Very often people will ask me what the best rep range is for building mass. It’s a simple enough question but like everything else with the body, the answer isn’t as simple as saying “6-9″. From my experience it has more to do with training age, time under tension, and movement speed.
As one trains, they get better at recruiting motor units and are therefore able to achieve the same workout with fewer reps. E.g. assuming the average newbie is able to recruit 40% of their motor units. As they perform reps, fatigue will set in and in order to continue the set, they will need to recruit different motor units. At some point they will fatigue all available units and the set will have to end. Assuming a 5% fatigue rate of the total units per rep, they will be able to do 12 reps before they fail.
Someone who has a higher training age will be able to recruit more units per rep. They will also have more muscular endurance which will delay fatigue. Assuming they are able to recruit 80% of the available motor units per rep and that 3% of them fatigue per rep, they will be able to do 7 reps before they fail.
The end result is that failure occurs after 7 reps for the trained athlete and 12 reps for the new athlete. Both achieve the same physiological state of failure; it just takes fewer reps for the trained athlete to get there BUT they’ll be using a much larger weight.
Time Under Tension:
Regardless of most other factors, the length of time it takes to complete a rep will impact the amount of work that one is performing and it will impact the amount of motor recruitment. 6 reps with 505 tempo will recruit more fibers than 6 reps of 211 tempo. When someone asks a question which is better for growth, 4-6 rep ranges or 12-15 rep ranges but doesn’t mention tempo the question is impossible to answer. 12 reps at 101 tempo = 24 seconds TUT while 6 reps at 505 tempo = 60 seconds TUT. You’ll get better size gains with 60 seconds than you will with 24 seconds, but you’ll be able to handle more weight for sets lasting 24 seconds vs. those lasting 60 seconds.
Some of the motor units *may* not be recruited at slow speeds while others will not be recruited when moving at a fast speed. IF you do not recruit a motor unit the muscle fibers that these motor units control will not grow. There is also a growing body of research that indicated that the growth potential is not the same for all fiber types / motor unit types – fast twitch appear to have the potential to grow more than slow twitch.
Putting It All Together:
Keeping TUT between 45 and 75 seconds and varying rep speed is key once training age reaches a certain point or, more accurately, once you gain a certain amount of control over your motor units. Remember too that the body adapts quickly to EVERY action that it has to perform so variety is key.