S&S Detailing- That's an interesting study you're taking part in!
Noting that, yeah, you *are* a big guy!...what does it take for you to burn 400 calories on the treadmill (speed, time, how hard you perceive it to be)?
Are you/they working from actual max heart rate or "age-related" maximum ("ARM")? Since I'm so old (50) I don't work off the ARM as it'd have me working way below what I consider a "real workout". At my age, that 170 bpm (what you do for your hard work and 100% of my ARM) is good and demanding, and what I do for my hard intervals.
I did a home-brewed version of determining my max hr (185) and it was *extremely* difficult; awfully half-@$$ed compared to what you went through, but I can't imagine working any harder and maintaining consciousness as I was starting to "lose time" as it was. I shoot for 90-some% of that for my hard intervals, and do almost all my cardio as interval training.