As people exercise, their bodies adapt and improve. That's why each time you run a certain distance, it becomes a little less difficult.

So how do you know when to make your workout harder? Should you increase speed or distance? Should you do something else?

Keep in mind that exercise is nothing more than stress inflicted on the body for the purpose of forcing the body to adapt to that stress. The difference between this stress and others is that you can control the amount of stress you expose your body to. As you challenge your body it becomes stronger and more capable of handling that amount of stress the next time.

Putting too much stress on the body will usually result in an injury, while not enough stress will prevent you from improving your fitness level. So how can you tell if you are working hard enough but not too hard?

Most fitness professionals use something called the FITT principle: When a workout becomes too easy, change either the Frequency, Intensity, Time or Type.

Frequency

This is the number of times you exercise per time period, usually measured per week. If you are currently exercising three times a week and your fitness level is not improving, then you could change the frequency of your workout and exercise four times a week. This will put some additional controlled stress on your body.

Intensity

Most people find it difficult to increase the intensity of all of their workouts in the same week. I suggest that you keep the intensity of most of your workouts consistent, but pick one day and call it a "hard day." On that day, focus solely on making your workout more challenging.

You can also interval train. Let's say your current workout is 45 minutes on a Stairmaster. Every five minutes, increase the intensity of the workout by one level for one minute. Then return the intensity back to normal. Eventually you should be able to stay at that higher intensity for more time.

Time

If you can't squeeze more workout days into your weekly schedule, try making, say, three of your days "long days" by adding 15 minutes onto the end of your current workout. This will increase your weekly exercise time by 45 minutes, which is virtually the same as adding another day of exercise.

Or try adding just five minutes to EVERY workout. If you currently work out five days a week, start doing pushups at the end of every workout for five or six minutes. Again, you will be squeezing the equivalent of one more workout into your week.

Type

If your workout is getting too easy or it no longer challenges you, perhaps it is time to switch activities. This is where cross-training could help. When you have been doing a single exercise activity regularly, eventually that activity will become easy for you.

Maybe 45 minutes of running was difficult when you first started, but now you can handle that without a problem. Now, it may be time to try a 45-minute aerobics class. The cardio benefit is the same, but your body will be challenged by the new movements. Different muscles may be required to participate in the class.

Varying your activity has other benefits, too. Individuals who cross-train are less likely to suffer from overuse injuries. Additionally, exercisers who multi-mode are more likely to stick with their regular exercise because they are less likely to get bored.

If you are happy with your exercise program, don't start making drastic changes. Exercisers who want to maintain their current level of fitness should continue their programs as they are.