How it Works: Air Displacement Plethysmography

Rachael and My How it Works on Air Displacement Plethysmography… Enjoy!!

Recommended further reading: : BOD POD Evaluation : FDA BOD POD Manual : Dual Energy X-Ray : Comparison of ADP, Hydrostatic weighing and electrical impedance

Epsom Salts: The Inconvenient Truth

Do you suffer from insomnia, muscle cramps, delayed-onset muscles soreness? If so, then your physical or massage therapist may recommend Epsom salt without even knowing why. In its multitude of benefits, Epsom salts are said to improve sleep, reduce inflammation, improve muscles cramps, wound healing and much more. Epsom comes from the name of the English town that the mineral compound, magnesium sulfate, was first discovered. It was extracted by boiling water from a bitter saline spring that people would soak in for great health benefits. The medicinal properties are said to be established by a chemist Nehemiah Grew in 1695, who acquired a royal patent for exclusive manufacturing rights. Today, Epsom salts are a main ingredient in most bath salts and soaks and can be found at almost any pharmacy or bath specialty stores. Before you go take a soak, what does the science say?

Figure one is a molecular model of magnesium sulfate

Well the inconvenient truth is that there is little to no research on the effects of bathing in an Epsom salt bath. There are endless blogs out there with endless claims about the health benefits of Epsom with nothing to back it up. For example, there’s a claim that it’s good for people who are magnesium deficient, but there’s no evidence the magnesium is absorbed through the skin. After searching for articles for magnesium sulfate, bath salts, Epsom salts, I have found not a single article that even investigates Epsom salt baths. The closest thing I found was a patent for methods of different bath soaks, which just boil down to saying put salt in warm water and soak for 15 minutes. Other bloggers seem to have the same problem when trying to find the proof, (in the references below is a link to another similar article.)
I did however find articles about other medical uses for magnesium sulfate. In the study from Maternal-Fetal Medicine Units Network, they wanted to test if magnesium sulfate could help prevent cerebral palsy in preterm babies. 2241 women at imminent risk for delivery between 24 and 31 weeks of gestation, were randomized into experimental (magnesium sulfate) and placebo (control) groups. Each group receive magnesium sulfate, administered intravenously as a 6-g bolus followed by a constant infusion of 2 g per hour, or matching placebo. After a follow up analysis, the rate of the primary outcome was not significantly different in the magnesium sulfate group and the placebo group. However, in a secondary analysis, moderate or severe cerebral palsy occurred significantly less frequently in the magnesium sulfate group. The risk of death did not differ significantly between the groups.

In my references, there is also another article about using magnesium sulfate to treat polymorphic ventricular tachycardia. Long story short is there is no evidence to support the claims people make about Epsom salt baths. I’m surprised there isn’t any actual science considering a folk remedy that been around for hundreds of years. We can only go by What other people say. I can see some merit to the muscle relaxation benefit because magnesium sulfate can be used as a laxative and magnesium chloride is a common ingredient in rescue inhalers, in both cases it relaxes muscles. But again, there is no research on it. The best thing to do is try it and see if it works for you.


Questions to consider

Do you use Epsom salt? If so, Why?

Do you feel it helps?

Any thoughts why there is no research?



Dwight J. Rouse, M.D., Deborah G. Hirtz, M.D., for the Eunice Kennedy Shriver NICHD Maternal–Fetal Medicine Units Network*

N Engl J Med 2008; 359:895-905 August 28, 2008 DOI: 10.1056/NEJMoa0801187


Treatment of torsade de pointes with magnesium sulfate.

D Tzivoni, S Banai, C Schuger, J Benhorin, A Keren, S Gottlieb and S Stern

Circulation. 1988;77:392-397, originally published February 1, 1988


Force That A Resistance Band Needs To Withstand To Induce a One Gravity Environment

In space, astronauts need to work out about two hours a day to combat the muscle and bone density wasting effects of zero gravity. With no gravity, how are you supposed to workout? Don’t you need gravity to hold you in place to do a workout? Until the technology for artificial gravity is invented, NASA had to get a little creative to help astronauts workout. A harness with resistance bands attached to either side (pictured below) is used to hold astronauts in place to run on a treadmill. To most effectively reduce bone density loss, the resistance bands must produce enough downward force to simulate the gravity on Earth, a 1G environment. To choose resistance bands that are suitable for this purpose, you must know the total force that they need to withstand. So, how do you calculate the total tension in each band so that there is a 1G environment?

Astronaut running on TVIS treadmill with harness (left). Simplified diagram of the tension in the resistance bands attached at the hips.


Assuming that the astronaut is of average height and mass (H=1.77 meters, m=80 kg), we can determine the downward force the required for the 1G environment and how high the attachment point is (hips). According to the anthropometric data on the document attached (see bottom for links), we know that the height of the hips are 0.53 of his total height, equaling 0.9381 meters.

To determine what a 1G environment is we use F=ma (where a is the acceleration due to earth’s gravity, 9.8 m/s^2). So now we know that the force in the y direction must equal 784 N. Since there are two bands, each band is only responsible for producing half of that downward force, 392 N.

Before we solve for the tension in the band, we need to know the angle theta. Using simple trigonometry, theta = sin^-1(.9381) = 69.73 degrees. You can also use the width of the treadmill (.7 meters) with the height to find the hypotenuse and then use the Law of Sines to find theta. Either way will get you the same answer.

Now we have everything we need to solve for T.

T_y = T * Sin(theta)

T_y = 392 N (downforce),     392 = T * Sin(69.73)

T = 417.9 N.

This means that each resistance band must withstand 417.9 N of force to produce a 1G environment for a man who is 1.77 meters tall and 80 kg.

Assuming average height and weight is good because most astronauts are very close to the average height and weight of US males. There is not a lot of storage space on the international space station so multiple harnesses is not really plausible. By choosing resistance bands for average height and weight will be able to accommodate most astronauts adequately. I also simplified the body to a point mass, this changes the the angle of theta a little. In this application, I think this simplification is okay because it wouldn’t change the overall tension by a by much. A safety factor would easily cover the change in force that the angle is responsible for.

This information can now be used as a metric to select the proper material and dimensions for the resistance bands in the harness.

Below is a chart of the anthropometric data mentioned above, and my hand-written calculations for this problem.

A Lap Pool on a String?

Swimming exercise method and tether therefor
Patent Number: US 5846167 A
Filed Dec 29, 1997 – Issued Dec 8, 1998
Inventors: You Ching Liu, Samuel O. Engels
U.S. Classification: 482/55; 482/111
6 claims

Figures 1-4 are diagrams of the front and back of the harness, the tether and how the two look when in use.

Are you a swimmer that doesn’t have access to a big lap pool? Do you struggle with keeping your hips up at the surface for the most efficient stroke method? If you answered yes to one or both questions, then the swimming tether is the patent for you. The swimming tether allows you to swim in a confined space, while promoting efficient stroking technique.

The tether can be broken down into three key components: harness, line, and anchor. The latter two components are simple, an elongated, Y-shaped line with a hook or clip at all three ends for two attachment points on the swimmer and one to a fixed supporting object. These two components alone allow for someone to swim in place without moving, due to the constant horizontal reaction force that the line provides. Basically, you are playing tug-of-war with the wall. However, the claim that these two components provide, anchoring the swimmer helps you swim in place, is no different from any other swimming tether on the market. What gives this product the advantage is the claim that the harness provides; gives you the ability to swim in a more efficient technique. The harness attaches the swimmer to the line in a novel way. The harness fits around the torso like a normal vest in the front, and protrudes a little lower than the waist in the back. There are two anchor points on the shoulders to allow for the translation of your swimming force into the line. There is a loop located in the middle of the back just below the hip for the line to be freely threaded through. That loop functions to keep the hips up for stroke efficiency. There is no horizontal force at the hips because it is not a fixed anchor point. The tension in the line induces a normal force on the hips which keep them in line with the shoulders.

I choose this patent because it gives me flashback of training for varsity swimming in high school. Doing pulling drills on sprint days included tying a rope around our waists with the other end tied to the wall and we would have to sprint our hearts out for 30 seconds. It was terrible because the constant tension on our hips caused our legs to sink making us work much harder with our shoulders to keep us afloat and propel us forward. “You want to train like you race” our coach always said, so why were we training with our hips down trying to stop ourselves from drowning?

This patent takes the benefits of swimming in place and perfects them. By attaching to your shoulders and threatening the line through the loop at your hips, it automatically positions you for the most efficient stroke. This product can benefit a wide range of people from athletes to average Joes. People with space restrictions can use this for simulating swimming long distances without a lap pool. Athletes can use it for better training by being able to focus more on technique rather than drowning. By allowing the user to stay in shallow water, use proper form with ease, and its simplicity, this product is ideal for elderly, novices, and injured people for rehab or strength training.

The full patent can be viewed here