Dr. Andy Galpin

Other muscles like the soleus, spinal erectors, other muscles say in your fingers or eyes are not necessarily going to change as much.

It would be very difficult to convert your soleus into an exclusive fast-twitch muscle fiber.

Because it's really meant to do one job, and the overall stimuli that you provide to it is fairly similar.

It doesn't really matter what kind of exercise or training intervention you put, how many plyometrics or sprints you do.

The vast majority of the time that your soleus is contracting, it's still because you're standing.

And so there's just not a lot of change that can happen there.

The VL is quite different since it is so important in all knee extension and really hip movement.

It's going to be hyper responsive to what you're asking it to do.

So again, can vary heavily within your body as well as from person to person.

And we'll talk about more of this stuff later, but as you've already heard me allude to,

Your fiber type, again, the proportion of fast and slow twitch, is hyper-responsive to lifestyle.

We know extensively for decades now that it will respond to both changes in exercise as well as the removal of that.

So some of the classic Dallas bed rest studies where they put people into bed where their head was something like six degrees below their legs, so you can imagine laying on a bed with a slight backwards tilt, so your head is a little bit below, your feet are a little bit above,

And we've done this for up to 50 plus days.

And what we're doing there is trying to simulate both space flight.

This was initially done in preparation for thinking humans were going to go to the moon or further for extended time, as well as it's actually representing a nice model of aging.

You can actually simulate upwards of a decade of aging in about 10 days of bed rest or so.

And so we get a really good grasp of what happens when you go into disuse, similar for if you have to go into a cast post-surgery or something like that.

So we see in those models of forced inactivity, as well as any number of exercise training studies, looking simply at individuals cross-sectionally, so those that have exercised for long periods of time throughout their life compared to those who haven't, any of these interventions you want to look at.

And we see hyper-responsiveness in your fiber type profile based upon your physical activity.