Unfortunately...It can happen.
Bad form and overload are the main culprits.
Although nobody ever wants to get injured, sometimes, it happens. This may be in the form of DOMS (Delayed Onset Muscle Soreness) at the start of your programme or maybe in the form of joint pain as your programme goes on. Or it could be something much worse... Either way, injuries are nearly always caused inside a workout by excessive overload or bad form.
Prevention is the best medicine.
It's important to prevent injury as much you can. By far the biggest cause of injury is bad form. The best thing you can do to prevent injury is to keep good form, paying particular attention to your back, neck and knees, throughout your workouts and ensure your body is fully warmed up when exercising intensively. You can do this either by easing into high intensity exercise gradually or by doing a separate warm up routine. If you pay attention to these things every time, you will massively reduce, or even eliminate, the chance of yourself getting injured in a workout.
Once you are injured, however, the best medicine for injury becomes rest. But only for the affected area. It is still possible to workout other areas of your body whilst you rest your injury simply by making your workouts focused.
Delayed Onset Muscle Soreness.
DOMS (Delayed Onset Muscle Soreness) is the most common injury related to exercise. It is very common for people doing HIIT style workouts for the first time. Most people have experienced it at some point in their lives. It can be mild or really quite debilitating. It's that dull, aching pain in the affected muscle a day or 2 after exercise, that becomes very tender to touch, and becomes stiff and inflexible, and painful to apply weight or pressure on.
DOMS is very common when you begin a new exercise program or routine, or even increase the duration or intensity of your current workout. Conversely to common belief, DOMS is not caused by muscle burn from lactic acid build up. The pain is caused by microscopic tears in your muscle fibres that occur in your workouts in the eccentric (lengthening) phase of muscle movements and the inflamation that follows as they heal. As the microscopic tears happen during the workout from overload, it can be very hard to avoid. With hindsight, you may realise that you should have lessened the increase in intensity or duration that caused it, to help to prevent it... But of course by then it's already way too late! By making your increases in overload small and continuous rather than a huge sudden leap is the best way to avoid DOMS. But of course, depending on your circumstance and equipment you have available, that isn't always practical so when it happens, it happens. This is absolutely nothing to worry about and is perfectly normal as it is part of the adaptation process for increasing your strength and endurance as your muscles recover and build. It doesn't stop it hurting though! Unfortunately, the pain can take several days to subside.
Rest is the best medicine, along with active recovery (gentle aerobic warm up exercises to increase circulation to the region) once the worst of the DOMS has subsided. A massage may also do some good in the same way as active recovery does. Stretching the affected muscles should also give some temporary pain relief.
So, given that rest is the best medicine, should you ever train with DOMS? Well, that all depends on your workouts focus.
When NOT to train whilst injured.
Use common sense here.
Obviously if you have broken bones, fractures, or are recovering from heavy bleeding or other such major traumas or severe medical conditions then any kind of exercise is not recommended and you should get checked out and cleared by a doctor before you undertake any kind of exercise at all.
When you can train whilst injured.
With most HIIT routines you can't train effectively at all with DOMS or joint pains. You either suffer and struggle on with complete inefficiency or sit it out for days until you have recovered.
One of the many strengths of The 4 Minute Max Outs is that you can continue to get your HIIT whilst injured thanks to the duel focus aspect of each Max Out.
This of course depends on the nature and place of the injury. The term injury here refers to muscle pain such as delayed onset muscle soreness (DOMS), muscle tiredness or strain, or joint aches. With The 4 Minute Max Outs you should be able to carry on training if you have muscle or joint pain in one area of the body simply by doing the Max Outs that focus on another, allowing the injured area to rest and recover.
For instance, if you had DOMS in your quads or calves, or joint pains in your knees, you could very easily do the Upper Burn and Upper Rush Max Outs to full effect and still receive maximum HIIT benefits whilst your legs get another day to recover. Likewise if you had a sore shoulder, arm or chest, you could do the Lower Rush or Lower Burn Max Outs to full effect and still receive maximum HIIT benefits whilst your upper body gets another day to recover.
This way you still continue to work your cardio system to maximum effect, by still getting that heart rate up, still burning those calories, and still creating the EPOC afterburn and production of testosterone and growth hormone to help shape and tone your muscles. And as you increase your circulation you also actively increase circulation to the injured area, without putting pressure on it, assisting recovery. Much better than just sitting it out.
Always apply common sense - Don't make your injury worse.
However, that said, if you feel that exercising makes your injury worse or adds strain to it in any way even by using focused Max Outs, it is better to stop altogether and wait until your injury has recovered before you return to exercise. Otherwise you wont receive maximum benefits from your HIIT and may actually injure yourself further through bad form.