In relation to tiring from swinging a sword for hours, knights, if we are to focus on them, trained from the time they were a child. All knights ever did was fight. Their role in feudal society was officially 'those who fight.' They trained in the use of swords, lances, maces, hafted-axes, bows and also practiced wrestling. Considering that all knights ever did was fight and train, we have to assume that the majority of knights became HIGHLY proficient in the use of melee weaponry. As well as developing enormous strength through continual use of arms and from wearing large amounts of armour for the most part when they were practicing, knights would have developed techniques for conserving energy with each swing whilst still maintain enough power to inflict damage. As well as building up muscle strength through continuous use of arms, their muscles would have adapted to the various movements they performed when wielding their weapons. Muscle strength alone does not determine how effectively one can swing a sword - rather it is the combination of muscle strength and adaptation of muscles in performing certain movements which allows for muscular power and endurance, giving one the ability to land fatal blow after blow for an extended period of time. Therefore, since no one here has undergone the rigorous training regime of the knight, no one can safely assume how hard or easy it was exactly for a knight to endure in battle.
There are many historical accounts of the ability of knights in battle; including cleaving men and their horses in two and generally being able to hit with such force they could penetrate armour as if it wasn't even protecting the victim.