What is the best rep range for gaining muscle mass?

There are a lot of ideas out there about the best repetition range for building muscle mass.  Some people focus on heavy weights and low reps.  Others focus on low weight and high reps.  While both methods work it is a little more complicated than that.  Building muscle is about creating an overload which forces the muscle to adapt by growing.  This is achieved by a combination of tension on the muscle and the total volume of work being performed.  

Heavy Weights & Low Reps
Extremely heavy weights, 90%+ of your one rep max, done for low reps (1-5) certainly has the tension that you need but volume is low.  As a result, more strength is produced than mass.  As an example, both powerlifters an olympic lifters spend most of their time in the 1-5 rep range and while they do develop muscle mass it is not on the same level as a bodybuilder.


Light Weight & High Reps
Weights below 60% of your max is great for building muscular endurance.  Anything above 13 reps will build capillary density and improve lactic acid tolerance all of which are great for endurance.  Unfortunately there is not enough tension on the muscle to produce maximum growth.  Some mass will be achieved but it will be primarily red cells and sarcoplasm.  This missed the fast-twitch white muscle cells, which have the greatest potential for growth.


Moderate Weight & Medium Reps
Bodybuilders, through years of trial and error, found the ideal repetition range long ago.  Moderately heavy weight that can be done from 6-12 repetitions, is what they intuitively gravitated to.  The weight is heavy enough that it produces enough muscular tension.  It also allows for enough volume that lactic acid and human growth hormone(HGH)is produced.  This mixture of tension and HGH seems to be the magic elixir which causes growth. 


Does this mean that we should stick to this range exclusively if our goal is to build and preserve muscle mass.  No.  Your body will adapt to a particular range in as little as three workout.  While beginners can stick to this range for a quite a while, as you adapt is becomes harder and harder to progress.  And since progression is the key to overload you will quickly plateau.  Simply putting in more and more effort, eventually will lead to burnout and over-training.

So what is the answer?
The answer is variety and change.  If you find yourself hitting the proverbial wall, one or more acute variables in your workout should change.  Perhaps he reason you are unable to progress on your regular workout is simply because you are just not strong enough!!!  In that case doing some work in the 1-5 rep range could be just what you need.  Or perhaps the reason you are stalling out is because you don't have the muscular endurance.  In which case some work in the 13+ rep range would be called for. 

Putting It All Together
A great solution is the classic Light/Medium/Heavy workout.  An example of a three days a week full body workout might look like this.

Monday- Light
Wednesday - Medium
Friday - Heavy
Monday - Medium
Wednesday - Light
Friday - Medium


As you can see there are two medium workouts for each of the light or heavy.  This is sort of an undulating periodization twist on an old school idea.