As a personal trainer, people are always asking me what the primary factor is for muscle growth stimulation.  With so many different workouts out there today, it's hard to know which ones are effective, and which ones are not.  Some advocate high volume and high reps, others recommend low weight, long rest periods, and high intensity.  With all these different avenues of training, there's really only one true factor that ensures muscle growth will take place.  If you don't follow this rule, you will not gain muscle and will continue to stay the same day after day - workout after workout. 

The rule is as follows:  Intensity - not duration, not volume is the primary factor that determines muscle growth.  The exercise being performed must be intense enough to literally "scare" the muscle into thinking that it must expand on its existing stores to survive - if it does not it will die. 

That's it, that's the rule to experiencing ever increasing muscle growth until you reach your genetic potential.  Intensity is the key to muscle growth.  The thing is, most people are not prepared "mentally" to perform exercises at the level required to stimulate a muscle to grow.  Most people are asking how they can make their workouts easier, the true champions are asking - "How can I make my workout harder."  It's the latter that goes on to create a highly polished, and well-built physique. 

The rule is especially important for intermediate trainees who've seemed to hit a plateau.  They have been training for awhile at this point and their body has simply become accustomed - it adapted - and it no longer perceives it as a threat.   If they don't "up the ante" by creating ever intense workouts they'll simply top out  in terms of muscle growth and continue to spin their wheels. 

The next time you're in the gym notice how you workout - are you using the same intensity you were using 6 months ago, a year ago or more?  If you answer yes to that question, without even seeing you or your progress I know you've made little to no gains. 

So what's a person to do?  Well, you got to get intense.  If you performing sets to muscle failure you may now need to add forced reps to your training routine.  Maybe you could perform compound sets - two sets for the same muscle performed together with little to no rest in-between.  Start thinking how you can make your workout harder - what could put more stress on the particular muscle being targeted? 

You'll quickly find that as your workouts get more intense you can't workout as long.  This is exactly what you want.  As you continue to get bigger and stronger your workouts should continually become more intense and shorter in duration.  In fact, most of the biggest bodybuilders today started working muscles 2-3 times a week and slowly convert to working a muscle once a week and sometimes less.  They know through years of experience that intensity combined with adequate rest is the most surefire way to build muscle mass. 

So, the next time you're in the gym start working out smarter by working out harder - when you begin to see the newly achieved progress you'll be glad with the results.