Despite the many reasons people give for exercising, the primary reason distills down to one foundational element – adaptation. We exercise to adapt, plain and simple. That adaptation is sometimes anatomical (bigger muscles), other times neurological (increase power), and still other times metabolic (increased endurance). Regardless of the type of adaptation derived, the same underlying mechanism drives adaptation: the stress – recovery – response cycle, otherwise known as the General Adaptation Syndrome (GAS).
First proposed by French researcher Hans Selye in 1936, GAS is the theoretical framework of how all organisms adapt to stress. Understanding GAS is fundamental to appropriate exercise prescription, because, without going through the full cycle of stress, recovery, and finally response, an individual cannot improve from exercise (whatever that improvement means to them). In some cases, if the cycle of GAS is not properly applied, not only will individuals fail to adapt to exercise, they will actually find themselves detraining or getting worse—not better. Although it is rare that exercise can become maladaptive, a lack of appreciation for the three stages of the GAS cycle increases the likelihood. Let’s breakdown GAS a little further to understand its application to exercise.
Stress
Simply defined as a state of strain or tension on an organism, stress is a normal part of our daily life. Sometimes this stress is emotional or mental, but in the context of exercise, the stress is physical (stressing anatomical structures or physiological systems). In order to be considered stress to the organism, the level of strain or tension must be more than the body is used to encountering. In fact, if the body was used to encountering it, we wouldn’t notice any stress or tension.
Practically speaking, we all know the way stress works. Too little stress is bad; it causes us to be apathetic, bored, and stale. Too much stress is also bad, as it can cause us to breakdown physically, emotionally, or mentally. Much like many things in life, there is a goldilocks zone in the middle where there’s enough stress to induce a positive change, but not so much stress we breakdown or so little stress we fail to improve.
This “right” level of stress produces a phenomenon that is referred to as “overload;” overloading being a strain or tension applied to the organism that is slightly more than it’s used to. When this stress is applied in this overloading fashion, we begin the cycle of Selye’s GAS model.
Put in a practical exercise context, it works like this. You walk into the gym one day, you pick up a barbell that is 50 pounds and you curl it ten times and set it down on the 10th rep, but you felt like you could have done 15 reps. In this example, no overload has occurred, therefore the biceps will not adapt (or it will adapt imperceptibly). Let’s say you realize it was “easy” and decide to work a little harder. You grab a 65-pound barbell and curl it 10 times, struggling to complete the 10th rep. In this case you placed an overload stress on the bicep that will begin a cascade of events that will likely lead to positive adaptation, provided you recover. Finally, let’s say you were really ambitious and excited to get bigger biceps. You then decided next time you’re in the gym you’d curl the 95-pound barbell as many times you can. You’re only able to lift it 2 times before the barbell crashes to the ground. In this scenario you’ve overloaded the muscle, but probably too much. This likely will lead to less of a positive response.
Bottom line: the first step in the process leading to exercise adaptation is placing an overload stress on the muscle a little more than it can handle to kick off the GAS cycle.
Recovery
During the recovery phase, a couple of things happen. First, performance declines. That performance could be strength, power, endurance, flexibility, aerobic capacity, etc. based on the specificity of the stress placed on the body. This decrement in performance is acutely normal during a short period after overload stress is applied. For novice exercisers, with the appropriate level of stress this timeframe for performance decrement is usually 24-72 hours. For more advanced trainees, the high level of stress required to drive adaptation might result in a recovery period that takes 7 or more days (much more for the very advanced, elite trainees of the world). These are approximate timeframes for performance decrement and certainly vary based on a number of factors.
While this decrement in performance persists, something else amazing happens: the body begins to heal itself, or in other words, RECOVER from the stress. This healing can be anatomical (synthesis of new myofilament to repair damaged muscle), neurological (improved innervation by motor units), or metabolic (increased substrate availability, such as increasing muscle glycogen stores or increasing enzyme content). Whatever the case may be, the healing or recovery is first trying to get the body back to baseline—where it was before the overload stress was placed on the body. After the body is back to baseline, something even more amazing happens (more on that below).
It should be said that a number of variables affect recovery from exercise. Genetics, nutritional status, sleep, training status, age, gender, level of overload, and many other factors determine how quickly recovery happens. Recovery is complicated, but importantly, recovery is only the second phase in the process and it just gets you back to baseline, which is by definition not where we want to stop. Again, we exercise to adapt and make progress, which leads us to the last stage of Selye’s GAS.
Response
All organisms respond to stress after recovery has taken place. When the stress is appropriately applied and recovery is allowed for, this response comes in the form of a positive adaptation, or the reasons we want to exercise. This positive adaptation is designed to better handle the stress the next time the body comes into contact with it. This specific positive response is a very deeply engrained evolutionary mechanism that has allowed us to thrive as a species for over 100,000 years. If we weren’t able to positively respond to environmental stressors, we wouldn’t have been very long for this planet.
In some cases, if the stress that is applied is too great or too frequent, recovery will be inadequate. If a stress is applied before recovery has fully taken place, improvement will not occur. If anything, performance will continue to decline and the stress of exercise will become maladaptive. While this state is not often reached by general population individuals, it is still conceivable that even these novice clients can slip into an overtrained state if proper stress and recovery principles are not applied. The job of the exercise professional is to ensure that the proper level of stress is appropriate for the client, while also ensuring adequate recovery has occurred in order to allow for positive adaptation to occur before another stress is placed on the body.
Putting It All Together
To summarize, we exercise to positively respond or adapt. The first step in that process is an appropriate level of overloading stress. When this occurs, function acutely declines specific to the system(s) we stressed. This triggers a recovery process to first drive us back to baseline (where we were before the initial stress occurred). Following recovery, we positively respond by enhancing our anatomical structures and physiological functions to better accommodate the same stress next time we encounter it. Pretty straightforward, right? Not so fast; there are a few more things to consider.
Once a stress has been placed on the body, and the recovery/adaptation cycle has been run through, the previous level of stress should no longer be as much of a stress (since the body just “adapted” to it). If that is the case, that precise level of overload stress will no longer elicit adaptation when encountered again. To ensure overload is achieved the next time, it must incrementally progress. This is the concept of progressive overload, or making the stress slightly harder (5-10% harder) than it was previously to keep the stress-recovery-response cycle churning.
Other considerations are principles like specificity and reversibility. Specificity states that adaptations will be specific to the specific stresses you place on the body. That is, doing indoor cycling is great for road biking performance, but not great for swimming performance. If the stress is no longer applied for an extended period of time, the positive adaptations that occurred as a result of the stress will go away. This is the principle of reversibility. Finally, it should be acknowledged that the stress-recovery-response cycle is very dependent on the individual. These individual differences must be considered in the context of GAS and overall exercise prescription.
Implications to Exercise Prescription
It becomes increasingly obvious that an appreciation and application of GAS is necessary to ensure proper exercise prescription (ExRx). To drill down deeper, we can examine the concept of GAS in the context of prescription of aerobic and resistance exercise modalities.
We’ll start with the similarities between the two and then we’ll discuss some of the subtle differences. First and foremost, stress is always defined practically as the aggregate of duration, intensity, and frequency of training. Increasing any of these three parameters equals more stress; decrease any and you have less stress. Typically, when prescribing and progressing exercise, one variable is increased while the other two are held constant, as increasing more than one variable can result in too much stress at one time. It is a good general rule of thumb to only progress an individual about 10% per week (slightly more in younger novice clients, slightly less in advanced or older novice clients). This 10% could mean 10% more volume, 10% less rest, or 10% more intensity (HR, load, etc.).
When we get more specific with regard to aerobic exercise, it is generally recommended people perform aerobic exercise 3-5 days per week, for 30-60min, at 60-90% of their MHR — or and RPE 6-9 on a 0-10 scale. Once the baseline level of “stress” is established (i.e., the first aerobic exercise prescription), typically duration is increased as the first progression (until it cannot be practically increased any longer), than frequency (again until it’s practical limit), and then finally intensity. This duration, frequency, and intensity progression is appropriate for novice trainees. As an individual transitions to intermediate and advanced levels, progression can take place with any of the variables, though still likely sticking to progressing only one at a time.
With strength training it is generally recommended that novice individuals train at a frequency of 2-3 days/week for each muscle, with sets of 6-15 repetitions, 2-3 sets/exercise, at loads that make the last 2-3 reps of each set hard to very hard (measured normally with 1-3 reps left in reserve). The individual should be resting 1-2 minutes between sets and 48-72 hours between workouts. At the novice level, progression can take place by first increasing reps and then decreasing rest between sets. From there, if it is practical, increasing from a frequency of 2 days to 3 days makes sense. Generally going beyond is not necessary except for the most advanced trainees. After volume (set and rep) progression and an increase in frequency, finally intensity (or load) can be increased.
Common Errors with GAS and ExRx
To round out this discussion on GAS and ExRx, let’s talk through some common errors in a very specific context.
The first error is doing too much too quickly. Start at a lower frequency, volume/duration, and intensity, and then progress at about that 10% rate weekly. Focus on volume/duration first, frequency second, and finally intensity last in your progression. Remember exercise is like cooking: you can always add ingredients, but you can never take away after you’ve put in too much!
The second error is failing to progress. The reality, as sobering as it is, is your last workout was probably your easiest workout, because if the next workout isn’t a little harder your body won’t be compelled to adapt. That said, strive to make each workout progressively more challenging for clients per the frame of reference above (volume, then frequency, then finally intensity). It should also be noted that the more advanced a trainee gets, the more often the need to step back and deload. Deloading basically means continuing to exercise, but doing so at a lower cumulative level of stress to allow the body to actively recovery, because the body nearly always recovers better actively than passively. In this case, not every workout is progressively harder. The specific tactics for deloading are beyond the scope of this article, but should be understood when working with more advanced trainees.
The last big error to discuss is failing to allow for adequate recovery time. Remember, we don’t positively adapt unless we recover. Training too frequently before recovery takes place short circuits adaptation, often stopping it dead in its tracks (and sometime causing an injury). With that said, aim for an appropriate frequency of exercise to ensure recovery takes place. We exercise just not for the sake of exercise, but to positively adapt. Understanding and applying the principles of the GAS framework individually to each client ensures positive adaptation always occurs.
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