This is an excerpt from Heart Rate Training by Roy Benson & Declan Connolly.
Several conditions may cause your heart rate numbers to skip all over the place. Normally, you will notice a steady increase or decrease in your heart rate that will pretty much match your effort. The heart rate monitor is not an electrocardiogram (ECG). However, it does use the electrical activity of the heart to give you a signal. Therefore, any time an irregularity occurs in the signal, it will be reflected in the heart rate number displayed.
One cause of irregular readings is a loose chest strap that allows the electrodes to slip and slide on the skin and cause electricity from the friction. This interferes with the transmitting of the electrical impulses of the heart muscle. You need to shorten the elastic strap. If you have a very small rib cage and narrow chest, just tighten the strap well past the halfway point to achieve the 6-inch (15 cm) gap even if it means that the extra material loops down. Put a safety pin in front of the slide to keep the strap from slipping back to the original halfway position. If this is a consistent problem, buy a shorter strap or permanently shorten your strap by putting a few stitches into it to hold it in the shortened position. Straps stretch over time and become loose. When this happens, we recommend buying a new strap.
Static Cling on Clothing
One cause of irregular readings is clothing worn over the transmitter. Shirts still charged with static electricity from the dryer can confuse the receiver. Nylon wind jackets can generate static electricity that also can interfere with the transmission from the chest strap to the receiver, although this is more likely to be a concern in dry or cold climates. Usually, sweat takes care of it pretty quickly. Females may find that underwire bras can be a source of interference. If you suspect that static cling is causing the problem, wet your hand and wipe it across the material in front of the transmitter, or just wait until you've run far enough for sweat to drain off the static. (This should take 8 to 10 minutes.) The signal is also transmitted better and stronger once you start to sweat.
Interference From Cross Feeds
Cross feed occurs when your monitor receives information from the transmitter of the person next to you. The solution may be to find a partner who isn't as scientific as you are (and therefore does not wear a monitor). If not, arrange yourselves so that you are both wearing your receivers on your outside arms. More advanced (and expensive) monitors have their own embedded codes to prevent cross feeds.
If you're a real techno geek, you may have other devices that are confusing your heart rate signal. When testing in the lab, we have seen devices such as pedometers and GPS systems interfere with the heart rate monitor signal. This is also the case when exercising indoors on a stationary piece of equipment with lots of information displayed on the console. In this case, either exercise outside for a change or take the receiver off your wrist and strap it to a lower part of the machine out of range of the console.
Shirts soaked with sweat can get heavy enough to bounce up and down in front of your transmitter and interfere with the signal to your receiver. If it is that hot and humid, remove the chest strap from under your shirt and refasten it on top of the wet shirt just below your chest muscles. The wet shirt will still allow the electrodes to have great contact with skin and be able to pick up the electrical activity of your heart muscles. In fact, this is also the solution if you experience chaffing from a strap that seems to slip up and down on your skin. Just wet your shirt first before starting the workout.
Radio frequency alarm systems installed in buildings can occasionally interfere with your monitor's own radio frequency as you go past. The solution: ignore them and keep going. The goofy numbers will return to normal as soon as you are out of range of the alarm system.
Read more about Heart Rate Training.