"Hypertrophy" has been shown to be elicited best in a 6-12 rep range. 6-8 is sometimes referred to as "functional hypertrophy" where it is a bit more strength based, 8-12 is *generally* the magic zone for bodybuilders.
Having said that, your muscles are split into 2 fiber types (there are actually further sub-groups, but we will stick to types 1 and 2 for simplicity). Type 1 or "slow-twitch" and type 2 or "fast-twich".
Type 1 fibers are more aerobic, so they primarily use oxygen as their energy source. This means that they contract slower, produce less force, and are therefore targeted with lower resistance, higher reps.
Type 2 fibers are more anaerobic, so they primarily use stored energy. This means that they contract quickly, producing higher amounts of force/power, and are therefore targeted with higher resistance and lower reps as they are great at generating force, but fizzle-out quickly.
Now, whenever you lift (or press) a weight, your body "recruits" muscle fibers to do the job. Obviously, the heavier something is, the more fibers must be recruited to move said weight. But, regardless of how heavy something is, your body NEVER recruits all of its fibers into any given lift. This may or may not be an evolutionary thing, as you don't want to completely fatigue a muscle 100% in any given action just in case you still need to use it in times of need.
When you are lifting a lighter weight for more reps, the body recruits *mostly* Type 1 fibers, while the type 2 fibers are *mostly* just resting or coming along for the ride (being lazy). Then, as you increase weight and lower the reps, the reverse effect happens, the type 2 fibers are doing the work while the type 1 fibers are being lazy.
If you are training for strength, there is no reason why you shouldn't be lifting heavier weights in lower rep ranges, as you want to improve strength. BUT, when training for hypertophy, you want to recruit as many fibers as possible to increase your chance of growth.
Type 2 fibers have a greater affinity for growth, but there is still room for growth with Type 1. So, what I believe Body Beast is attempting to do is target both types of muscle fibers with the varying repetition ranges. First, you start off with 15 reps to target the Type 1 fibers, then as you increase weight and lower reps, then you are targeting the type 2 fibers. Not only that, but with training a wider range of rep ranges as you do, you recruit more overall muscle fibers, eliciting a higher chance for muscular growth.
Make sense?
Quote of the day:
"Success is doing ordinary things extraordinarily well."
~Jim Rohn
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