The human body is always attempting to maintain a stable and constant environment. This is also known as homeostasis. It is good that we don't have to worry about controlling our heart rate, body temperature, or digestion, but when it comes to getting into shape, we have to overcome homeostasis because our bodies attempt to stay where they are and they don't like it when we rock the boat.
There are 4 variables involved in the overload principle when developing a conditioning program. They are frequency, mode, duration, and intensity. The proper use of these components can help you overload your body and overcome homeostasis.
Frequency- This is the first component and obviously refers to how often one engages in a particular exercise or activity. The minimum frequency should be 3 times a week if you want to get results and improve. Many people work out 4-6 times a week depending on their time commitments, goals, and the exercise they participate in, which leads to the next variable, which is mode.
Mode- Is the type of activity you choose to participate in and could include things like running, swimming, yoga, weight lifting, etc. Your body will require more time to recover from more intense activities, but easier activities like walking can be done daily without any problem.
Duration- This is the length of time that one spends performing an exercise. Any amount of cardio or aerobic activity is good, but if someone really wants to start burning fat as a fuel source, it is ideal if they exercise for over 40 minutes. Resistance training is a different story. I think a person can get a great workout in half an hour or less, and they don't need to spend all day in the gym. One thing that will determine how long you exercise is the last variable of intensity.
Intensity- Of all the variables, this one is often said to be the most important. The intensity is the amount of energy you expend while exercising and is what really determines your results. You can walk at a snails pace on a treadmill for hours without accomplishing much. If you don't exert yourself to the point of sweating or increasing your heart rate, then your intensity is too low. Intensity can be determined by measuring your heart rate. Obviously, if someone is a beginner or getting back into shape, they will want to ease into it and gradually build up their intensity to avoid injury. When you work out, make sure you are not just doing the exact same thing each session. If your body has adapted to your usual routine, then you may want to bump up the intensity to get better results.
When you design an exercise program, make sure and choose activities that you enjoy so you will stick with them, but you will also want to take into account the above principles in order to get the most of your workout.