It all comes back to the question: What are you trying to do? If you're training in sword fighting, then use a sword. Just repeat the same sword cut over and over again. Try to go as fast as you can without losing accuracy. Repeat.
In some amount of time, your forearms will tire. When they tire so much that either you're starting to slow down or you're starting to see your accuracy decline, that is time to end practice and/or move on to something else. Come back in two days when your forearms have recovered. Don't come back the next day, because it's not enough time to recover, and you need to be recovered in order to improve both your strength and your form. You always want to train fresh, or else you're training poor movement.
That's an improvement over the traditional method, which just has you repeat something until your arms feel like they're dead. You don't improve as quickly that way. You always need to be fresh, or else you're going to drill into your mind / body an incorrect and inaccurate way of performing the technique. Sword training requires a high level of accuracy. You must keep your mind focused. Don't let yourself get sloppy.
If your goal is to make as forceful a strike as possible with your sword, that is another thing altogether.
First of all, strength isn't what you should be targeting. You can be strong and slow. You don't want that. You want speed and accuracy primarily, with strength secondary. Also, strength with speed equals power.
You might reason that adding a weight to the end of your sword or simply making sword heavier is the way to go. It is not. Think about it. What does that achieve? Sure, it makes you use your muscles more, and that could make certain aspects of your sword technique stronger. But it results in muscle imbalances when you go to use your sword without weight.
Why is that? Because, gravity is a force that always acts downwards. But what you really want is a force that opposes your sword cut in whatever direction you happen to be cutting. So for when you're slashing the sword left to right, you want a force to push against your sword from right to left. And vice-versa. Simply adding weight will just increase downward force instead. And so the muscles on your arm, shoulders, back, traps, etc. will merely work harder to keep your sword up, rather than trying to increase strength in the direction of the sword cut.
That's not ideal, and it will only lead to imbalances of strength. The muscles that hold a sword up are different from the ones that pull the sword through a lateral slash.
There really aren't many sword cuts that will be directly improved by merely adding weight to the sword.
A better way to train is use some kind of a pulley system, attaching a rope to the end of the sword, and the other end of the rope attached to weights through a pulley. Then complete the slash so that the rope is constantly pulling against your sword in the direction that it is slashing.
But that's kind of hard to setup properly, since you're talking about multiple pivot points and a high level of complexity.
Instead, get rid of the sword altogether. Just grab the rope in your hands and perform the sword cut motion in such a way that it feels like the rope is pulling against you continuously throughout the cut.
For lateral sword cuts, you'll work the obliques a lot this way. In fact, a lot of sword technique requires a strong torso. You can work obliques and the "core" part of your body using other exercises as well. But the rope pull is a great way.
For overhead straight down sword cuts, get right under the pulley and pull down. This is actually a pretty standard weightlifting exercise called, oddly enough, the "pull down".
If it seems like the resistance is good for only 1/2 the sword cut, that's because the rope can't change its angle as you're changing your angle. That's a limit to this kind of equipment.
So to get around that, you want to just perform that part of the movement that the rope / pulley system can resist against uniformly and continuously. Stop the movement when it begins to get off track. And just train that part of the movement over and over again.
Then isolate the remaining part(s) of the sword cut movement, and practice that/those separately with some other configuration of the rope / pulley system.
You can also try bungee cord or rubber bands for resistance. My only problem with them is that they're hard to position properly and safely, and they only give you a fixed amount of force. In other words, you can't add weight to them as you get stronger. But maybe you don't need to add weight. It depends on what you're trying to get out of it.
One other thing. Before you start doing any of these kinds of rope / pulley exercises, you should consult a personal trainer in a weightlifting gym that's equipped with rope / pulley machines. You want them to show you the proper form, so that you don't injure yourself or develop bad habits. You can even tell him what you're trying to do with the sword cuts, and see what he says. He might not know a thing about martial arts, but he does know proper movement. And that can really help you.
Alternatively, a lot of the sword fighters of the past used to practice sword cuts using a short wooden staff (hanbo or boken). They would practice the sword cut using the staff against a hard object, like a tree or a wooden post wrapped in rope or something. This is a less optimal and less direct way of improving power (strength + speed). It makes the sword cut feel light and easy until you hit the target hard, and then you suddenly have a huge load on your muscles. That works sub-optimally, because it's actually a kind of isometric exercise. Isometric exercises are different from dynamic exercises like I described with the rope and pulley. They have different benefits, but dynamic is superior for sword cuts, in my opinion, because it gives you resistance throughout the entire cut rather than just at the end. And this method of hitting a rigid object can lead to injury pretty easily.
Anyway, hope that helps. Good luck!
Just to clarify, what I'm saying is that whenever you actually use the sword to practice sword cuts, make sure your primary goal is accuracy - accurate to the form. You want to go as fast and strong as possible (as powerful as possible), but without sacrificing one bit of your form. Your goal is not to gain muscle from sword cut exercises, but to drill proper technique into your muscle memory (working on your body's nervous system). This is especially true because you should NOT be doing these cuts to complete exhaustion, because going to exhaustion will cause your form to become messed up, and that will just drill into muscle memory the wrong form. So pay attention to form primarily, whenever you do sword work.
Your grip strength will only increase a little bit this way when you do the sword cuts, so it's unsuitable as a way of gaining real strength.
To gain strength, you use the weighted supplemental exercises with the rope and pulley that I mentioned. When you do these exercises, you do not have to pay attention to sword form. You're not holding a sword, and you're not performing a sword cut. You are merely working the muscles that you use during the sword cut.
You still want to utilize the whole body when you do it. Do not isolate one muscle group. You want the whole body exercised at the same time (a complex weight lifting exercise).
When you do these supplemental weight exercises, make sure you do them after your sword practice, not before. Whenever you work with the sword, your muscles should be fresh and non-fatigued. That also means never working on a sword two days in a row. Put a rest day between the two days.
Some other good weight training exercises for sword work are: the dead-lift (great for building forearm strength and a whole lot more), the bent-over row, the pull-up / chin-up, dips, oblique crunches, the plank / flip-flops, and the lat pull-down.
When you're doing sword work, your instructor will correct your form and will point out problems. Some problems to watch for: 1) Being off-balance, 2) Over-extending yourself in some direction or over-rotating, 3) Allowing antagonistic muscles to get in the way (don't tense areas of the body that are not directly involved in the sword cut), 4) "Winding up" before a strike (you never want to telegraph the start of your cut), 5) Blinking, clenching teeth, grimacing, etc.
Anyway, just wanted to clarify those points since my message. Didn't know if I was making myself 100% clear.