This of course doesn’t mean that you have to train till your arms fall off or till you drop dead under the bench. As a matter of fact it has a lot to do with the way the human muscle works.
Now, then, if you go into the gym one day and decide to work a specific muscle group you’ll aim to work that certain muscle-group as thoroughly as possible. At least that’s what you should aim for.
In order to understand how training to fatigue works, and why it’s good for a muscle group to be worked out this way, one should fundamentally understand how human muscle works and the processes that take place inside it during consecutive contractions.
During the course of the first rep (successive contraction and extension) the body calls in some muscle fibers to take care of the task. As they do take care of it, they use up fuel from the muscle and oxygen, during a process of oxidation they produce lactic acid which builds up in the fibers doing the contraction.
( the oxidation is by the way the burning sensation one feels when working out the muscles)
As lactic acid is built up in the fibers used during the first contraction, other fibers are called in by the body to take their place for the second rep. These too get fatigued (‘filled’ with lactic acid) so, still other fibers are needed for the 3rd rep, and so forth and so on.
The number of muscle-fibers the muscle-group can make use of, is limited though, and when there are no more left to call in, failure occurs. That is the inability to do even one single rep with the weight that’d been used.
During the rest following such a set, the organism ‘rushes to the scene” bringing in more fuel, more oxygen filled blood, and tries to carry the lactic acid away as fast as it can. The blood that is rushed there, gives one the ‘pump’ feeling: the swollen aspect and feel of the muscles. In a few minutes the body ‘cleans up the damage’ so another set can commence. This way one can be sure that all the muscle fibers get their workout. If you don’t train to failure you can be certain that the muscle group isn’t thoroughly worked out and stopping a set just because you’ve done a certain number of reps, you set out for yourself before starting, is a sure way to underachievement.
There’s another important thing as far as this type of training is concerned: when exactly - during which rep - failure occurs.
One needs to carefully calibrate the amount of weight one uses for a set. There are several factors that can determine a person to use smaller or heavier weights, which in turn determines when failure happens. If you work for definition, the smaller weights will mean more reps before failure kicks in, if you work for mass, the heavier load will make fatigue appear a lot faster, in a few ( 8-10 ) reps. One, however can use a load so heavy that he’ll find himself unable to execute another rep after the first 3-4 are done. Now, 3-4 reps are just not enough for all the fibers to get called upon, in this case he’ll need to decrease the amount of weight a bit.
It’s all a careful balancing act, which everybody needs to learn to do, by trial and error.