In any exercise program, there is almost always a point where the body gets used to a given workload or intensity and starts to plateau. You may notice you are not losing weight at the same rate as before, that you can't bench as much, or no matter how hard you try your 5k times are not getting faster. When designing an effective exercise program there are three principles of program design that must be followed to produce desirable outcomes, keep the body from plateauing, and avoiding injury....here are the three principles:
The Principle of Specificity:
The principle of specificity basically states that the individual must train in a specific way to produce a specific adaptation or training outcome. For instance....if I am training a client who wants bigger chest muscles I am going to emphasize exercises like the bench press that recruit the pec muscles. Another example...if I am training a runner to run a fast 5k I am going to place an emphasis on getting that person running....long runs, intervals, pace work, etc....there would be little reason to have this person in the weight room (like the first example) when time would be better spent running.
The Principle of Overload:
The principle of overload refers to assigning a training regimen that is of greater intensity than the person is used to. Without the stimulus of overload, even an otherwise well-designed program greatly limits an individual's ability to make improvements (NSCA Essentials of Strength and Conditioning, page 380). A good example of this principle in action is weight training. If I want to increase the intensity (or overload) I could increase the weight, assign more reps and sets, decrease rest period between sets, or assign more weight training sessions on a weekly basis. Another example, going back to our 5k runner, would be to increase weekly mileage, add intervals to an otherwise long, slow distance training regimen or to increase training days. When this principle is properly applied overtraining is avoided and the desired training adaptation will occur.
The Principle of Progression:
Similar to the principle of overload is the principle of progression. If the training program is to continue to producing higher levels of performance/results, the intensity of training must become progressively greater. Progression, when applied properly, promotes long-term training benefits. Take for example a plyometric program...one could progress by adding more drills, changing the difficulty of the drills, increasing foot contacts, etc...The main point is that progression is based on the individual's training status and is introduced systematically and GRADUALLY.
In conclusion, when all three of these principles are applied properly to a training program the results will keep moving forward while the body adapts to a higher workload. Over training and injury will be avoided, and in a sense, the body will never hit that plateau.