Injuries will definitely take a toll on your body in any sport, but they can be debilitating in high-impact Jiu-Jitsu. For this reason, it’s important to differentiate what is considered high-risk training with an injury.
In the case of a bad knee injury or a torn ACL, the bad news for the jiujiteiro is that you are better off not training at all. It’s important to heal fully, and follow through with any required rehab steps, or even surgery, to let your body ease back into normality.
There are some BJJ practitioners out there who will actually train with a torn ACL, but invest thousands of dollars in a custom-made “robocop” brace. This is definitely not the sort of neoprene wrapper that you can easily purchase at a local pharmacy, but rather a rigid carbon fiber brace that clamps onto the thigh, with large hinges. It’s none too pleasant to roll with anyone who is wearing one — it constantly pushes up against your body, and might even puncture you.
The only honest way to potentially train with an injured knee and a brace, is to find partners who will be understanding of your injury, and willing to work with you. Again, it’s well to remember that any jiujiteiro is putting themselves at a very high risk of further injury by continuing to train, even with a protective brace.
Lighter injuries are more conducive to training. If the knee is simply just a little banged up, a BJJ practitioner is better off to train on the bottom. If you’re standing, for instance, you are an easy target for a reckless to partner sweep you, or perform some crazy de la Riva guard and crank your knee sideways. It’s safest to be on the bottom, and stick to playing butterfly and half guard, best showcased by legendary BJJ figure, “Gordo”.
You should know how to assess the severity of the injury. Here is how to do it: