As you exercise your core temperature goes up, your body reacts and increases blood flow to your skin and in effect reduces the amount to your muscles. To keep the same flow to your working muscles your HR will have to go up. Similar to a having a higher HR when working out on a hot humid day. It's all relative. I ride an elliptical at times and do long steady state workouts on it. I get up to say 155 (my 90%) and hold there. Eventually my HR starts creeping up and I ease off the effort to keep it there. I've used a fan but I can't say it changed much for me but others say it helps. Hydration goes down during the workout probably adds to the effect.
Last edited by u235; 06-15-19 at 01:12 AM.