At the 2025 Tour de France there was an open secret being whispered through the peloton: most stage winners were wearing a thumbnail-sized sensor clipped to their heart-rate strap.
The device, made by Swiss company Core, measures a rider's core body temperature in real time - a hidden layer of physiology that can tell whether a rider is thriving or overheating. According to Core, 17 of the 21 stages were won by riders using their sensor-prompting speculation that it offered a winning edge.
