About this idea of training to failure, or sub failure, or in Tbombz more specific theory -- 2 reps short of failure, and the risks of injury it might, help to delve into a bit of common sense. Or at least what seems to be common sense.
The idea of lifting weights is to subject your body to an overload that it is not accustomed to with the idea (and hope) that it will stimulate an adaptive response. In this case, bigger and stronger muscles. It seems to me that if you are always doing things that are already easy, doing something that your body is already accustom to, this will not happen. If, for example, you can do 8 reps on the bench with 205 lbs and failing on the 9th rep but all you do in practice is always stop at the 8th rep, or in Tbombz case, stop at the 6th rep, never trying for that 9th rep, never trying to exceed your functional ability how will that stimulate an adaptive response? How will that signal to the body that a change is needed? What does it need to adapt to? Of course Tbombz will say that it is simply a matter of increasing the load. But if you can only do 8 reps at 205 lbs but stop at 6 reps, what difference does it make if you increase the load to 225 lbs when you probably could only get 4 or 5 reps but stop at 2 or 3?
And training to failure does not increase the risks of injury provided there is no degradation of training form. If you are sloppy with the squat you will hurt yourself and stand a much greater chance of injury regardless of weight though of course the resistance matters.
You get hurt when you exceed the structural integrity or tensile strength of your body be it muscle, tendons, ligament or bones. So it is the force generated that increases the chance of injury which decreases as the you preform a set there by making it less likely that you will injure yourself as you get closer to failure -- provided form is consistent.
Going back to the 205 lbs on the bench for 8 reps -- failing at the 9th -- you are your strongest during the first rep. Force is high but intensity is low. You could easily lift more than 205 lbs for that first rep. As you progress through your set strength, and consequently force, diminishes though intensity rises. Finally, on your 8th rep you can barely generate that 205 lbs though your intensity is at it's highest.
Because you actually get weaker as you approach failure you stand a much less chance of exceeding your structural integrity and tensile strength provided form is consistent.
Another point is that of course if you train heavy, going for the low end of the rep scheme to be able to manage a heavier load, you increase your chances of injury but even then form is much more important. Explosive movements dramatically increases the chance of injury regardless of the weight because of the force generated. Similar to why you can sit very comfortably in a car going sixty miles an hour as long as you accelerate slow enough. But if you go from zero to 60 mph your neck will snap off or, more commonly, go from 60 mph to zero (such as when you slam on your brakes) you will go through the windshield if you are not wearing seat belts.
You can take you fist and push as hard as you want on a brick wall and you will not injure yourself. Punch the wall as hard as you can and you will break your hand. It's the difference between velocity and acceleration.
