In order for your muscles to grow, you have to stimulate them
with a high intensity training stress and then allow them to move
through the three phases of the recovery process.

There seems to be a number of differing variations of high intensity as
it relates to bodybuilding, so let's make sure we are on the same page, okay?

High intensity refers to the percentage of momentary muscular effort
being exerted.

If you can curl a 100 pound barbell for 10 repetitions and an 11th
repetition is impossible, you have performed high intensity training
and have trained with 100% intensity and to the point of momentary
muscular failure, if, and this is a big if, you also gave it 100%
mentally, not just physically.

And that makes high intensity training a completely different kind
of animal.

This is what is meant by the phrase, "training to failure", as
advocated by high intensity training enthusiasts. In other words,
if you are doing a set of barbell curls and you perform as many
repetitions as possible until you can't complete another repetition
in good form, you have trained to failure.

There is another important point we need to touch on in the above
definition and that is the ideal of volitional effort. You see,
your muscles are voluntary - they will not contract (work) unless
your mind tells them to do so.

You see, intensity, as it relates to bodybuilding, is the
possible percentage of momentary muscular and volitional (mental)
effort exerted. You can not train with 100% intensity (train to
failure) without doing so mentally as well as physically.

The fastest way to make progress would be to stimulate
your muscles with high intensity weight training (in order
to trigger the adaptive response) and then minimize the
stages of the recovery process. The way to minimize the
recovery process would be by performing the least amount
of exercise needed to trigger the adaptive response - one
set per exercise per body part.

One set per exercise per body part is the least amount of
exercise we can do and still stimulate muscle mass and strength
increases. By performing just one set, you limit the drain
on your body's finite energy reserves.

Every set you perform eats in to your limited recovery ability.
Your workouts need to be hard, brief, and infrequent if you want
to make maximum progress in a minimal amount of time.

Once you've performed a high intensity set on an exercise,
you've triggered the adaptive mechanism that will allow
over compensation (increase in muscle mass and strength).
It only takes this one attempt (one set) to make progress.
Performing any more sets will only drain away your body's
limited recovery ability.

If you remember, your muscles adapt and grow larger and
stronger by being exposed to an anaerobic stimulus that
challenges their current existence. In other words,
progressive overload through high intensity weight training.

If you train intensely enough, most of you will not duplicate
or outperform what you did in the first set. For example,
if you train to failure and manage to do 10 reps with 200
pounds in the bench press, you will not be able to duplicate
this performance. You may get 6 or 7 reps on the second set.

So what would be the purpose of this set, since it does not
come close to challenging your body's current strength level,
and we already know that is necessary in order to cause the
adaptive response we are looking for. All you've done is
create a much greater inroad into your body's recovery ability.
Although it doesn't seem like much, by adding a second set,
you've increased the volume for that body part by 100%!!

However, this is where things get muddy for the skinny
hardgainer. For you, it may be necessary to add another
set or two. Here's why -

As a skinny hardgainer two things are likely - your
muscles have low neuromuscular efficiency and you have
more slow twitch fibers.

With lower neuromuscular efficiency your body may need
a second set to fire enough muscle fibers to trigger the
adaptive response. Thus, a second set could be of benefit.

In addition you need more time under load as discussed in
the previous report. In order to satisfy this as well as
make up for lower neuromusclar efficiency, you may need
to add a set or 2 to each exercise in order to make enough
of an inroad to trigger the adaptive response mechanism.

Keep in mind, there are a lot of variables when it comes
to designing a proper weight training program. I am not
stating that one set per exercise is the most effective
way for you to train all the time. But it is extremely
effective, if (and that's a big if), you work at a high
enough intensity level, something very few people are
able or willing to do.