Every evening in the gym, the ladies (and a few guys) can be found working purposefully on bikes, treadmills and elliptical trainers. The whir of machinery fills the air and determination is fired by visions of losing weight and getting in really good shape.

Some casually browse the pages of old magazines, read the latest thriller or chat with friends as they dutifully put in their "forty five minutes" of "slow and steady". They are well-intentioned but clueless. I question the results of their efforts but decide it is unwise to offer unsolicited advice.

One lady, however, is focused and pays attention to what she is doing. She periodically refers to the treadmill's glowing red display and adjusts her pace accordingly. Good for her! She is monitoring her heart rate.

What she knows, that the others don't know, is that your Heart Rate is a dead accurate indicator of just how hard your body is working. This is the best way to make sure that you are exercising vigorously enough to improve your fitness level but not so hard as to overexert yourself.

This is also a great way to lose weight and get really fit in the shortest amount of time. What you have to do is exercise at the correct intensity within an optimal range. This is your Target Heart Rate (THR) which is expressed as a percentage of your Maximum Heart Rate (MHR).

You can calculate your personal Target Heart Rate, but this gets a bit tricky. You have to know your Resting Heart Rate (RHR) and plug it into a couple of formulas: (MHR – RHR X (50% - 80%) + RHR = THR) where (Age – 220 = MHR).

You also have to know how long to exercise, at which point in the optimal range (50% to 80%) and you have to monitor your heart rate while you are working out. There are charts that estimate your THR for you but their accuracy can vary, up or down, by as much as ten percent.