In basic terms, Progressive Overload is the principle of ensuring that one is always pushing themselves out of their comfort zone when exercising. This principle involves continually increasing the demands on the musculoskeletal system to continue making gains in muscle size, strength and endurance; in order to improve, you must make your muscles work harder than what they are used to.
The body is a complex and clever system which likes to work comfortably and efficiently. It does this by adapting to the demands it is presented with over time. When you exercise and push yourself out of your comfort zone, for example by lifting heavier weights, the body adapts to cope with this. These adaptions do not occur on a small scale and are much more significant than being simply visual; the body adapts to be healthier, stronger and more aesthetically pleasing from the demands you choose to push it through consistently. Consistency must not be confused with doing the same thing as the same level of intensity often; in order to make further adaptions, the level of intensity that one exercises should gradually increase – this is progressive overload. ‘Gradual’ and ‘progressive’ are keywords, if one is to overdo an exercise there is a couple of risks that they could encounter. The first is exercising to the point of poor technique, this is a mistake because the muscles you are trying to improve will not develop in the correct way, muscular imbalances may occur and compensation from other areas of the body may lead to results not coming through as efficiently as they could. The second risk factor is injury; although quite obvious it can lead to a major setback, delaying expected progress and throwing you out of routine which can be difficult to return to due to loss of motivation.
Although ‘Progressive Overload’ is usually a term referred to in strength or weight training, there is no reason it can’t be repurposed towards any other fitness goal through the increase of weight, frequency, duration or number of repetitions of an exercise.