As completely nerdy as this sounds, I have always found the idea of the Neuromuscular Junction completely fascinating. The idea that the brain, in the tiniest fraction of a second, sends an impulse from to the muscle via a nerve. This all happens billions of times each day!
The motor control center in the brain is located in the cerebellum which is close to the brain stem. This area of the body is one of the most primitive and first to develop in the growing fetus. The cerebellum is the beginning of the neuromuscular system. It is connected the muscles by motor units. There are many cells in a motor unit. Once a motor unit fires, ALL the cells in the motor unit contract. This is called the “All or None Principle.” The quantity of cells in a given motor unit is determined by how much precision is required for the specific action. Muscles that execute precise, fine motor control (like the fingertips) have fewer cells in their motor unit, while muscles that perform bigger tasks (like the quads for jumping) have more cells in their motor unit.
If a small force is required, only a few motor units of the muscle will be involved, a larger force will use more. That is intuitive. The part that is less obvious and more interesting is the following: Light effort involves “early firing motor units.” When more work is added, more motor units are added but the initial ones firing STILL work with minimal force. Additional motor units that are added work with medium force and finally the last motor units work with maximum force. It is like a running a relay and putting your slowest pacers in first, then the average ones, and then the fastest!
Take home message? Light workloads (no hand or ankle weights, very little weight bearing) work the same “early firing motor units” each time. To work the entire muscle we add the weights or increase the intensity and duration of weight bearing activity.
More to come...stay tuned for the an explanation of "muscle memory" and how we experience it in our every day tasks.
McArdle, W. D., Katch, F. I., & Katch, V. L. (2006). Essentials of exercise physiology. Baltimore, MD: Lippincott Williams & Wilkins.