Last two days at the gym:
1 mile on treadmill
20 min on elliptical
20 min on bike
2 x Fish Game! on rowing machine (about 1600 meters, 10 min)
30 min elliptical
30 min bike
So, I don't know about your gym's machines but at the gym I go to, each machine has a little heart rate chart on it. On the left side of the chart is a set of values that correlate age and optimal heart rate for "fat burning". On the right side of the chart is a set of values that correlate age and optimal heart rate for "cardio". The reason I'm going to the gym is to "get healthier" and more specifically to lose this damn gut! So, while the "cardio" side of the chart seems intriguing and is useful, I should probably be more interested in the "fat burning" side of the chart.
Well for my age, it says that I should be at a heart rate around 130-140 for fat burning and 150-160 for cardio. At this point I usually tilt my head to the side and give a hearty "Whaaaa?". See, when my heart rate is at 130-140 I'm barely even moving in my opinion. My breathing is only slightly heavier and I definitely don't even break a sweat. At 150-160 I'm starting to "feel it" and start heavier breathing and sweating, which to me feels like I'm doing an actual workout. So where's the discrepancy?
After reading multiple websites about heart rates and the many heart rate "zones" I feel like I finally understand the low rate. It turns out that at the lower heart rates your body burns a higher percentage of fat and a lower percentage of carbohydrates (sugars). The actual number varies widely from site to site, but that's the general consensus. A couple of examples with made up numbers:
In your "fat burning zone" you burn 100 calories. Lets say 50 of these are from carbohydrates (immediate energy source) and 50 come from fat (stored energy).
In your "cardio zone" you burn that same 100 calories. Let's say 70 of these are from carbohdrates and 30 are from fat.
So you can see the "fat burning zone" is more efficient at burning fat. However, the point to note is that while the fat burning zone is more efficient, you will generally burn total calories much slower than in a "cardio zone". Take this example from this website:
So, let's say you're exercising at a fairly low intensity that burns, oh, 100 calories in a half-hour. Let's say that 70 percent of those calories come from fat. Your neighbor, however, is working out much harder, outside the magical "fat burning" zone: She's burning up, say 300 calories in that same half hour, but only 50 percent of those calories are from fat. Now do the math. You're burning a higher percentage of fat, but 70 percent of your 100 calories equals 70 fat calories burned. Your neighbor, on the other hand, is burning a lower percentage of fat, but she has burned up 50 percent of 300 calories, or 150 fat calories, more than twice what you've burned in the same period of time!
So you can see thats it really a personal preference. It makes sense that they balance out. Work long and easy and burn fat. Or work short and hard and burn fat. Either way, it goes back to my original mantra which was -- "as long as you are getting your heart rate up, you're doing well!". It's just a matter of how high you go and what your time schedule and preference is. In any case, I am glad to have that mystery solved!
Sources: (1 2 3 4 5 6)