First of all, when most people are reading or writing about this, they are thinking of a “muscle”, i.e. bicep, triceps, etc. However, a muscle is made up of many, many fibers, and it is the injury to these fibers that results in muscle growth.
True, if you COULD do curls, for example, until you could not do any more, you would almost certainly be stimulating muscle growth. It’s the professional bodybuilder or weight lifter who’s likely to do that extremely stressful and painful thing.
What about us exercisers who just want to get stronger, more physically fit, and maybe get some bigger, showier muscles in the process? What if WE don’t want to go to the extremes of the professional or are just not ready or able to go that far? Are we doomed to be 97 pound weaklings all our lives?
Not necessarily, and here’s why.
Remember those individual muscle fibers that make up the big, whole muscle… the one with a name? As you work out, if you use enough weight, you will be exercising those individual fibers, some of which are not very strong to start with. As you exercise rep after rep, you will be working, and tearing, those weaker fibers, and they will “fail”.
Other fibers will take their place, and, some of them will “fail” in time too as your workout proceeds.
Also, during various positions during the muscle expansion and contraction, various “bundles” of muscle fibers will come into play, taking over the movement, or relinquishing it to other bundles.
So, assuming you use “enough” resistance, or, do enough reps, you WILL be taking various muscle fibers and muscle fiber bundles to failure. This failure will result in muscle growth, as the experts have been telling you.
Too, since many of those big, named, muscles, have various sections, i.e. the bicep having two “heads”, working with different exercises can cause growth in those areas which can help the entire muscle work with heavier weights and do more repetitions.
For example, I, myself, found that by adding triceps extensions to my workout routine, I experienced muscle growth in previously poorly trained areas which allowed me to use heavier weights in presses and push ups, which, in turn, caused muscle growth in sections which had not been progressing previously.
So, yes, you do probably need to exercise to failure for muscle growth, but, unless you are trying to be a professional bodybuilder, or compete in weight lifting competitions, just keep trying to progress and do your reps, and add alternate training for additional growth in size and strength.