joking aside this is actually true regarding heart rate and it's important to exercise at all brackets. A close personal friend of mine is a regular marathon runner, does triathlons, training for a half ironman etc so his endurance at 50-85% heart rate is not in question but after we took up squash together for fun he quickly realised his high intensity endurance was much lower lol. We peak 600-700 calories per hour doing that shit.