Rep ranges are a funny thing. Lots of people talk about doing this many reps for this many sets but really have no idea why they are doing that particular number or the effect it will have on their bodies.
Generally, lower repetitions like 3-8 are most productive for creating absolute strength. Repetitions in the medium range like 10-20 produce anaerobic strength endurance. The higher rep ranges between 20-40 produce aerobic strength endurance.
The reason for such varied ranges even within a category of rep range is because people will take varying amounts of time to complete the rep. We know that it is not the actual number of reps but the time that the muscle is under tension that really causes the adaptative response. So if you do 4 reps that take 6 seconds each and I do 6 reps that take 4 seconds each, we have both worked in the 3-8 range and caused that adaptation response that would be caused by putting a muscle under 24 seconds of tension.
Now on to fiber types. There are actually 3 major types of muscle fibers in the body that we are concerned with. Type 1, type 2a and type 2b. Some people have natural tendencies to do well at certain activities more so than others and this is because of the ratio of one muscle fiber type to another, among other things. Type 1 muscle fibers (or slow twitch muscle fibers,sometime called red) are your endurance muscle fibers because they are very resistant to fatigue and injuries. The sad part is that their ability to produce power is very low also.
Type 2a muscle fibers (fast twitch muscle fibers - intermediate - sometimes called white) are much larger and stronger than type 1 muscle fibers. These fibers have a high capacity for glycolytic activity and can produce high force output for longer periods of time.
Type 2b muscle fibers are the extreme end of the power scale. These are the survival fibers. The whole purpose for 2b muscle fibers is to allow enough power and strength to survive emergency situations. Sixteen percent of an inactive persons body is 2b fibers. Now even though these fibers are powerful, they have no resistance to fatigue or injury. Fact of the matter is that if and when you need to use these guys, most untrained people will damage that tissue beyond repair.
Now I've said all that to say this. Research has show that it is the white muscle fibers, the type 2a and 2b that give the greatest returns in size and strength when trained. These white muscle fibers are for high force output, that means lots of calories burned along with a tissue that requires alot of calories to stay alive.
So what about the red muscle fibers, the type 1? Well your body becomes efficient at what it is needed for. So if all you do is only cardio and other high rep activities, your body is going to favour the development of type 1 muscle fibers and in the end rob you of your ability to achieve your maximum strength, size and power. What if strength, size and power are not your goals? Well not having at least some of those attributes will result in a supressed metabolic rate. That means you will not burn as many calories in a day. This means that if you are on a weight loss program, you need to work the type 2a muscle fibers in the 10-12 rep range in order to boost your metabolism and stay strong and healthy.
Now the body can only handle increasing levels of stimulus in a particular area for a given amount of time. This is why in good programs you will find several totally differant workouts, each stressing a differant muscle fiber type for a given period of time. An individualized program has the majority of the workout time spent in the phase that best effects the goal that is trying to be achieved.