Physical trainers are always urging women clients to speed up and men to slow down. A heart rate monitor can help you find your ideal exercise level. Some exercise equipment (e.g., treadmills) comes with built-in monitors. But there are three portable varieties you can use while jogging, biking, rowing, etc: chest strap plus wristwatch, wristwatch only, and chest strap plus earphones. Polar offers twenty different chest strap plus wristwatch models (from $60 to $400--better prices at eBay). The watch connects via wireless to the chest band. A dab of messy conducting gel may be required, but they give a continuous readout of your heart rate, so you can vary your exercise level on the fly. You can also learn a lot about what calms you (e.g., petting a dog or cat) or stresses you out (e. g. most business phone calls). A wristwatch-only model like the Micro Touch MIO Heart Rate Monitor watch ($129) doesn't require a chest strap. You put two fingers on its terminals to take a reading. But it doesn't provide a continuous reading.