Bioelectrical impedance analysis (BIA) is a method for measuring body composition, including muscle mass, body fat, and total body water. Alternating low and high-frequency electrical currents are sent through the water in the body via contact with electrodes to measure impedance.
The impedance is used to determine total body water, which can then help derive your fat free mass-the portion of your body that does not contain fat, including your muscle and of course body fat. The best thing is you can keep track right on your phone!
(This is offered for free with your Gold or All-Star membership).