Dr. Andy Galpin

Now, all of those fibers within that motor unit will be of the same fiber type, so fast twitch or slow twitch.

And they're not necessarily laying next to each other in the muscle.

They are spread out both superior and inferior.

They're deep within the muscle belly.

They're kind of all spread out there.

And this allows you to control and not have spastic movement.

So when you activate a motor unit, not only does this contract all of the muscle fibers in that unit, it contracts them all fully.

That's called the all or none principle.

And so when you flip the switch and turn on that motor unit, you pass that activation threshold.

it activates and contracts all the muscle fibers in that unit to their fullest potential.

What you can't do is turn a muscle fiber itself on at 60% or 70% or 80%.

So the only way you relegate force production across an entire movement is to actually turn on or off more or less motor units.

The example here, say I would like to take my hand

and reach it up towards my eyeball and lightly touch my eye.

Now, I recognize and realize this is only going to take a small amount of force to move the weight necessary to move my hand to lightly touch my eye.

If I were to do that too hard, I would smash my finger into my eyeball and that would hurt.

And so what I do is I start with what are called low threshold motor units.

So these are motor units that are very susceptible to firing or contracting.

And they activate, in this case, very slow and metabolically efficient muscle fibers, the type 1 muscle fibers.

And so what it says is I kind of start with the least amount possible.