The "spike" ie a product of how much the work interval drives the HR up, combined with how long the rest period is that allows the HR to recover. That's the main difference in the top two -- a short rest interval, so HR recovery doesn't really being to take place and then work is done again... much more like (though still a few differences from) steady state cardio.I was really surprised just how smooth those top two are, especially the top one. I thought there’d be a way more pronounced spike for each single! (With my very little knowledge of HR training or monitors)