Dr. Andy Galpin

Physiology is very lazy like this.

And so I'll start with the least amount of force necessary.

And this allows me to move very slowly but with a lot amount of control.

And now if I realized that actually I had a five-pound weight attached to my wrist and I had to lift my hand with more force than I thought I needed, I would turn on more and more motor units.

And if I realized that I had that five-pound weight and then I added a 10-pound ball to my hand and I need more and more force production, I would slowly turn on more and more motor units.

As we continue up this cascade, again, the size principle is telling us we start with the lowest threshold, smallest motor units.

So this is not only tends to be slow to muscle fibers, but it's also oftentimes, but not always, motor units that have less muscle fibers in them.

The amount of muscle fibers in a motor unit can vary highly.

Several of them in your eye can be as low as a couple of dozen muscle

muscle fibers in the motor unit, and those in your glutes might be several thousand muscle fibers.

So even if I activated one motor unit in my eye, I would get a small amount of force.

One motor unit in my glute might produce hundreds, if not higher, amount of force per motor unit because I have more muscle fibers in that.

In addition,

The muscle fibers might be of larger size in the glute and smaller size in the eye because we need more motor control there and less force production.

Something like the eye, again, is meant to be highly precise with its movement, very low margin of error.

I don't need to get there particularly hard.

I need to get there sort of quickly, but I need to be under control.

The glute, in this example, this is your butt muscle, any of your large butt muscles.

They don't necessarily need to have a lot of fine-tuned precision with the movement.

They just need to contract with a lot of force.