Why does my weapon arm get so tired during bouts, and how do I stop it dropping?
Start with technique, not strength: most arms that give out mid-bout are working far harder than they need to. A tense shoulder, a white-knuckle grip and an arm held high and stiff in front of you will exhaust almost anyone within a few touches, especially in épée, where the weapon is allowed to weigh nearly 770 g against under 500 g for foil and sabre. Fix the way you hold the weapon first, then build the endurance to back it up.
Check these with your coach. Your shoulder should sit down and relaxed, not creep up toward your ear. Hold the grip like a small bird: firm enough that it cannot fly away, loose enough not to crush it, with your fingers doing the work and your palm soft. In en garde, keep the elbow bent and roughly a hand's width in front of your side, so your whole arm carries the weapon instead of the shoulder alone. A grip that does not fit your hand makes you squeeze harder, so ask about a different size or style. Have a teammate film a bout: tension is easy to see on video and hard to feel while fencing.
During a bout, rest whenever you can. When you are well out of distance, lower your point a little and let the arm relax, then bring it up as you close in to attack or defend. Fencing in absence of blade, without constantly engaging or beating, also spares your arm. Look at your habits too: long preparations, letting the opponent lean on your blade, and running down the clock all keep the arm loaded. If you feel it fading, break distance and take a breath before you lose control of your point.
Then build endurance, and steady practice beats cramming. Fencing a few times every week all year does more than a hard month before each tournament. Add two short strength sessions a week: push-ups, planks held in the push-up position, dead hangs from a bar, rows and light shoulder presses for the muscles that hold your arm up. Fence some longer bouts in practice, to 10 or 15 instead of only to 5, so the end of a long direct elimination bout feels familiar. On competition days, keep eating and drinking between rounds.