One of the first things that I wanted to learn when I was studying to become a personal trainer was how and why certain rep ranges did certain things to your body. Why do I do 8-12 reps to get bigger, but 1-5 to get stronger? Why isn’t it the other way around? I’d seen plenty of programs on the internet that told me how many times I had to lift something, but none that told me why.
The reason why different rep ranges matter is something called the SAID Principal; it stands for Specific Adaptations to Imposed Demands. This basically means if you try to lift a heavy weight once, your body will adapt to make it easier to lift that weight next time. That’s what weight training revolves around, imposing a demand on the body that will elicit the desired adaptation.
But why does lifting heavy weights for low reps elicit one adaptation, while lifting moderate weight for high reps elicits another?
Think about it like this:
Simplify the muscle into two parts The myofibril is the engine of the muscle. It’s what makes your muscles strong.
The sarcoplasm is the gas tank of your muscle. It holds the fuel to power the engine.
Let’s say you wanted to drive your car for a mile at 100mph. Your engine would have to be powerful enough to make you car go that fast. Now if you wanted to drive a car for a mile at 200mph, you would need a more powerful engine to make it go faster Likewise, if you wanted to lift 100lbs once, your myofibrils need to be strong enough to lift that weight. They would then need to get stronger in order to lift 200lbs once.
Now that your engine can go 200mph, let’s say you wanted to drive at 200mph for 10 miles. Your gas tank would need to be big enough to hold enough fuel to power the engine for that long. Likewise, if you wanted to lift 200lbs 10 times, your sarcoplasm has to be big enough to hold enough glycogen (muscle fuel) to power the myofibrils for that many reps.
This relationship between your myofibrils and sarcoplasm (engine and gas tank) continues in this way. To lift a heavier weight, your myofibrils need to be stronger, and to lift a weight more times, your sarcoplasm has to be bigger so that it can hold more fuel.
So as your myofibrils (engine) increase in strength, your sarcoplasm (gas tank) has to increase in size to power them.
This is why elite bodybuilders are huge, while elite powerlifters are so strong. They primarily train in a certain rep range because they know that range will cause a specific part of their muscle to adapt in a certain way. This of course is a simplified explanation of how muscles work.
Bodybuilders are still pretty strong. They need a powerful engine to move such heavy weight 8-12 times; the same is true for powerlifters and their gas tanks. The point is the specificity of what they train for.