Sometimes I think the hardest thing about fencing is not what you must do, but everything you must NOT do. Body parts that need not to move. The need not to think, but to feel.
Take your shoulders, for example.
Shoulders are perhaps one of the friendliest parts of the body. They shake when you laugh loud and hearty, they're available for a friend to lean on, and for some reason known only to them, they want to take on the stress of everything else in your body- perhaps the origin of the phrase "to shoulder your grief," or when someone has "the weight of the world on his shoulders."
Sometimes, this is a good, helpful thing.
Not so much in fencing.
Controlling the point of the sword requires very small, very precise movements. That requires the coordination of some very small muscles, muscles you don't even know you have.
It might seem logical to suppose that using small muscles would require less energy than using large muscles. That might be true, it IS true, once you are able to use only those muscles required, but that ability takes a long time to acquire. Until then, not only do you have to move the right muscles, you have to NOT MOVE anything else.
But your shoulders want to help you. Those tiny muscles fatigue easily, and as soon as they do, the surrounding larger muscles step in to help out. First muscles in your forearm, and then your biceps and triceps, and very soon, your shoulders.
You may be able to easily see that large muscles, like those in your shoulder, simply cannot have the required precision of the smaller muscles in your hand. If not, take my word for it for now. Large muscles are for "gross motor skills," small muscles are for "fine motor skills." In plain English, this means that when your shoulder attempts to "help," what mostly happens is everything is thrown haywire, and your point ends up somewhere in Toledo. That, and all that large muscle movement burns a lot of calories, and wears you out quickly.
Not helpful. Good intentions of your shoulders notwithstanding.
Your shoulders like to help in other ways, too.
For instance, when you lunge, you are attempting to move the point of your sword closer to your opponent's body, and to do so in a way that preserves your aim, so you hit what you are aiming at. In order to do that, you must keep your upper body in the same alignment, from the beginning through the end of the move.
But lunging, especially when you are first learning, requires a significant expenditure of energy. Again, this means you get tired quickly. It also requires a high level of coordination between a large number of body parts, and it requires some flexibility, both of which must be acquired through practice. Lots of practice.
Most people do not enjoy the feeling of muscle fatigue. Most people refer to that sensation as "pain." Most people avoid pain.
So most people, after one or two lunges, will start to shorten their lunge, in an attempt to avoid what they expect to be a painful experience.
When they do that, they suddenly fall short of their intended target. And when they see that they will fall short, their shoulders step up to the plate and say "Aha! I can fix this! I'll save you!" and the shoulder of their swordarm will extend forward in an attempt to close the distance that their feet are suddenly not taking.
Remember what I just said, about maintaining your aim by maintaining the alignment of your upper body?
Your shoulder does not give a damn about this. You can argue with it for a very long time, trying to convince it to stay still, and it will not believe you. (Yes, I talk to my body parts, doesn't everyone?) Shoulders are not easily convinced that they are not, regardless of their good intentions, your savior. They will rant. They will insist. And they will keep trying, and keep trying, to bail you out.
And they will get tired. Very tired.
Be kind to your shoulders. Do not ask them to do what your feet should be taking care of. Do not ask them to take up the slack for your fatigued hand muscles.
Practice a new movement only until you begin to fatigue, and then rest. Be precise in your practice. Go slow. Learn the pattern of the movement. Speed is unimportant.
"Most people practice something until they can get it right. A master practices until he can not get it wrong."