There are two important reasons why cardiovascular exercise should be part of any well-designed fitness program. One is obvious. Our heart is the organ responsible for our very existence and this form of exercise causes it to adapt to a higher level of function. This is a positive change that cannot be gleaned from other forms of exercise (strength training or stretching, for example). Secondly, cardio or "aerobic" exercise also produces a significant energy outlay because a continuous lengthy period of muscular activation is undertaken. The end result is the ability to significantly impact energy balance and create a deficit that must be satisfied by accessing existing body fat stores.

For cardiovascular exercise to yield optimal benefit, it must be performed at a level that challenges current capacity. Simply stated, working in your comfort zone won't do. Will you use energy if you maintain a non-challenging pace? Sure, you use energy when you do anything, including sleep! Will you use much? Not on your life!

Even if a slow pace is adequate for expending the amount of energy that suits your goals (very long durations, for example, can compensate for low energy expenditure per unit time and result in a sufficient energy outlay over the course of a session), it won't do the trick when it comes to bringing your cardiovascular system to a higher level of development. When that is the objective, degree of effort is the key. But determining the level you should be working at to satisfy this requirement is not so simple.

Many exercisers monitor their pulse to assess their aerobic training intensity. Unfortunately, there are numerous drawbacks associated with this method. Those who must exercise with existing pathologies can certainly benefit from placing an upper limit on the amount of cardiac stress they encounter. But in this case, a specialist would make the determination of what that restriction should be. For the rest of us, predicting the proper heart rate to work at is wrought with assumptions.

The first major supposition upon which the target heart rate is based concerns the highest beating rate your heart can achieve. A common prediction formula requires subtracting your age from 220. Unfortunately, the estimate derived from this simple calculation carries a rather large standard error. This means there is quite a bit of variability such that the range within which most people's actual maximal heart rate will fall in relation to this prediction is large. So, other than providing a ballpark figure, this calculation won't help you much.

The only way to definitively identify your maximal heart rate is by measuring how many times your heart is beating when you're working as hard as you can. Obviously, for the sake of safety, this is ideally done in a supervised setting. Accordingly, an exercise stress test is typically done in an exercise laboratory or physician's office. Once you know your maximal heart rate, figuring out what fraction of it you should be using when you're exercising is also challenging. No matter how ambitious you are, you can't simply take it to the max, because doing so ensures there will also be a considerable amount of anaerobic energy transfer contributing to your effort. And when energy must be gained by virtue of a large input from this process, you cannot sustain the pace for very long.

Target heart rate zones are typically assigned as a percentage of your actual or calculated max. But this prediction is also problematic. Specifically, the percentage that accurately defines the relative degree of effort associated with exercise is also highly variable between individuals. For example, a well-conditioned endurance athlete might be able to perform for a long period of time at 80 percent of her maximal capacity, while a couch potato could be on borrowed time once he exceeds 50. It simply cannot be predicted with any reasonable degree of certainty.

The best way for apparently healthy exercisers to determine where they should exercise along the spectrum from total inactivity to maximal effort is by understanding the physiological responses that define the levels they'll encounter along the way. Recent investigations in the burgeoning field of oxygen uptake kinetics have identified distinctive zones known as intensity domains. You can discover each of these by analyzing how you feel when you're working at various discrete levels of effort from low exertion to high. Multiple tests in an exercise laboratory can also give you this information. Once determined, levels of effort that define these domains can be correlated with the heart rate that is present at that workload. But this is really unnecessary because once you discover the appropriate perceived exertion (how you feel as you exercise within a particular domain), you can bypass the middleman (heart rate) and use this innate feel to determine where you should be exercising in accordance with your goals.

Fred Dimenna, a Certified Strength and Conditioning Specialist and lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at www.freddimenna.com or email him: mrnatural@yahoo.com