It very much depends upon how you define ‘bad for you’. In my line of research, we do a lot of work with people who have metabolic diseases – people who might be overweight or obese, who have diabetes, or have cardio-vascular illnesses. We go through a rigorous screening procedure in which we look into every aspect of their medical history. Some of these individuals are going to have an increased risk of adverse cardiac events. If you don’t go through a screening, especially looking at heart function, it could be that intense exercise could trigger adverse cardiac reactions, even a heart attack.
If someone falls into that bracket, possibly with a history of heart disease in their family, then it’s important that they’re properly screened by a medic before they embark on intense exercise. It’s not unheard of that, with proper screening beforehand, people who have had heart attacks can still complete high intensity interval training in safety. The important thing is to be properly screened and monitored.
https://www.youtube.com/embed/2R86JV8gbJo?wmode=opaque
"A dead-lift for instance can, if done badly, cause severe injuries"
For people without the problems I outlined, I’m very much an advocate of any form of exercise at all, as long as it’s done correctly. But exercises done badly can be very bad for you. A dead-lift for instance can, if done badly, cause severe injuries. Just the other day I saw someone dead-lifting 130 kilos and he had a cat-like curve in his back. It is horrifying to watch and it will almost certainly result in severe lower back injuries. But a dead-lift performed with proper form is an excellent exercise. In short, in general it’s exercise done incorrectly that has the potential to be bad for you.