More Than You Ever Cared To Know About The Repetition.

What is the magical allure of a 5 x 5?  Or a well performed set of 15 reps?  How do I choose how many reps to do in a given set and what is the rationale for using one or the other?

Moreover, why do you never (intentionally) see a set of 17? Or a set of 9?

One could argue that rep numbers are a matter of human beings’ preference for “ideal” numbers – 1, 3, 5, 10, 12, 15 – those numbers seem “nice” somehow; more intrinsically …pleasing… than a set of 4, or a set of (heaven forfend) 13 reps.  But rep selection has nothing to do with the human heritage of symbolic numbers like 3.  It has to do with time.

The numbers of reps you do is determined by your set time.  Long ago, exercise physiologists figured out that the time frame for anaerobic work (i.e., the energy system in which intense muscular contractions can take place) was anywhere between 30 seconds and 120 seconds.  Let’s keep that thought in mind and do a little math:

Assume that you perform a rep by taking 2 seconds to lift the weight and 4 seconds to lower it.  That’s a total of 6 seconds per rep.

Assuming you perform each rep uniformly, you would get 5 reps in 30 seconds and 20 reps in 120 seconds.  In other words, the majority of set/rep schemes falls squarely in the range of optimal muscular stimulation.  Not only that, but if you do a few additional calculations, you find:

• 8 reps = 48 seconds (0.75 minutes)
• 10 reps = 60 seconds (1 minute)
• 12 reps = 72 seconds (1.25 minutes)
• 15 reps = 90 seconds (1.5 minutes)

Which are nice, round, easily-measured time intervals, whereas 17 reps = 1 min and 42 seconds, which is just plain weird.

Serendipity is a beautiful thing.  Of course, you could argue Thomas DeLorme chose 10 reps because 10 was a nice round number.  But I like to delude myself into thinking he was merely being a good scientist.

Some of you might argue, “Wait – I saw this big guy lifting and he sure as heck wasn’t doing 6 second reps.” 

The speed of the rep doesn’t matter (for the sake of this argument) – what matters is the total time you spend “under load.”  So long as you’re falling in that optimal range*, you’ll stimulate muscular growth.

Back to the original question: How do I know how many reps I should do in a set?  The answer: It depends.  That will vary according to your goals (e.g., trying to lift the heaviest weight possible once vs. preparing for an MMA fight), but for most people (who just want to look and feel better), any rep range that allows you to fall within that 30-120 second range will suffice nicely.

Stated differently, it doesn’t matter whether you do 5 reps, 10 reps, 20 reps, or 1.25 reps, so long as 1) you’re working hard on those reps and 2) the weight you lift increases over time.

Q: “What about the whole low reps for mass, high reps for definition argument?”

A: Low reps will allow you to lift heavier weights than do higher rep ranges.  Since intermuscular tension is paramount in increasing muscle mass, it makes sense that consistently lifting heavier weights will result in greater increases in muscle mass.  Incidentally, your muscles will also grow with progress at higher rep ranges; it’s just more difficult for most to achieve the kinds of hefty weight loads that really stimulate big muscle gains with higher reps (20 rep breathing squats, anyone?).  Less weight lifted = less muscle built. It’s not some special property of the rep range, and performing a greater number of repetitions won’t magically burn off all the subcutaneous fat that is hiding your muscles from view.

*Addendum: Some savvy readers will recognize that certain populations like Olympic weightlifters still manage to get muscularly impressive while falling far below this 30-120 second set time (try 1.25 seconds).  Note that while not falling strictly into that time frame, weightlifters still lift lots of heavy things, and they do so many, many times.  Additionally, the weights they lift increase over time.  So in their case, they’ve got all the parts of the formula except this theorectical “optimal time frame for hypertrophy.”  Muscular growth happens to be a side effect of the practice they do for their sport; they aren’t purposely trying to increase their muscle mass.