Exercise equipment has gotten so advanced that I can't help but wonder what it will do next. If you're like me, the most important motivators are features like the number of calories burned or the distance walked. But here's a disappointing bit of news: if you use "calories out" data from your treadmill or elliptical trainer to justify your daily indulgence, it's possible you're not burning off quite as many calories as you think.

The number of calories your equipment says you're burning may be off by as much as 15 percent. Here's why.

The equipment you use may not be capable of computing based on personal factors such as your fitness level or percentage of body fat. In other words, a machine that computes calories burned based on the fitness level of the "average" person may not be true for you if you're not average: either you work out frequently and intensely, or you're out of shape and just beginning to exercise at a mild pace.

As with automated systems of every kind, the information provided by your exercise equipment is only as good as the information you enter. The more data you enter, such as age and weight, the more likely it is that the machine will provide an accurate estimate of calories burned. Another common mistake gym-goers make is resting their upper body on the equipment's handlebars or rails when fatigue sets in, which skews the results.

If you're trying to decide which piece of equipment to use for maximum return, experts suggest varying the equipment you use and selecting exercises you enjoy. If you treat yourself to variety and enjoy what you're doing, you're more likely to stick with your regimen over the long term.