I Didn’t Know That. . .

Q: How do the heart monitors on exercise equipment work?

A: Just like a regular electrocardiogram used by your doctor, according to Chris Clawson of the Life Fitness division of Brunswick Corp. But the monitor on fitness equipment uses the equivalent of only one lead--compared with five or six for a regular EKG--plus a ground lead, and searches only for the strongest electrical signal emitted by the heart, timing how often it recurs. Some monitors use a strap to attach a lead directly to the chest, close to the heart, where it is easy to detect the signal. The problem is more difficult with the hand plates on most exercise equipment. The plates are farther from the heart, and the heart’s electrical signal can be overwhelmed by the electrical signals from muscles. In some systems, the measured heart rate can be 20 to 30 beats per minute different from the actual rate. Newer equipment patented by Life Fitness uses sophisticated digital processing techniques to screen out signals from muscles, giving a more accurate reading.