I am doing intervals at 90% max. heart rate 2x per week (trying to replicate this study [PDF]) However, on some days the heart rate does not go up with intensity. I always run up the same hill - on some days the heart rate goes up quickly to 170 and I can keep it there with sub-maximal effort (if I push really hard, it goes pretty quickly to 185). But some days it barely gets to 160 and I cannot push it higher even if the effort feels harder.
At first, I thought that it could be symptom of tiredness or upcoming illness but it does not seem to be the case - sometimes I feel great after a few days of rest, I slept well, looking forward to pushing it hard... and the heart rate just does not go up. I assume that if this was caused by some heart condition, it would be like that all days. But it fluctuates seemingly randomly - one training it goes up easily, the next week it does not (in which case I stop exercising after the first interval and just take a walk). Any idea what could be the cause?