The idea of working a set of repetitions to the point of muscle failure has been around for decades. The claim is that this practice maximizes the rate of muscle growth. The claim is that a lack of sufficient intensity of effort will only tire you and trigger no development.
It should be said that working to failure is anything but easy. For example, I have found that I didn't actually know what it really meant until quite recently. The reason is that, as an unassisted beginner, I first had to overcome deficits in my ability to deliver intensity of effort (form, unknown personal limits, etc). This was in no way apparent to me early on and meant that, although I was working to the point of failure, I was not able to give everything I had even if I wanted to. I had to learn to do so. This is OK but something to expect and look for. Why?
Because gradually I got to the point when I could put more into an exercise but didn't. I had fallen prey to another difficulty of working to failure, the rut. Pushing oneself to the limit every single time is, as someone put it, masochistic and counterintuitive. Here is an example:
Imagine you take a math test and you fail it. The score tells you know some math but not enough. That's OK. You study more and retake the test. The thing is that the test has become harder and you fail it again. The score tells that you know more math than before but still not enough. Imagine this situation going on indefinitely. You will become very, very good at math but you will fail the tests nonetheless.
Working to muscle failure means that, no matter how strong you get, you will never pass the test that you put your body through. In fact, that is precisely the objective. You don't want your metabolism to ever consider that it can handle the "situation" it is faced with. Rather, you want your metabolism to always "think" it needs to grow more muscle in order to match environmental demands.
You fall into a rut when you fail to challenge yourself sufficiently, when the test you put your body through is not demanding enough.
