows – this last hardship being the most alien' (p.38). Constantly being equipped for combat might have had some impact on the soldier's physique, but I think that 'eating poorly most of the time' would be far more aesthetically influential. Le Jouvencel elsewhere gives us descriptions of feasting, but this depiction of misery needs to also be weighed when we are discussing a knight's lifestyle. Building a goal physique with access to modern food sources is difficult enough and we need to remember that knights did not always enjoy the proper nutrition.

Towards the end of the text, we also get a joke about the physical importance of keeping men-at-arms properly employed. To set the scene, Jouvencel has been chosen to lead all the recently unemployed soldiers to a different place. The king and his advisors are discussing when they should launch this mission:

'‘True,’ said the chancellor, ‘they’re sitting around getting fat. If they don’t get moving, they’re going to be too fat for their armour, and you’ll have to equip them all with new gear!’ And the king replied: ‘At least it won’t cost you anything, chancellor!’ Everyone burst out laughing. (p.191)'

So we also need to consider that not every soldier was as diligent as Boucicaut in keeping himself in shape. We should note that Le Jouvencel is discussing men-at-arms rather than officially dubbed knights, and such men are far from ideal models of behaviour in the text. Regardless, this joke gives us another insight into how they might have looked. We might also (briefly) imply that these men might see their waistlines growing during peacetime because they were no longer burning the extra calories associated with warfare whilst also having a more reliable supply of food.

To conclude this little ramble through these two sources (others are available), the physical capabilities of a medieval knight undoubted would have varied based on an individual level. Boucicaut's biographer gives us one, probably exaggerated, extreme, whilst Le Jouvencel tells a rather different story. There should be little doubt that a diligent and well nourished knight may well have had a considerable physique, but these were definitely not universal. After all, knights were individual human beings beneath their armour and were similarly victims of circumstance.