CPT Muscles

General Adaptation Syndrome in Fitness Explained

Kinsey Mahaffey
Kinsey Mahaffey
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The human body is amazing. It constantly responds to its external environment to overcome and thrive. If you picked up a slightly heavy dumbbell to perform biceps curls, using a weight that you could only perform 6-12 repetitions of and continued to do so each day, your body would respond by increasing the muscle size and strength of the biceps to overcome that challenge until it is no longer challenging.

This is a simple example of muscular adaptation. An adaptation that fitness professionals rely on to help their clients get results.

Although the biceps example simply explains the general concept of adaptation, it's important to understand the process of how the body responds to external stressors to see the desired adaptation.

This is where General Adaptation Syndrome, or GAS, comes in. Understanding GAS can help a fitness professional craft a workout plan that will challenge their client (in a good way) and help them to see results while also minimizing the risk of negative outcomes, like injury due to over-training or progressing too quickly.

This article will give you a better understanding of what GAS is and how to leverage it for better fitness outcomes.

What is General Adaptation Syndrome?

General Adaptation Syndrome, GAS for short, describes how the body responds to stress physiologically. Canadian physician Hans Seyle is credited with proposing this theory in 1936 to describe the process for how an organism adapts to various stressors.

Stress can come in one of two forms: physical (such as a resistance training workout) or emotional (such as work or personal-life stress). For this article, we'll focus on physical stressors, specifically, exercise. Prolonged exposure to negative stress (like over-training or lifting too heavy before you're ready for it) can lead to negative physiological consequences, like stress fractures or injury to the muscles.

On the other side of the spectrum, prolonged exposure to positive stress, known as eustress, can lead to positive physiological outcomes, like improved fitness and muscular strength or endurance. The body will respond to the stress that is placed on it and a physical adaptation will occur over time in response to that stress stimulus.


What Are the Three Stages of GAS?

The way that the body responds to stress, according to the GAS model, can be broken up into three stages:

1. Alarm Reaction Stage

This is the initial reaction to the stressor. In the initial 6 to 48 hours after exercise (the stress stimulus), clients may experience fatigue or joint stiffness, and after 24 to 48 hours delayed-onset muscle soreness (DOMS) may kick in. The alarm reaction stage stimulates an increase in oxygen and blood supply, neural recruitment to working muscles, bone formation, increased joint load and tolerance, and connective tissue strengthening.

The benefit to this response is that over time when exposed to small doses of the same stressor, the body will adapt to overcome these challenges. The key to inducing a positive response is to use the principle of progressive overload, increasing the intensity or volume of exercise programs using a systematic and gradual approach.

2. Resistance Development Stage

Consistent training will move the client into the resistance development stage. This is the stage where clients will begin to adapt to their training in a way that enhances their performance. The human body adapts to repeated training sessions by increasing its ability to efficiently recruit muscle fibers and distribute oxygen and blood to the proper areas of the body. By applying progressive overload, the client will continue to improve their performance and ability to overcome the challenges that they are faced with.

For example, if they are training in Phase 1: Stabilization Endurance Training, their body will respond by improving in balance, joint stability, and muscular endurance. If they are training in Phase 3, Muscular Development Training, the client will be able to tolerate an increasing volume of work and heavier loads, resulting in muscle development and increased strength.

3. Exhaustion Stage

The exhaustion stage describes prolonged stress or stress that is intolerable, leading to exhaustion or distress. Some negative consequences of this stage include:

• Stress fractures
• Muscle strains and ligament sprains
• Joint pain
• Emotional fatigue

Applying progressive overload and allowing for sufficient rest between sets and/or sessions as needed can help reduce the risk of entering this dangerous stage.

The SAID Principle

Another commonly known adaptation principle is the SAID (specific adaptation to imposed demand) principle, also known as the principle of specificity. While the GAS model describes a general physiological response to stress stimulus, SAID is a principle stating that the body will adapt to the specific demands that are placed on it.

When it comes to workouts, the body will adapt according to the movement patterns and acute variables (sets/reps/tempo/rest/frequency) that are used consistently. This is one of the reasons that we stay in each phase of the OPT model for 2-6 weeks, to allow for adaptation. When considering the SAID principle, it's important to keep mechanical, neuromuscular, and metabolic specificity in mind when programming.

Mechanical Specificity

Mechanical specificity refers to the weight placed on the body and the specific movements performed to produce specific muscular adaptations. For example, high repetition, low weight movements produce muscular endurance, while low repetition, heavyweight movements produce muscular strength.

Neuromuscular Specificity

Neuromuscular specificity refers to the speed of muscle contraction and exercise selection to produce stability, strength, or power adaptations. Choosing an unstable, yet controlled exercise performed at a slower tempo will help a client develop stability. To develop strength, the exercise will be performed in a stable environment with a heavier load and moderate tempo. Muscle contraction speed varies with each of these demands placed on them.

Metabolic Specificity

Metabolic specificity refers to the energy demand placed on the body. You can vary the metabolic demands to produce aerobic (reduced rest periods and prolonged work periods) or anaerobic (increased rest periods and more demanding or high-intensity work periods) adaptations.

GAS helps us to understand how the body adapts to stressors and SAID helps us understand what specific movement or response we're asking the body to adapt to.

What GAS Has to do With Designing Workout Programs

As fitness professionals, we can use the GAS model to our advantage to help our clients see results without suffering the negative consequences of exhaustion. When it comes to programming, GAS helps us understand why it's important to periodize, or systematically cycle through, different phases of training.

We plan our clients' workouts based on the physical adaptations, or outcomes, that they want to achieve. To help them achieve their goal, it's important to make a plan that will systematically help the client achieve the physical adaptations that they're seeking without stalling out (staying in one phase for too long) or burning out (reaching exhaustion because of intolerable amounts of stress placed on the body). To avoid either of these scenarios, the client can stay in each phase of the OPT model for 2-6 weeks, depending on their goals and/or rate of progression.

Using the GAS model, we can expect a client to experience some fatigue, joint stiffness, or DOMS after the first couple of sessions while they're in the alarm stage. After a few weeks of consistent training, the symptoms of the alarm stage should lessen, and the client should enter the resistance development stage and begin to see improvements in their performance. Their performance will continue to improve until their body adapts to what they're doing, and they (possibly) hit a plateau.

Currently, or the end of 6 weeks in any given phase, it's time to move the client into the next phase of training, whether that means advancing to the next level or cycling back to an earlier phase to change the stimulus. Cycling through the phases every 2-6 weeks will allow your client enough time to recover from more difficult phases and to continue to see results as the stimulus continues to change.

Keeping the GAS model and the SAID principle in mind while programming will help you better develop a fitness plan for your client that helps them reach their goal effectively and efficiently.


Sutton, B. G. (2022). Nasm Essentials of Personal Fitness Training. Jones & Bartlett Learning.

The Author

Kinsey Mahaffey

Kinsey Mahaffey

Kinsey Mahaffey, MPH, is a Houston-based fitness educator, personal trainer and health coach who developed her commitment to lifelong fitness while playing Division I volleyball. She’s passionate about helping others cultivate a healthy lifestyle and enjoys educating other fitness professionals who share this vision. She’s a Master Instructor and Master Trainer for NASM. You can follow her on LinkedIn here.


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