Without doubt, one of the most frustrating parts of jiu-jitsu (after 'getting your ass kicked and not being able to do anything about it') is dealing with injuries. Unfortunately it's something which will happen to most people who train, and usually quite a few times.
Before anything else, I can't stress enough that if an injury seems really bad (lots of pain, massively decreased range of movement etc) you should seek medical attention. The more time you leave between getting injured and seeing a medical professional the worse things can get; longer healing time, harder to treat, more complicated surgery, worse infection potential etc. So if something seems bad, go to hospital!
Lots of minor injuries can be treated immediately using the R.I.C.E. protocol and then icing can be used to help recovery along with anti-inflammatories and stretching/massage... although you should only do these things if you understand what you are doing and have experience of the medical care provided by professionals. If you get things wrong, you can make stuff a lot worse.
The next thing to look at is injury prevention. There are plenty of things you can do which should reduce the frequency and severity of injuries...
- Make sure you tap. There is no excuse for being hurt by a submission in training if you could have tapped.
- Learn when you have to rest. Some injuries you can train with, but some you have to rest (more on this later).
- Supplement your training. The best stuff for injury prevention is to increase strength and flexibility. A basic powerlifting routine and some yoga classes will cover this.
- Know when you have to stop training early. A slight muscle pull can easily turn into a full tear if you decide to finish the round of sparring, or do one more round. This could be the difference between a week off and months.
- Don't try to be a gym hero. You don't need to fight every sweep or pass to the bitter end. If a leg or arm or anything is in a dodgy position, let your partner move on.
However, jiu-jitsu is a contact sport and you will probably get injured at some point. Not all injuries are equal though, they can vary dramatically in severity and some you can still train with fairly normally, some you can train if you modify the way you spar and others mean you have to rest end of story. Knowing what injuries you can train with takes experience, so it's always worth being overly cautious when you are a novice. The more experienced you are the better you can control sparring too, and so can manage to train with worse injuries than novices. Whatever the case though, always think about the risk/reward balance; is starting back a few days early worth the potential risk of setting yourself back by months? Jiu-jitsu is a long-term activity, so don't ruin that by taking pointless risks early on in your training.
If you are training with injuries the most important thing to remember is you can always tap. You don't have to be caught in a submission, it could just be a position which is awkward due to an injury, or maybe your partner has just gripped an injured wrist or ankle. Again, don't be a gym hero, tap and restart. One thing I dislike is before you roll telling your partner "Don't attack this cos I'm injured" etc, just say you are injured and might have to tap for no apparent reason so your partner is ready for it. If you can roll then just roll, there is always the option of tapping to prevent further injury.
You can even train with fairly major injuries if you are experienced enough and sensible about it. It might mean that there are positions you have to avoid or even only a handful of positions you can comfortably use. The most important issue with this is that you need to be experienced to be able to do these things. When you are a novice and have a major injury your best bet is definitely to rest it until it's fully healed.
One thing I would never recommend is taking painkillers to be able to train. All this does is mask the pain signals and can easily lead to further injury.
Probably the worst thing you will experience in jiu-jitsu are the times you have major injuries and have to miss training for a long period of time. Making sure you have proper medical treatment will reduce the amount of time off but some things can still take months or a year or more to recover from, so how can you deal with this? Obviously different people cope with adversity in different ways, but beyond anything else I think the most important thing is to remember that you will heal. Injuries which are impossible to heal 100% are thankfully very rare, and even most of those injuries will still heal enough to allow you to train again.
So if you're out for a long time, what things can you do? As I said, it will vary for each person but outside jiu-jitsu I like to do the things I normally would. I never watch many videos, just the big competition broadcasts, and I think it's a bad idea to watch lots of instructionals as you won't be able to try out any of the stuff you see for a long time. I think it's more likely to make you more frustrated. For the same reason, I don't think it would be a good idea to buy any new gear. Try to just live your normal life outside jiu-jitsu, don't preoccupy yourself with jiu-jitsu and remember that you will heal!
There are things you can do while injured. There is no reason you can't attend classes to watch (as long as your instructor will allow it, but I don't see why they wouldn't)... this will help you still feel part of the club and obviously lets you see your friends who you train with. Depending on your injury there may still be some things you can do on the mats too. In the past I had a very bad shoulder injury which put me out of action for 8 months. In that time I use to go to the academy and do as many solo drills as I could; stuff like backward, forward, sideways rolls, shrimping, sit-outs etc (I might do a video of this sort of thing at some point). Anything I could think of which didn't cause problems to my shoulder (it could bear weight but I couldn't move my arm much). As my shoulder got better I increased the intensity of these sessions (moving faster, more reps, for longer etc) so that when I got back to training I had at least a little bit of cardio. However, as dealing with time off is more of a mental issue, you really have to find your own way to deal with things.
Knowing when you are safe to go back to full training can be difficult to judge, and it's not something anyone else can tell you whether you're ready for. You have to make a judgement call based on what it feels like to move the injured body part. A good way to safely check is to drill something which involves moving or putting strain on the injury and then building up the intensity/resistance of drilling.
You also need to be prepared for returning to training after a long period off. It's not easy and it's a big cause of people quitting training totally. I have seen a lot of people over the years who get injured, spend months off the mat then return only to disappear after a few sessions back. Why does this happen? Quite simply, it's another ego issue. When you spend a long time away not training everyone else is, so you can easily find yourself being tapped by lower grades and people you used to tap. This is no reason to stop training though, it's just another hurdle to overcome and is no different to what everyone faces when they first start training. If you are a coloured belt you need to realise that any pressure you feel to perform is only something you make up in your own head; if you get tapped just suck it up and keep training.
The worst part of returning to training is lack of cardio. Even if your injury allowed you to still do some cardio training, it won't be the same as jiu-jitsu cardio and you will find it tough when you first start back. The only way to deal with this is to get stuck in straight away and not avoid hard work. As long as your injury has healed enough to allow it, you should be sparring every round as soon as you're back. If you just do a couple of rounds then sit out because you're tired your cardio will take a long time to come back, but if you just suck it up and do every round you will soon be back to the level you were before.
The last issue with returning from a long hiatus is a loss of skills. Personally, I don't think skills decrease that much with time out (unless you are very inexperienced). You will lose quickness of thought and your reactions won't be as good as they were, but your general skill level will remain and it won't take you long to get back to normal the same as cardio. I always use the analogy that I haven't ridden a road bike now for around 15 years, but I know I could jump on one and ride it without problem. Obviously riding a bike involves less technique than jiu-jitsu, but it is a useful example.
So basically in summary; you are gonna get injured, treat it properly and get back on it as soon as properly. But remember, fingers and toes don't count ;-)