reading & thoughts for you ...
"They offer no indication of actual calories burned. The only
way to determine that is by using an (expensive!) machine
that analyzes inhaled and exhaled gasses. Such machines
usually exist only in clinical settings."
http://www.cnn.com/HEALTH/heart/9909/07/cardiomachine.accuracy/index.html
http://www.polarusa.com/healthclubs/Own/own_zone2.asp
some ... "uses scientific data from ACSM! These numbers were calculated using Metabolic Values. The Classification of Energy Costs of Human Physical Activities Compendium classifies physical activities by energy expenditure. The results have been compiled from an extensive number of respected pieces of research, which have been carried out."
for better 'accuracy' ... "Units that take into consideration your weight will be fairly accurate in estimating caloric expenditure. It is important that you enter your accurate weight when setting the program on the machine. It is also important to perform the exercise correctly in order to fit into the equations used to predict calories burned. For example, on a treadmill that means not holding on the handrails. ON an elliptical trainer it means only using the rails for balance, not to hold your body weight."
"The displayed calorie count is probably not complete fiction.
I ass-u-me that it is based on statistical data, either provided
by academia or by the manufacturer, relying on parameters
of the machine (e.g. incline and speed) as well as personal
parameters (e.g. weight). For example, Polar claims to do
its own studies in order to build statistical models for the
calorie counts displayed by Polar HRMs."
http://sparkpeople.com/resource/fitness_articles.asp?id=536