This is a short clarification on how progressive overload actually works. It has come to my attention that this very essential concept is misunderstood by many, this is my attempt at explaining the concept and trying to make it as clear as possible.
Overload is traditionally described as any exercise that is hard enough to challenge and break the physical and chemical balance of the body. A stimulus intense enough to disrupt homeostasis. The intensity of the stimulus must meet or exceed the required threshold in order to make the body begin the adaptation process.
Overload is not the intensity of an exercise, but the combination of the variables of training that meets the required intensity to challenge a structure of the human body, such as the skeletal muscle or the cardiovascular system. Overloading these structures initiates a signal cascade that, provided the correct circumstances, culminates in training adaptations.
Regular application of a specific exercise overload enhances physiologic function to induce a training response. Exercising at intensities greater than normal stimulates highly specific adaptations so the body functions more efficiently (McArdle et al., 2010).
Intensity is often confused with load, and this confusion is understandable: according to one of the textbooks that I own, intensity is defined as a synonym of load (Chandler & Brown, 2013). But load is only one of the training variables used to achieve a desired intensity. Furthermore, intensity is not universal but always relative to the individual and its current exercise capacity.
Overload is a term that refers to the variables of training. An overload is an exercise performed with load, volume and frequency manipulated such that the individual trainee is challenged enough, and in the correct way, to obtain the desired training adaptations.
In the context of resistance training, intensity is prescribed either with percentages of the one-repetition maximum (1-RM), with the RPE scale or with the RBF scale described in this blog post.
In the context of aerobic training, intensity is prescribed either with percentages of the maximal heart rate of the individual (HRmax) or with the RPE scale.
Intuitively, progressive overload may be assumed to be the increase of overload over time. Once again, this concept is abused by textbooks, causing much confusion even among students of exercise science, personal trainers and strength coaches. This abuse may also be the result of the incorrect definitions of overload and intensity.
Popular books and some textbooks treat progressive overload as a condition that leads to further adaptations, the cause of long-term strength gains with resistance training, or the cause of long-term VO2max gains with aerobic training ( Chandler & Brown, 2013; Bell & Dauphinais, 2014; ).
This leads to people thinking that the principle of progressive overload means that the overload must increase over time in order to drive adaptations. This is wrong. The stimulus must increase over time because the individual trainee's exercise capacity is expected or is observed to increase, rendering the previous stimulus unable to produce an overload any longer.
Progressive overload is the result of an overload. It is the condition that requires changes in the variables of training to cause further overload because the values of the variables of training used previously would not meet the intensity threshold required to cause training adaptations any more.
As mentioned in this blog post, in the application of the autoregulated double progression, strength is expected to improve. This improvement is what requires the increase in volume and load. The objective of the autoregulated double progression is to maintain intensity at the desired level without relying on rigid and inaccurate a-priori estimations.
Most research articles make the exercises more difficult on predetermined occasions, and are not concerned with the condition of the test subject in each session.
Researchers know how progressive overload works but are faced with other challenges in the process of organizing a trial and must reach compromises: they assume every trainee will improve their performance by an estimated amount and use that estimation to standardize overload progression.
This forceful progression is utilized out of necessity: the methods of a scientific experiment must be as standardized as possible. This method is viable for the scope of research, and in short-term clinical applications.
You do not need to forcefully add load to the bar, or to add sets or repetitions in order to obtain results. As long as the intensity is high enough, then that is enough.
Sometimes adaptations may take longer than expected, and it often has to do with factors that are out of our control. Adding weight to the bar potentially pushes you closer to failure and if your ability to recover and adapt was already compromised, adding more stress will potentially only make your situation worse.