Takeaway Points:

The General Adaptation Syndrome model was designed to describe the body's reaction to a stressor, and has been used to describe how our bodies respond to exercise. However, it seems unlikely that this really explains everything very well.
The Impulse-Response model more generally describes how bodies adapt to and react to exercise, both in the short and long term.
The repeated bouts effect helps explain how our bodies adapt to exercise and require greater levels of training in order to continue improving.
While many of the takeaways of this post are relatively simple when distilled into basic wisdom, it's surprising how rarely this basic wisdom is actually applied in many training programs.

How do our bodies adapt to exercise?

This seems like a pretty basic topic. It’s unfortunately also one that many exercisers (and many trainers) never really learn about. In fact, when I took my first personal training certification, I was literally never taught this information. It’s clear that our industry has a long way to go.

The most commonly known model of adaptation is based around the model of General Adaptation Syndrome (GAS), which is demonstrated in a graph below.

GAS-general-adaptation-syndrome-selye-homeostasis-stress

GAS was originally used to model the body’s response to an anxious or noxious stressor, such as a sudden loud noise, a nagging boss, ingesting a poison, or another car swerving into your lane. There’s an initial “drop” in readiness where you’re surprised and frightened, before your body can react - then, your body quickly amps up your adrenaline to mobilize a “fight or flight” response, focusing your attention, making it easier for you to combat the stressor. If you can’t manage the situation quickly enough, this heightened state of attention quickly wears you out - and you can reach into an exhaustion phase where your readiness rapidly diminishes.

Repeated bouts of stress can further worsen this graph - you’ve got lower highs and lower lows. After you’re exhausted, your homeostasis is lowered until you fully recover by de-stressing sufficiently (sleeping, relaxing), so when you’re starting from that exhaustion phase, your next alarm/resistance stage won’t go as well. You react more slowly, and tire more quickly. Regular stress will really mess you up.

While GAS is primarily designed to describe the body’s response to an acute toxic stressors, it was then adapted to describe the stress of exercise as well. After all, it seems similar - we warm up (alarm), we start to work out (resistance), and if we push it too far we get excessive fatigue and potential injury (exhaustion). However, the reality is that exercise is not the same as psychological stress, so the model (while initially compelling) isn’t really accurate. After all, GAS may (sort of) explain how the body reacts to a single training session, but it doesn’t describe what’s going on in the long term.

In order to create a model that’s more relevant for fitness, the Banister Impulse-Response model (IR) was developed, which is demonstrated in the graph below.

The IR model essentially uses two specific factors (fitness and fatigue) which are added together to get a sort of total - performance. In response to a single training session, fitness is increased, but so is acute fatigue. Initially, the fatigue is high enough that there’s little point to working out again - your total performance is lower. In the long term, both fitness and fatigue decrease, but fatigue decreases quicker than fitness does - so after a bit of rest, the net performance becomes a positive.

This is all well and good, but of course this only models a single session of training and its recovery. If we were to train for a single workout, we’d see that bump in fitness/performance after recovery, but this, too, would decay over time. So, we have to perform multiple sessions, taking advantage of that sweet spot where performance increases to get in a harder workout.

This time, we workout harder, and the corresponding response is bigger. Over time, we can add up multiple sessions to continue to increase. Here’s a graph of what’s happening in the long run with multiple training sessions.

This makes a lot more sense than GAS, and describes how people respond to exercise much more accurately in the long run. It also helps explain why some people don’t react to exercise as well as others - if your recovery is compromised by poor sleep, then you’ll see your fitness take longer to decrease, and your performance will take longer to become apparent. If you’re working out too frequently, you’re training again before you’re fully recovered, and this is compromising your ability to provoke a greater stimulus. Over time, if you're not fully recovering from one workout to the next, even if this doesn't have any acute impact on fitness, this may cause fatigue to build up over time until you need some time off to fully recover.

If you’re not progressing your workouts properly via periodization, then you’re basically just repeating the single training session graph over and over again with your fitness/performance peaking only to the same height each time and not adding up. This means that you can establish a “basic” level of fitness, but it won’t increase much over time.

Due to the principle of specificity, adaptations to exercise are highly specific to the lift used, the weight used, the velocity of the lift, and many other factors. So, workouts must be kept somewhat consistent (practicing the same things repeatedly) in order to provoke further reactions - otherwise, you’re basically just training an entirely different quality (with its own IR curves) each time you work out.

It also partially helps explain how we get injured. If your current level of performance looks more like the left side of the multiple bouts graph, but you’re trying a workout that’s more suited for the right side of the graph, then you’re pushing beyond your current abilities, and you’re likely to get hurt. It’s only by building up to that higher level of performance that we can handle it.

Conversely, if you’re on the right side of the graph, less difficult workouts will feel easier and fail to provoke much of any further reaction. Think of a guy who can bench press 400 just messing around with the empty 45lb bar. It may be useful for other reasons (warming up), but it’s definitely not a workout for him, and no matter how many times he bench presses 45lbs, it’s not likely to build much muscle for him or increase his strength, even though this may be more than enough to cause someone else to improve.

Another concept that helps to explain adaptation is the repeated bout effect.

The repeated bout effect basically says that over time, as muscles adapt they grow more resistant to further adaptation. This helps explain why (as seen in the IR model above, and in periodization in general) we need a greater stimulus each time we work out - because if not, we’re not overcoming our muscles’ greater resistance to exercise.

This also explains why soreness decreases over time and individual workouts feel easier. As we exercise, it damages the muscles, and they become resistant to further damage. Soreness when starting an exercise routine is high, but this becomes less and less the more we become adapted to exercise.

To simplify things:

To make things simpler, think of it like this.

You work out, providing a strong stimulus that is both positive (increasing further fitness) and negative (exhausting and physically stressful).
If that stimulus is compromised in some way (the positives blunted, the negatives increased), then you will also see compromised results. You've got to carefully manage both the positives and negatives in order to further improve.
After the initial workout, your fitness will be temporarily reduced until you recover. Working out again before fully recovering, or having poor recovery in general, will compromise your results.
If you don’t work out consistently, then fitness and performance will decay over time.
If you do work out consistently, and increase the stimulus progressively, you will continue to improve over time.

This may seem a bit simple in terms of its basic takeaway. After all "work out, rest, work out harder, repeat" doesn't seem like it should be revolutionary or anything.

But I’ll tell you that it’s shocking, even the number of professionals who don’t seem to understand these very basic concepts. Stuff like “muscle confusion”, super frequent workouts (although this can be done, if individual workouts are easier), purposefully and pointlessly exhausting workouts (which worsen your recovery time), and many more, all violate these basic concepts.

It’s important to remember that science is malleable, and that this doesn’t “prove” that there’s only one specific way to work out. There are numerous reasons why we can’t just repeat the same workouts endlessly forever, continuing to improve. This is, at best, a simple concept for explaining how things happen - there’s still plenty of ways that this model can be bent and broken.

However, if your own plan isn’t properly designed to take advantage of the way that these models describe our bodies and their response to exercise - chances are that it’s suboptimal, and that there’s room for improvement.

How Do Our Bodies Really Adapt?

Takeaway Points:

How do our bodies adapt to exercise?

In order to create a model that’s more relevant for fitness, the Banister Impulse-Response model (IR) was developed, which is demonstrated in the graph below.

Another concept that helps to explain adaptation is the repeated bout effect.

To simplify things:

However, if your own plan isn’t properly designed to take advantage of the way that these models describe our bodies and their response to exercise - chances are that it’s suboptimal, and that there’s room for improvement.

Enjoy this post? Share the gains!

Further Reading:

How Do Our Bodies Really Adapt?

Takeaway Points:

How do our bodies adapt to exercise?

In order to create a model that’s more relevant for fitness, the Banister Impulse-Response model (IR) was developed, which is demonstrated in the graph below.

Another concept that helps to explain adaptation is the repeated bout effect.

To simplify things:

However, if your own plan isn’t properly designed to take advantage of the way that these models describe our bodies and their response to exercise - chances are that it’s suboptimal, and that there’s room for improvement.

Enjoy this post? Share the gains!

Further Reading:

Why Science Doesn't Prove Much Of Anything: Understanding Falsificationism

Can Humans Predict The Future?