I can't attest first-hand, but apparently the favorite among several competing schools of thought is that "high-intensity training" is the most productive method. There are several books by Darden on this topic. I actually tried high-intensity training for a while (I'm fairly certain I was doing it correctly, too) and promptly got a severe tendinitis in my left medial bicep. The tendon was just not tough enough to take the loads I was imposing on it by lifting heavy weight to failure in relatively few reps.
I was able to increase the intensity of workouts after my arm was fully healed (hoping something else doesn't give out), because it does make quite a bit of sense as an approach to building new muscle, I think.
In any case, it's important, I think, to be alert to your own body and to be able to discern the difference between various post-workout aches and pains, some of which are actually good news and some of which are very bad news indeed.
Muscles that are not conditioned to exercise hurt differently from those that are. The first is a longer-lasting, rather more painful hurt, the sort of thing that makes it hard to walk for two days after the first time you do squats or lunges or leg extensions. The ache of tired but practiced muscles usually goes away in 24-36 hours. The first kind, so-called delayed-onset muscle soreness or DOMS, may take 48-72 hours.
Keep in mind that muscles do not grow while you're stressing them; rather, they grow during the recovery from stress, and a muscle that hurts has not recovered enough to accept more stress safely.
In general, it's probably a good idea to wait at least 48 hours (more if it's still hurting) between exercising a given muscle group or body part. The bigger muscles (quadriceps, say) usually recover more slowly than the smaller ones (biceps, say). The main point here, though, is that pain which goes away in a day or two is not a bad thing, provided you don't try too hard to blast through it with more exercise.
There is another kind of pain. Fortunately, it's usually bigger and sharper, and it does not go away in a day or two. If you have this kind of pain, you must lay off (right away, too, not at the end of the current set) and get medical advice promptly. Failure to do exactly that probably set my program back six or more months.
I kept right on doing dumbbell curls with 30 and 35 lbs (very heavy weight, for me, at that stage) after the pain started, noticing, of course, that it was getting worse after each workout, but thinking I would "work through it."
Then I started to read up on injury, and other people in the gym told me to be very careful, because tendinitis ignored becomes a long-lasting, possibly permanent, disability.
Concerned that I har perhaps destroyed my program before it had properly gotten started, I went to the doctor (the diagnosis was as expected), who told me to do absolutely nothing with that muscle, if possible, to take 600 mg of ibuprofen (for its anti-inflammatory powers, not its analgesic ones) three times a day, and to continue this regimen at least a month, maybe longer, until things got better.
How would I know what better was? Here, a very experienced lifter at the gym was helpful. "Be totally ridiculous about the weight when you try curls again -- 5 or even 3 lbs would not be too little. Remember, you want to strengthen the tendon, not the muscle. If you have even a little twinge, stop at once, and wait a couple weeks before trying again."
As it happened, after a month I was able to do 8-lb dumbbell curls, but not 10-lb ones, without discomfort. After a couple weeks of that, I felt bold enough to try the EZ-bar (20 lbs with no weights on it) because of the better hand position. That too worked, as long as I did the curls on a preacher bench and watched very carefully in the mirror to achieve perfect form, going very slowly up and more slowly down.
I told the same fellow I mentioned above about this, saying that I was going to give it a year, if that's what it took. "Remember that you said that," he replied. Urk!
Well, all that was near the beginning of the gym effort for me. Now it is years later, I'm 68 instead of 60, have had some age-related ailments to deal with (arthritis in my knees, some degradation of the lower spine), as well as a two-year break from going to the gym regularly.
Going back to the gym, which I did in the summer of 2003, is a major part of dealing with advancing age, and actually is more important than ever if one has the kinds of age-related problems I've been having, I think.