Any training program should begin, first and foremost, with setting a goal or goals. Secondly, you should determine either your 1RM (1-rep maximum) or 10RM. If you look around online, you can find many different ways of calculating such a thing. You can choose to either test yourself to determine your 1 or 10-rep maximum, or you can try various testing procedures to estimate your 1RM or 10RM.
Testing your rep maximums is not necessarily imperative, but it can be helpful to accurately choose proper resistance to hone in on your repetition goals.
If you decide to calculate your 1RM or 10RM, you can then follow this table to aim for specific reps:
%1RM - Number of Repetitions
100 - 1
95 - 2
93 - 3
90 - 4
87 - 5
85 - 6
83 - 7
80 - 8
77 - 9
75 - 10
70 - 11
67 - 12
65 - 15
So, a quick example on how to use this table: If you can bench press 250 pounds 1 time, you should use 218 pounds for 5 reps. Make sense? Good, let's move on.
What training goal should I aim for?
Hard to say, that is usually up to the individual, but for most folks who are just looking to "get in shape", usually the hypertrophy and muscular endurance categories are mostly used. Both of these categories allow for a fair amount of strength increase, but also allows folks to burn calories, etc. Individuals who are completing P90X are generally training is that rep range as well.
Strength training and power training (for the most part) is targeted for specific sports and athletic performance. Not only that, but as I will explain later, strength and power training repetition goals should only be used by experienced resistance-trained individuals, done with safe, proper, equipment, monitored by a certified strength and conditioning specialist, and be performed as "core exercises".
Strength vs. Power Training
I don't intend on making this a science lesson, so I will do my best to explain this in simple terms. Strength is the amount of force a muscle or muscles can generate to move a load or weight. For example, an individual may be able to max out their bench press at 250 pounds. By lifting that 250 pounds, once, they can generate a lot of strength to move the weight up one time, but the repetition may be slow.
Power, on the other hand, is being able to generate force at a much higher rate. For example, an individual who can bench press 250 pounds, 1 time, would want to drop down in weight, in order to press the weight at a faster rate.
Strength training can be valuable in many facets of athletics, however, in many cases, power is far more valuable to perform better. Let me use this example, Usain Bolt has very strong legs. I am assuming he uses strength training to make his legs strong. Having said that, there are many individuals who have just as strong, if not stronger legs than he does.
The difference here, is his ability to generate force, or power. He is able to generate a tremendous amount of power to propel himself down the track.
"Core" vs. "Assistance" Exercises
Strength and power repetition ranges are intended to be done by "core exercises". A core exercise is one that recruits one or more large muscle areas (chest, shoulder, back, hip, thigh), involve two or more primary joints, and receive priority when one is selecting exercises because of their direct application to sport.
"Assistance exercises" on the other hand, usually recruit smaller muscle areas (upper arm, abdonminals, calf, neck, forearm, lower back, or anterior lower leg), involve only one primary joint, and are considered less important to improving sport performance.
Strength - High-load, low-repetition training to improve overall strength of the muscle(s) being trained. Should mainly be targeted by "core exercises". Long periods of rest between sets to allow full recovery of muscle fibers in order to produce maximum amount of force every subsequent set. (not applicable to P90X)
Power - High-load, low-repetition training similar to strength training but designed to increase the overall explosiveness of the muscles. Also designed to be targeted by "core exercises" only. Also intended to have long periods of rest between sets to promote full recovery. (not applicable to P90X)
Hypertrophy - Higher repetition goals when compared to strength or power training. Shorter breaks as well, as the intention is to cause a higher metabolic demand in the muscle fibers to promote muscular growth.
Muscular Endurance - Low rest periods, higher rep goals, lower loads. Ideally used to improve the aerobic efficiency of the muscle fibers. Should be used by endurance athletes to improve muscular efficiency. Not intended to improve overall strength or power, however.