That's interesting chris... so does exercise over 128bpm not burn much in the way of fat (only glycogen presumably?)? Also, how does that scale with RHR and body weight? My RHR is 48 and weight is 61kg (13 more than I was two and a half years ago), so would that be higher or lower for me? I often ride up over 160bpm when climbing (no HRM so can't tell you precisely, but counting beats says it's close to or beyond 3 per second which is obviously 180)