How Many Reps?  In How Much Time?
Brad Carlson – Certified Muscle Activation Therapist and certified personal trainer with: ACE AFAA and NASM

If you had 60 seconds to do one set of repetitions, would it be better to do 15-20 reps or 8-12 reps?  Obviously more is better right? Think again.  If you were to do 15-20 reps in 60 seconds, you would do 1 rep every 3-4 seconds.  If you do only 8-12 reps, you’re doing one rep about every 6 seconds.  That’s almost twice as slow and you’ll complete fewer repetitions in those 60 seconds. 

So why would you want to do less repetitions in that 60 second time frame?
 “It’s not what you do, but how you do it!”…..so when it comes to your body, you better do it right. Moving the resistance faster takes away the intensity and increases the risk of injury and soft tissue damage.  Keeping tension on the muscle and moving it under control, recruits more of the muscle fibers, therefore, increasing muscle stimulus.  This increased muscle stimulus is what gets the muscle fibers to respond to size and strength gains (with the help from rest, hydration and proper nutrition).

Use 2/3 of the weight you can normally do for say, a biceps curl.  Now do the curl and move your arm up and down faster than you normally move for a few repetitions.  Now, with the same resistance, move slow and controlled really focusing on tightening the muscle and squeezing it at the end of the range.  Then lower it slowly keeping tension on the muscle throughout the range.  Feel the difference?  Think about how many less repetitions your going to do and yet, how much you can increase your results and reduce your risk of injury.