I have sustained so many injuries, 100's of injuries in fact, since I started cutting more seriously. Now I can tell you that exactly 2 of those have resulted in me drawing blood. The rest have been much more mundane, all stemming from my muscles not being able to stop and start the blade exactly when I needed to. Trying to stop a blade from hurtling through the air, in order to bring it back in the opposite direction and then swing it back again in under a second whilst maintaining correct control, can cause some serious strain if you're not used to it. If you overdo this for too long or over extend your arms, or put too much power into a cut then POW! It's ouchie time and that unfortunately and frustratingly, leads to timeout, sometimes lasting weeks or even months depending on how badly you mess up. The sad fact of the matter is that as much as you might believe that you only need the strength in the core of your body, this only applies to people who have already made these mistakes, happen to have that strength in their arms or train more carefully. If you're like me, then you'll train for longer than you should, you'll have no teacher to tell you when you're doing something wrong and you will, inevitably end up overdoing it. For this reason, a little premptive strategy is required. I've taken a leaf from the books of a thousand martial artists for this and looked at the 'Suburito'
Ok, stop. What the hell is a Suburito?
A Suburito is basically a more heavily weighted bokken. Yes, people will jump on me for saying that, but thats essentially what it is. Suburi is the practice of your sword swings and the 'to' post-fixed to the end of it means that it's the sword like object used to do this practice with. Bokken are usually relatively light in comparison and are used because they are blunt and therefore a lot safer, they are a lot lighter than a sword and if needed they can be used in paired practice with little risk of taking someones limb off ;) With Suburito the weight makes them unsuitable for paired practice but fantastic for developing the muscle needed for cutting with a live blade.
There are many good options for purchasing suburito and they're all fairly cheap as well but it is possible to make your own. I have done so myself and although the end result is little more than a heavy, roughly carved wooden practice post, it does do it's job. I have found that the basic Jodan, or straight downward cut with multiple repetitions a few times a day has definitely helped with my day to day cutting over a very small space of time. The plain fact of it is that we're not used to putting these stresses on our wrists and forearms and a tool like the subirito is a brilliant way of incorporating simple practice like our downward cuts, whilst placing the emphasis on good technique on each individual cut as well as training the muscles that we need to do this.
Having used the downward cut as an example I feel the need to mention that at the moment, I have found no real benefit to practicing anything other than this with the suburito. This could just be because it is early days with my 'heavy stick' training but attempting upward diagonal cuts felt strange and awkward up and above the additional weight. This is something that you're going to have to find out for yourself I think but I have certainly noticed improvement in all my cuts with a live blade after following the simple practice of 50 good cuts in the morning and 50 in the evening. squeezing a few in the middle of the day is good practice too I've found.
So, the Suburito, carved wooden fence post, heavyweight bokken, call it what you will. It's a fantastic training tool for technique and confidence and one that I heartily reccomend. A muscle injury time-out is no fun at all and using one can seriously reduce the risk of that happening.