We can’t really answer the specifics of how much force is required. However, if you look up decapitations as capital punishment in Europe, you will find a host of information regarding how well a headsman could decapitate their victim. There’s a great deal of scientific data from the time on it and that’s worth a look. It’s also worth reminding everyone that the guillotine was invented as a method of execution not because it was more expedient (that was an unfortunate byproduct) but because it was considered to be quicker and more humane.
For reference, the average commoner (and occasionally noble) got the axe. Royalty (and occasionally nobles depending on country) could opt for the more “humane” sword. As a method of execution, swords were sharper and less likely to miss, so the death was quicker and cleaner. The headsman would often miss the first or even second swing with an axe. The axe was commonly blunt or carried a dull blade and would get stuck in the spine. The headsman might have to swing his axe a few times in order to completely remove the head from the body. It was both a terrifying and agonizing way to die.
The second part of your question relates to strength. We’ve talked a lot about upper body strength being less important compared to body mechanics. You don’t need to be a weightlifter to be an effective fighter (it is in fact less effective) and that is very true when it comes to weapons like the sword and the axe which rely heavily on momentum and a sharp edge over upper body strength.
Part of the reason this is a difficult question to answer is that there are multiple different kinds of swords and axes and they all go about decapitation in their own way. With an axe, is your character using a one handed axe or a two handed axe? A long hafted axe like the bearded axe that exists in a class similar to the claymore/zweihander (german, means two hands) will have no issues decapitating someone (assuming it’s wielder can wield it correctly) but will do so in an arcing pattern and come in on a diagonal instead of horizontally. A thrusting weapon like the rapier will drive forward on a direct line through the throat (and probably won’t bother with a decapitation because dead is dead). The longsword is better as a cutting weapon and could certainly go cleanly through the neck, provided it didn’t get caught on or deflected by the spine. It’s easier to aim between the vertebra than at them for a clean strike.
Many warriors may not choose to go fully through the neck at all and instead opt for a partial decapitation by going across the front of the neck through the wind pipe, esophagus, and carotid artery that are unprotected by bone.
Instead of focusing on physical strength, focus on how the weapon behaves by looking up the specific one your characters are using. It’s also worth noting that, for medieval warriors, it’s the armor that builds the body type. A heavy, bulky upper body will be common among warriors who wear plate mail because they must be able to fight while wearing it without become exhausted. This required a strong upper body and rigorous development of the shoulder muscles. This will also be true for both male and female warriors.
A warrior in lighter armor will develop a leaner body, but will be as effective at wielding their weapon because, again, greater physical strength is not what makes it effective.