Hi.
This is Dr. Dale Wagner, an Exercise Physiologist
in Health Physical Education Recreation Department
at Utah State University.
We're here today in the lab to do a body composition
measurement using air displacement plethysmography.
It's better known or more commonly known as the BOD POD.
It's a pretty simple device.
You'll see that doesn't take a lot of technician skill,
nor a lot of client participation.
It makes it easy to use.
That's a huge advantage.
It is a laboratory method.
So we cannot just cart this device around with us.
And as a laboratory method, it's reasonably accurate,
fairly accurate.
It has similar accuracy to underwater weighing.
but a much easier procedure than underwater weighing.
So as with any of our body composition methods,
we get some preliminary information.
First, we've already collected that on our subject.
That would be basic descriptive data like age and height.
So we take that information, already entered it
into the machine.
And now, we're ready for the test.
So as you can see, Jen, our client,
she's wearing minimal clothing with a swim cap.
That's critically important to this measurement.
The more clothing that you have on,
anything that is compressible will
create an error in the machine.
As part of our test, there's a self-calibration procedure.
So what's really happening here is
it's measuring the volume of the chamber while it's empty.
And then if we put any object inside the chamber,
like our calibration cylinder or a subject,
that's going to take up a certain amount of volume.
And that's really what this device is measuring.
It's measuring volume.
If we know the person's mass, which
we're going to measure on the scale and we know their volume,
then it's easy to figure out body density.
Body density is just body mass divided by body volume.
So we place our calibration cylinder inside the chamber.
OK, and now, we're ready for our subject.
And the first thing we're going to do
is have our subject step on the scale.
We're going to get her mass.
It's going to be critically important
for us to measure body density.
And now, we're ready for the BOD POD test.
And you can have a seat in there.
And it's a simple measurement.
We're just going to have our subject sit calmly
in the chamber, breathe normally.
I'm going to close the door.
The door is really only closed for about 20 seconds.
And then we open it up and get a second measurement.
And that'll be it.
Are you ready?
Yep.
OK.
So the BOD POD works off of a pressure/volume relationship.
And we have a known pressure and a known volume in the chamber
when the chamber's empty.
And then, of course, once we put a subject or a person
in the chamber, that's going to change
the pressure and the volume within the chamber.
And that's really how we're calculating the person's body
volume.
It doesn't take very long.
Her first trial is already finished.
So we're going to open up the door and do a second one.
And this will be it.
We can do a more sophisticated measurement
that we sometimes do for research
that includes what's called a thoracic gas volume
measurement.
If we put a little breathing tube inside the chamber,
we can have the subject breathe through the tube
and then do a breathing maneuver.
The purpose of this would be to measure the amount of air that
is in her lungs that will affect the person's body volume.
We're not going to do that right now.
We often just estimate that.
There's a prediction formula that we
can use based on the person's age and their height that
will give us a prediction of their thoracic gas volume.
But for research purposes, we would actually measure it.