The amount of repetitions are decisive for your strength and progress. Fitness trainer and model David Rich explains all you need to know about repetitions:
"When determining the amount of reps to do you must first determine what your goals are. Low reps (1-8) produce the greatest amount of absolute strength. Medium reps (9-20) produce more anaerobic strength. While high reps (21-40) produce mainly aerobic endurance strength. As a bodybuilder variety is the key for muscle growth, so you will want to train in a wide rep range. This range can go anywhere from 1-40 reps with 90% of your training between 5-15 reps. When lifting heavy, around 8 reps or less, you will be using your white muscle fibers more. As you get into the higher reps, around 15 or more, you will be using the red fibers more. While 10-12 reps produces the most volume. Knowing when and how to change these reps is a major part of making results.
Think of every rep as a set and every set as a series'. Don't rush through the reps so you can get to the last one. Instead focus on every rep as if it were an entire set. Focus on proper movement and form, focus on feeling the muscles work, and focus on getting the most out of that rep. Then put that same focus into the next one. This will allow you to get maximum results from every workout."
David Rich's website: http://davidrichfitness.com/
