I used a triac to control the power going to the fan, and used an Arduino 101 to control the triac. I programmed the app myself to read heart rate, power or speed. And then send a BLE signal to the Arduino to control the triac to set the fan speed.
It was something I did while I was playing about between jobs. It ended up taking a lot of work, but kept me occupied.
Buying a Headwind would be quicker. And maybe cheaper. But not as fun😁