The Transcendental Size Principle Since the mid-twentieth century, two neuroscience gods, Dr. Denny-Brown and Dr. Henneman, have been vying for claim to one of the most important neuroscience principles ever uncovered: the Size Principle.
What the Size Principle tells us is that there's a fixed, orderly recruitment of motor units. The weakest and slowest motor units are recruited first in any task. It doesn't matter if you're picking up a pencil or squatting 1000 pounds, the smallest motor units are recruited first. The smallest motor units are also the slowest. As more force is required, larger motor units are recruited. The larger the motor unit, the stronger and faster it is.
The largest motor units, however, fatigue fastest. You can't recruit the largest motor units for long before they exhaust and drop out of the movement task. But not all loads require the same level of motor unit recruitment — that's important to understand.
In essence, the smallest motor units contract slowest while the largest motor units contract fastest. This fact is based on the physiological properties of the motor unit, along with the organization of the nervous system.
As a neurophysiologist I must mention, though, that the actual speed of contraction between a small, slow motor unit and a large, fast one really isn't the reason why you can lift a barbell faster when you recruit the largest motor units. The reason why you can lift a barbell faster when you recruit the largest motor units is because you're also recruiting
all of the other motor units. This is an undisputed truth straight from the Size Principle.
It's perfectly logical, after all, that the largest and fastest motor units are most difficult to recruit. Think about the "fight or flight" response. When you're in the fight or flight mode it's easier to tap into the largest motor units. That makes sense given the fight or flight mode was, in ancient times, conducive to getting your head torn off by a raging beast. Such a situation requires you to high tail the hell out of a cave as fast as possible. As Charles Staley recently said to me, "The Size Principle makes you believe in God."
So the ultimate key to bigger, stronger muscles lies within a thorough understanding of how the nervous system controls and regulates motor unit recruitment.
Final Words
I want you to put on your thinking cap and really ponder what I've said. Did I create more questions than I answered? If so, I succeeded. There's a new training philosophy that's about to be uncovered, and don't forget, you heard it here first!
HOWEVER, I don't want to leave you hanging completely. Here's one practical tip I'd like to leave you with, followed by a question I'd like you to ponder. First, the practical tip:
Focus on how fast you can lift a load. Once the speed slows down, terminate the set.
Now the question (this is the key to the concept that will change everything):
If the last few reps of a high intensity set really do recruit extra muscle fibers, then why aren't the last few reps the easiest?
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