The General Adaptation Syndrome (GAS)

Different types of training can cause different types of adaptation in the body. We all know from personal experience how efficient we can become at performing specific movements if we train them persistently enough.

But what is it that makes us better and better at doing something, to the point of becoming second nature?

Let’s see why training works in the first place and how a body’s survival mechanism can help us to better plan our workouts.

The General Adaptation Syndrome

Complex physiological processes take place in our organism when it’s exposed to stress. Since we are interested in their overall effect, though, it is possible to simplify things and consider them in the context of a single overarching theory.

This theory is the General Adaptation Syndrome (GAS), proposed by Hans Selye in 1936.

Selye based his theory on observations of animals and microscope examinations of stressed cells. In his experiments, he injected mice with irritating substances and observed changes in their hormonal status.

At first, he thought he had discovered a new hormone but was soon proved wrong when every substance he injected produced the same symptoms.

Selye later realized it was not what he was injecting but the injection itself that was producing the symptoms, and that the changes he observed in his test subjects were their natural response to a “noxious agent”, as he first called it. He later coined the term “stress”, cornerstone of his theory.

The basic premise of the GAS is that the body, after being exposed to a stress, goes through a specific set of responses (in the short term) and adaptations (in the long term).
Reading the definition carefully, we find three major characters: a stress, a set of responses and a set of adaptations.

Stress And Homeostasis

The word “stress” usually conjures up the idea of something unpleasant or frustrating, such as a demanding job or an unsatisfying relationship.

The reality, though, is that pretty much anything can be considered a stressor because anything has the potential to interact with the body and to mess it up.

For the sake of this article, we will simply define stress anything with the potential to disrupt the body’s homeostasis.

Homeostasis is the continuous self-regulating process by which the body tends to maintain internal stability. This mechanism is always at work and maintains the stability of our internal environment in response to changes in external conditions.

Because we still carry the same genetic makeup of our ancestors, when something disrupts the homeostasis the body perceives a life-threatening menace and undergoes a series of internal processes in order to compensate and to ensure its survival.

As we will see, it actually goes a bit further than mere survival, but before that, we must go back to the GAS and talk about specificity and tolerance.

Specificity And Tolerance

Premise to the GAS is the concept of specificity. In his definition, Selye mentions a “specific set of responses and adaptations”. What we mean by specific here is that a certain stress will induce a certain type of responses and a certain type of adaptations.

If the stress changes, even the responses and the adaptations will change along.

Lift a heavy weight? You get stronger. Spend time in the sun? You get a tan. Specificity.

In addition to its specificity, Selye considered every stressor to be inherently toxic. Toxicity exists along a continuum which goes from innocuous to deadly. What determines the toxicity of a stressor is the tolerance of our body for that stressor.

Tolerance sets the threshold below which a stressor is innocuous, and above which it can result as lethal. A body genetically well equipped against a certain type of stress will have a higher tolerance for that stressor, resulting in a larger dose of stress needed to disrupt homeostasis and “intoxicate” the body.

Tolerance is not fixed yet. As a matter of fact, repeated sub-lethal exposures to a stressor lead to a tolerance of subsequent exposures for the same stressor.

In other words, by exposing to a tolerable amount of stress the body will become better at resisting it. Next time, a larger dose will be needed to disrupt the homeostasis.

The Three Stages Of The GAS

According to Selye’s theory, once the stressor is applied, the body can only go through three possible stages.

The first two stages contribute to the survival of the system, the third represents the failure of the system to adapt to the stressor.

Stage 1 – Shock

This stage is the immediate response of the body to the onset of stress. The amount of stress placed is enough to disrupt the homeostasis, this shocks the system which goes in a state of alarm and activates its defensive mechanism.

Stage 1 is triggered, the body is now in survival mode.

With all of its effort, it strives to protect itself from the external threat and to preserve life. Priority is given to vital functions, resulting in a general suppression of those metabolic processes which would waste precious resources.

These short-term responses last until the stressor is removed.

Selye thought that the first stage had, in general, a duration of approximately 48 hours. Today, we know this phenomenon to be highly subjective and closely related to stress and tolerance.

Stage 2 – Adaptation

Once the stressor is removed, the body quits responding to it. It feels out of the danger zone and reactivates all the normal metabolic functions which had been suppressed.

The body has now a chance to repair the damage induced by the stressor and undergoes a series of physiological processes and deep structural changes.

These are the long-term adaptations we find in the definition of the GAS.

But Stage 2 is not only about repair. Mindful of the shock which just occurred, the body also attempts to equip itself for possible future exposures to the same stress. It enhances tolerance against that stressor so that next time it will be harder to disrupt homeostasis.

Even the duration of Stage 2 is highly subjective. Let’s just say for now that the higher the tolerance against a stressor is, the harder it will be to induce Stage 1 and the longer it will take to complete Stage 2.

Stage 3 – Exhaustion

If we place too much stress on the body, its recovery abilities won’t keep up and exhaustion will eventually occur. This is something we want to avoid.

In general, getting into exhaustion could cause the death of an organism. This is what happens when a stressor overwhelms tolerance and the body can’t adequately adapt.

The stress capacity is simply too much to handle, resulting in a massive intoxication of the body.

Our hope here is that the stressor exceeded tolerance only so much that we still have a chance to survive! Major amounts of rest are needed to detoxify and recover from the stress event.

Also, because the body is now off of balance, there is a loss in tolerance for that stressor.

The Stress Continuum

Having covered the three stages of the General Adaptation Syndrome, let’s examine how they apply to a real-life scenario.

Within the parameters of Selye’s theory, there are only three possible outcome pathways following a stress event: no change in tolerance, tolerance gain or tolerance loss.
Disegno senza titolo

No progress

The first possibility is that stress capacity is too little when compared to tolerance. As a result, homeostasis won’t even bother to trigger adaptation because no adaptation is required to cope with the stessor.

No adaptation, no change in tolerance, no progress. The body is just fine as it is.


The second possibility is that a sub-lethal amount of stress exceeds tolerance. In the diagram above this is referred to as “appropriate stress” because it disrupts homeostasis (Stage 1) and allows the body to adapt (Stage 2).

The result of the adaptation stage is an increased tolerance for the stressor and consequently a progress in the body’s general condition.

Loss of progress

The last possibility is that a lethal amount of stress exceeds tolerance. Homeostasis is heavily disrupted and adaptation is urgently needed yet at the same time inhibited by the excessive intoxication in process (Stage 3).

The result is a loss in tolerance hence a loss of progress.

Partial Conclusions

In this first part of the article, we’ve seen that anything can be considered a stressor and that what determines its impact on the body is the tolerance of the body for it.

A stressor induces specific changes in the body, which becomes more efficient at responding to that particular stress.

The appropriate amount of stress creates a state of shock on the body which undergoes a series of responses and adaptations to never experience that shock again.

If we place too much stress we cause the exhaustion of the organism and we lose tolerance. On the other hand, if we place too little stress homeostasis won’t be damaged and the body won’t even bother to adapt.

The general lines of the GAS we’ve seen in this first part of the article are true in general and work for any kind of stress. In the second part, we will see how they apply to the specific context of training.

Liked this article? Share it!
Share on Facebook
Tweet about this on Twitter
Share on LinkedIn
Email this to someone

Leave a Comment

Join My Community

Learn The Best Knowledge, Backed By Science, For Transforming Your Body