Using an advanced system of body fat measurements, Alabama trainers were able to put Derrick Henry on the perfect diet.

Derrick Henry is big—that’s a fact. And while you may be able to discern that with your naked eye, the training staff at the University of Alabama needed to quantify it for themselves.

So when Henry showed up on campus, he squeezed his 6' 3", 240-plus pound frame into a Bod Pod. The device, which looks like a space ship escape pod from a sci-fi flick, uses an extremely high-tech system to measure exact body fat percentage through body density.

Henry is already built like a battleship, and the Bod Pod employs a scientific method similar to the one used to measure ships. When a boat is placed in the water, its mass is determined by how much water is displaced—not unlike measuring your weight by seeing how high the water rises when you get into a bathtub.

The Bod Pod swaps out water for air. It’s called Air Displacement Plethysmography: Once inside the Pod, a computer changes the air pressure and measures how the volume of air inside the chamber is affected. An empty chamber would see no change, but once a person is inside, the air reacts to that person’s exact body composition.

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A person’s fat and non-fat mass responds different to air pressure, which allows the computer to determine exact body fat percentages in a way that is more accurate than old-school calipers. It also gives a database of results that can be measured and compared over months or years, giving trainers the ability to chart and map gains and losses of their athletes with incredible precision.

All of this measuring told the Alabama staff that Henry was in ridiculously good shape—too good of shape, if such a thing exists. With such low body fat percentage, Henry was at an increased risk of cramping during games, especially in torrid Tuscaloosa.

That meant Henry needed to put on fat, which was no small feat for a guy blessed with a freakish body and an incredible metabolism. Luckily, Alabama’s professional-grade nutrition staff was able to put together meal plans for Henry, grabbing him after his legendarily intense lifting sessions to make sure he had the right kinds of protein shakes or fruits and vegetables in his system at the right times.

From first glance, Derrick Henry may have looked like the perfect football player when he showed up on campus. But thanks to the Bod Pod, the staff was able to identify a potential problem and snuff it out, en route to a national title and Heisman Trophy for the biggest running back in football.