I have a pretty basic exercise bike at home, which has a little screen that displays information about my workout, specifically speed, time, distance, heart rate and a calorie counter (calories burnt, I presume). I don’t have any instruction manual for this thing, and there doesn’t seem to be any way to program any information into it.
There are these metal bands on the handlebar which, when I place my hands on them during the workout, display all that information on the screen. If I remove my hands from them, the heart rate and calorie counters go blank.
I’m curious about the mechanism that measures my heart rate, and specially how it calculates the number of calories burnt. How accurate is it? Aside from the statistics of my workout (time, speed, distance), wouldn’t the number of calories burnt also depend on my physical condition - age, weight, general fitness levels, etc.?
The one I had, Tony Little’s Gazelle Exercise Fitness Machine, (a stand-up cross-country skiing simulator) the calories were just a generic constant times the time. No accounting for your speed, your weight, etc.
I presume the values are about as good as those government tables that equate 1 hour of brisk walking with some calorie count, again ignoring your own weight and any accurate speed or terrain variables.
On mine, the pulse rate was calculated by a simple infrared light meter that notices changes in blood flow in your thumb. This is how hospital machines do it now as well, only they put a rubber “thimble” over the index finger. So I suppose it’s accurate.
If you are only touching metal, then it’s probably a surface conductance change that it’s looking for. Any change within a certain range is counted as a pulse.
If it’s not just based on the time you’ve been on there, it’s most likely based on your weight and speed (and maybe the heart rate, but that would be pretty fancy). Accurately estimating calories burned outside of a calorimeter is extremely difficult, so don’t expect too much.
Although the display may blank out the heart rate and the calories expended when you remove your hands, the process used to monitor calories expended is probably not dependent on the heart monitor. In all likelihood, the calories expended depends on the difficulty setting of the bike. That’s it. Now, it is true that the personal factors you mention are somewhat related to the calories expended; still, rough estimates can be made. In your case, weight really means nothing since you are sitting down while exercising (varying your weight won’t change the load) as opposed to, say, a stairmaster. In that case, you are raising your own weight as you climb. I dunno about age; my guess is that that’s a factor only in the sense that younger guys are generally in better physical condition. General physical condition can make a difference, although it’s not how you think. If guy A does the exact same workout on the bike as guy B, but guy B is in better shape, guy B burns more calories even though guy A may be more tired when the workout is over. Ironic but true. Also, guy B’s metabolism will decline slower after exercise so that the post-workout calorie burn will be greater. Again, this will be true even though guy A will feel more tired and more tired for longer.
BTW, although you didn’t ask, the common formula used to determine maximum heart rate (220 -age) is quite a rough estimate. For example, maximum heart rate is exercise dependent, i.e., your maximum heart rate on an exercise bike will be different than your maximum heart rate on a treadmill even though both are aerobic exercises. The MHR will be higher on the treadmill. This is because a treadmill uses more of the muscles in your body and thus places a greater load on your heart.
Damn, I forgot: the heart rate monitor measures your pulse by monitoring the very low voltage produced when your heart beats. It takes that voltage and uses an IC chip called an instrumentation amplifier to multiply it many, many times. The tough part: getting a instrumentation amplifier with the necessarily high CMMR (don’t ask). The easy part: the signal is then cleaned up and counted with a simple digital counter. You could actually build one yourself. Scientific American had a how-to a coupla years ago.
Thanks for the answers. I had the heart rate monitor bit pretty much figured out myself, but I couldn’t understand its corelation - if any - with the number of calories burnt.