Want to give your strength training a real lift? It helps to understand a little something about repetitions, the number of times you perform a particular movement. The rep range you use dictates how your muscles adapt to your routine. In fact, by knowing the benefits of three key rep ranges, you can choose the strategy that's best for the results you want.
Low repetitions (1 to 5): Work muscles hard
This rep range allows you to use the heaviest weights, which puts your muscles under the highest amounts of tension. This increases the number of myofibrils in your muscle fibers. Myo-what? Myofibrils are the parts of your muscle fiber that contain the contractile proteins. Think of it this way: When there are more of these proteins to contract, your muscles can generate greater force. That's why 1 to 5 is an ideal rep range for building strength. And, of course, more myofibrils increase the size of your fibers, making your muscles bigger.
High repetitions (11 or more): Boost muscle endurance
When you use higher reps, your muscles have to contract for long periods of time. This increases the number of mitochondria in your muscle fibers. Your mitochondria are energy-producing structures that not only burn fat (the more, the better!) but also lead to greater muscle endurance and cardiovascular fitness. What's more, these structural changes boost the fluid volume in your fibers, adding size to your muscles.
Medium repetitions (6 to 10): Best of both worlds for muscles
With this approach, your muscles are under medium tension for a medium amount of time. Consider this just what it is: a mix of low- and high-rep lifting. So it helps you improve both muscle strength and muscle endurance. You might say it strikes a good balance between the two. However, if you use this rep range all the time, you'll miss out on the greater tension levels that come with lower reps and the longer tension time achieved by higher reps.
Keep in mind that these rep ranges don't work like an on-off button; they're more like a dimmer switch. As you move up and down in reps, you're simply dialing back the benefits of one and emphasizing those of another. Use them all.
Source: Pure Matters http://resources.purematters.com/fitness/strength-training/how-many-reps