Heart rate

Supernoob

Dear readers,

I am a beginning to train train for my first triathlon. My height is 168cm and weight is 62 kg. I regularly run 5km under 20 min. However, during each training and/or race my heart rate drops after 15 min (consistently) from 165+ to around 120. I do finish my trainngs and races. I have no symptoms shortness of breath, chestpains, dizy or other cardiac symptoms

My hometrainer trainings consist of 1 hour trainings at around 160 to 180 watt. My heart rate does not drop below 160. I have no medical conditions (that I am aware of) and am a healthy 28 year old male.

Do other people have similar experiences?

kind regards,