People exercise for a variety of reasons -- from weight management to improved levels of strength for sports. If you want to improve any aspect of your physical performance, you need to adhere to the exercise principles. These principles can be thought of as signposts to success.
Your body must be stimulated to get fitter. This means that you need to apply an overload to either your muscles or your cardiovascular system. Only by asking your body to do more than usual will you trigger adaptation. This means that, over time, your workouts must increase in difficulty, intensity or duration. Your body is only as fit as your last workout. To become fitter and/or stronger, work a little bit harder from one workout week to the next.
While overload is vital for triggering an adaptive response within your body, overload without recovery can result in a reduction in fitness. Exercise causes the breakdown of tissue in your body. When you rest, eat and sleep, your body goes through a rebuilding phase called anabolism. Anabolism occurs during breaks between overload. Training too hard and too often means that your body is rarely anabolic. To avoid this, take a couple of days off from intense exercise per week and also take periodic recovery weeks when both training intensity and volume are reduced.
Your body is a remarkably adaptive organism and, providing there is sufficient overload and recovery, will adapt to any form of exercise stress it is exposed to. However, the adaptations your body will make are specific to the types of exercise stress to which it is exposed. If you run a long way, your body will improve the systems responsible for transporting oxygen to your working muscles. If you lift heavy weights, your body will make your muscles bigger and stronger. However, some fitness attributes are diametrically opposite. For example, if you do a lot of running training, your body will attempt to help you become a better runner by shedding excess unwanted weight in the form of muscle mass. If you also engage in heavy strength training, your body will try and increase your muscle mass. Combining running and strength training sends opposing messages and, as such, your body will find it difficult to adapt to either stress. This can affect the results you experience from your training.
Unfortunately, you cannot store fitness. If you take a prolonged break from your regular workouts, your body will begin to revert to its pre-trained state. This does not happen overnight but takes a few weeks of inactivity. A short break of a week or so will have a negligible effect on your fitness but longer breaks will result in a steady decline in both strength and cardiovascular fitness. To maintain your fitness levels over time, you must be consistent in your training efforts.
Periodization describes the need to make a long-term fitness plan if you want to work toward fitness-related goals. Athletes use periodized plans to help them peak for important matches, races or competitions. A periodized plan will include periods of high intensity and high volume training but also incorporate periods of rest and recovery. Like a dance routine, periodized plans often use a three steps forward and one step back approach. The two steps forward being increased levels of intensity and training volume and the one step back being a period of rest and recovery. This pattern in your workouts will make overtraining less likely.