I always heard when I first started lifting "Lift heavy to get big.. lift light to get defined". Now let's talk about how that is not entirely correct. Lifting light weights for high reps is going to build muscle endurance..not size. And you need muscle for definition so that theory is out. Why don't light weights work for muscle growth? Well, without going into too much boring detail of muscle fibers and how they work.. Type 1 fibers have a very limited ability to hypertrophy (muscle growth) and these are the fibers you are activating when lifting light weight for high reps(anything over 15 reps really). Type 2B fibers have the greatest potential for growth. So how do we activate them?? GO HEAVY! they are only activated under heavy resistance. How heavy? how many reps? ... Keep it within the 6-12 rep range using weight you can only lift in good form to said amount of reps. Anything under 6 reps and you are lifting for power at that point. You still activate the type 2 fibers but power lifting does not allow for sufficient time to activate all motor units in an orderly fashion therefore diminishes the hormonal adaptations of the entire span of muscle fibers. Power lifting does play a beneficial role in ones routine though. Especially squats and dead lifts. These heavy lower body exercises increase serum testosterone concentrations and growth hormone more than any other exercises. So to sum things up on how you should lift to grow properly I'll break it down like this..

Lifting really heavy (85-95%) of your 1rm will increase power better than any other lifting volume. However, it doesn't supply for adequate muscle growth.

Lifting too light (anything over 15 reps) and you will only increase muscle endurance because this isn't a heavy enough load to activate those type 2 muscle fibers for hypertrophy.

So what's the idea range??

The idea is to do resistance training that is high in intensity utilizing 6-10 repetitions, heavy resistance, and a maximum rest period of 60 seconds between sets.This will maximize serum testosterone and growth hormone levels,allowing for successful recovery,adaptation,and muscle growth.