Dr. Andy Galpin

And so the way that these motor units are set up is in large part determinant of how they produce force and then again what the function of that muscle actually is.

So to go back to what we were talking about a second ago, if I'm trying to produce this finger into my eyeball and I need to produce more and more force, I slowly activate more motor units, right?

If I've realized the point where I need more force beyond that, I will increase and start activating the higher threshold motor units that also now start to tend to be faster twitch motor units.

And if I need extreme amounts of force, I will activate the highest threshold, which are the highest and probably where we're going to get into our type 2x fibers if we have any of them at all.

And so what happens then if we're playing the story backwards is any movement at all in a muscle almost always includes the slow twitch motor units.

But only when I start demanding high force production do I activate and engage the fast twitch muscle fibers.

If I then go weeks, months, years, or decades without doing anything of high force, those motor units just never get activated.

Eventually, what can happen is denervation, which means the nerve stops attaching to the muscle fibers.

Many of the muscle fibers die off.

We talked about that earlier.

The amount of muscle fibers in a muscle reduces with aging if you don't know anything about it.

And then also we see changes in motor units.

So not only is our ability to produce force go down, but our motor control goes down because we've lost the amount of motor units going into a muscle.

Sometimes we can see what happens called muscle fiber type grouping.

And so we'll see a nerve innervation, an axon that is gone, if you will.

The muscle fibers are still viable.

And so a neighboring motor unit will actually obtain them.

And if they were, say, a fast twitch muscle fiber, but now they're in a slow twitch state.

motor unit, they will convert that fiber type over to slow.

And so you see these big patches or groupings of slow fibers, as opposed to them being spread out throughout the muscle, which reduces our motor control and smoothness and muscle contraction.