Walk into a typical gym and you're sure to find a wide range of training styles.  Some people spend 15 minutes or more doing the same exercise, while others bounce back and forth between machines circuit style.  One serious exerciser might count his repetitions meticulously, while another goes on feeling alone.  Who is right? Or more importantly - who knows why?

Sets and reps are the main training variables in an exercise routine.  "Reps" refer to the amount of repetitions performed of an exercise and sets refer to the "chunks" of reps, separated by rest intervals.  Multiply reps times sets and you have the total number of times you performed an exercise.  The general oversimplified mindset is that sets of 8 reps will make you big, 10 reps will make you strong, and anything above 15 reps will tone you up.  The truth is that these numbers are simply a means to an end.  A more resourceful way of approaching your routine is to think and plan in terms of time under tension (TUT), or the length of time a muscle is working.

Here are two examples:

To shape and tone her legs, a woman performs 20 leg presses in a flash, finishing her set in about 22 seconds.  In the pursuit of big biceps, a gentleman picks up a heavy bar bell and performs 8 reps at a quick pace - done in about 18 seconds.  Neither of these people will achieve their desired outcome due to an overemphasis on reps and a total neglect of time under tension.  Indeed, the time of the set is so brief, that they are eliciting more of a power adaptation in the muscle. 

Combining Reps with Time Under Tension

To get the best results, begin by choosing the proper TUT protocol and apply it to your desired set/rep scheme.  Here are the TUT guidelines:

For power or strength - 5 to 25 seconds.  

For muscle growth - 30 to 70 seconds.  

For endurance - 60 to 100 seconds.  

In regards to the above example, a better way to stimulate growth in the biceps would be to do curls in a 3 seconds up, 3 seconds down fashion for a total of 10 reps.  This will equal 60 seconds of TUT per set - triggering a growth adaptation. The female in the above example would be better off increasing the resistance and going for a 15 rep set of 2 seconds press, 2 seconds negative, 1 second pause in the bottom position.  This 5 second repetition would yield a 75 second set - creating endurance and stimulating a bit of muscle growth.  

TUT for strength is small due to the necessity of heavier weights. Strength reps do not necessarily need to be sped up or slowed down, rather, the weight necessitates lower reps - 3 to 5 range.  Power based exercise is kept to a low TUT because the body can only create energy for high-power moves in short bursts.  Fast ballistic movements can be dangerous once fatigue and lactic acid begin to accumulate.  

Of course, there are two other important training variables to consider.  First, there is an inverse relationship between the number of sets and reps - more repetitions must fit into fewer sets and vice versa.  Also, rest time between sets goes hand in hand with TUT.  As a general rule, rest 2+ minutes for strength, 1-2 minutes for hypertrophy, and 30 to 60s for endurance.

 Applying these fundamental rules of exercise physiology will make for more productive workouts and may even reveal some strengths and weaknesses you didn't know you had!