I sat down one morning and decided to solve one of the great mysteries of the universe: Why don't more Americans exercise?

A 2010 study published in the American Journal of Preventive Medicine found that only 5.07 percent of the 80,000 Americans questioned in phone surveys from 2003 to 2008 had engaged in "vigorous" exercise during the last 24 hours. Vigorous, as defined for the study, is any activity that causes the exerciser to reach 70 percent of his or her maximum heart rate.

To estimate 70 percent of maximum hear rate without a heart rate monitor, gradually increase the pace of an aerobic activity and engage in a conversation with a partner. When the pace reaches the point where you run short of breath at the ends of sentences, you've probably hit the mark.

After 40 minutes or so of pondering something nearly as provocative as how did the ancient Egyptians built the pyramids, I realized the question, "Why don't more Americans exercise?", really didn't address my true concern. Why even attempt to get inside the head of people who can't get motivated by the more than 20 health benefits associated with regular exercise?

I was more interested in why so many people who start exercising stop sometimes even before that three-month trial membership to the local health club runs out. In many cases, it has to be something a bit more complicated than the overused excuse: a lack of time.

After all, I know many cyclists who work 60-plus hour weeks and still ride thousands of miles a year.

So I began to consider the times when I felt less than motivated to work out. Sometimes it was because my previous workouts were so ambitious that I felt burned out. Other times, it was because I felt as if my last few workouts were not productive.

Lack of motivation, mental burnout, and a loss of productivity: My guess is that if you've abandoned an exercise regiment in the past, one, two, or all three of those conditions played a factor. If that's the case, here's the most likely reason.

A lack of recovery.

Exercise recovery takes many forms. While most people associate it with taking a day off from activity or getting extra bed rest, it's really far more than that.

It's an ongoing process from the time the last workout ends until the next one begins. It includes not only what you eat, but also when and how frequently.

Immediately after vigorous exercise, for instance, the cells of your body are low in glycogen (the energy source carbohydrates become) and accept it readily. Other times your cells aren't quite so willing. So if you follow a bout of vigorous exercise with an energy drink containing a moderate amount of protein and carbs (a pint of low-fat chocolate milk works surprisingly well), you'll be far less likely to feel as stiff or sore the next day. (A hot shower where you stretch the affected muscles aids recovery, too.)

If your subsequent meals are high in complex carbs and protein and low in simple sugars and processed foods, the recovery process continues. And if you eat every two to three hours after ambitious exercise, you're doing everything nutritionally possible to have your exercise build you up rather than break you down.

Another way to aid recovery as strange as it sounds is to do more exercise. Increased blood flow expedites the recovery process, so the day after your toughest workout in weeks it's actually more beneficial to lightly pedal a stationary bike or do a high-rep but low-intensity circuit training workout and then stretch than to do nothing more than remark how sore you feel walking up and down steps.

But the physical elements to recovery are relatively easy to address. A far greater challenge is to know when you need a mental break. That's because exercise serves so many different purposes besides burning calories and improving health.

But if you're exercising primarily for those two aforementioned reasons, err on the side of caution. If you think you need a day off, take it.

If you can't muster any motivation that day to hammer away at the weights, take a walk with a friend.

Remember your goal is to still be exercising years from now. You don't ever want to view the process as a burden.

Just as importantly, you need to understand the ebb and flow of your body.

An article about a lack of recovery is well suited for the middle of June because many seasonal exercisers have been back at it for about six weeks. Six weeks is generally seen as the amount of time you can continue to make improvements on one exercise program before you plateau. Then it's time to take an "easy" week.

So if you've been running for about that amount of time, this week might be a good time to replace your four weekly runs with three gym sessions and a leisurely walk before pounding the pavement once again.