in case you were wondering, like me:
Athletic Wearable Trackers (GPS Vests)
Function: Many professional (/college) players wear a tight, cropped vest (resembling a sports bra or harness) directly over a base layer or skin.
Technology: These vests hold a small GPS pod between the shoulder blades that tracks distance covered, top speed, heart rate, acceleration, and physical exertion for sports science and performance monitoring.