With preseason underway across the Premier League, you're going to see a lot of photos of players training with their now accustomary black vests on. A few years ago, these garments were an absolute rarity, and, apart from some sophomoric comments about male players wearing sports bras (how dare you), it was generally assumed that they measured heart rate and yada yada yada.
The story went something like this: you return from the offseason having enjoyed a few adult beverages and burgers, you do some preseason fitness training, the physio comes over and tells you that you're now fat.
Everyone is satisfied with this meager explanation of what the black vests do. But, in actuality, the tech inside those small garments is capable of a lot more than that.
Have you ever wondered why the players wear those black vests in training?
— Crystal Palace F.C. (@CPFC) July 7, 2017
As this video shows, it's not just some guy with an iPad app seeing who can run the fastest and who's the most fat.
As well as tracking heart rate and speed, the GPS device can also record distance covered, acceleration and deceleration, dynamic stress load and other statistics using the GPS module, a heart rate receiver, long range radio and 3-D accelerometer, digital compass and gyroscope.
These statistics are then used to design performance plans for each individual, to prevent injury by monitoring workloads and to help design and facilitate individual training sessions.