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Inside The Body As It CrossFits

From your first “3-2-1-Go!” CrossFit’s ever-changing physical demands begin sculpting your body.


It changes you. Almost instantly. You become simply different; better. From the moment you encounter CrossFit, your body works to meet the sport’s various (and constantly varied) demands. Your heart gets more efficient, your muscles get stronger and your body gets leaner. Understanding how your body makes those adaptations involves a little digging into exercise science, but you’ll be rewarded with the ability to better appreciate exactly how your body has changed and how CrossFit makes you, well, a better version of yourself.

On the Inside

Our bodies are almost constantly experiencing stress, and we’re very fortunate that they’re enormously able to adapt to it. We’re not even talking about the idiot who cut you off on the highway or the ridiculous deadline your boss handed down or the dinner with your in-laws that’s looming on your calendar, though the body does react to those kinds of stressors, too. Instead, we’re addressing physical stressors. Like all exercise, CrossFit causes stress to the body, and a process called the general adaptation syndrome (GAS) forces the body to adapt to it, via three main stages.

The Alarm Stage

In the first stage, the body encounters an “unaccustomed” stressor; that is, a physical demand it doesn’t have the capability of meeting completely. (Think of your first thruster, muscle-up or pistol.) This puts stress on cells (like muscle cells) involved in the processes that are called on to meet that demand. When a cell is stressed beyond its capacity, it gets damaged. In the case of muscle tissue, cells (also called muscle “fibers”) are disrupted. The cell membranes are degraded, which causes swelling, and you spend the next day or so moving gingerly and wincing from the resulting delayed onset muscle soreness (DOMS).

The Resistance Stage

Within the next 36 to 48 hours, your body begins transitioning out of the alarm stage and DOMS subsides. It has now gauged the damage, responded to it and can begin the repair process during what is known as the resistance stage. Your body actually responds in a very logical way to this stress event. It has just encountered a demand it was ill-prepared to meet. That stressor caused damage, and the body, which is in a constant battle to achieve stasis, doesn’t want to go through the alarm phase again should you re-encounter that stressor in the near future. 

So, during the resistance stage, muscle cells not only repair themselves to the level they were at before, but they also complete a repair process that will ensure they’re better able to meet the demand of that stressor. The mechanism by which a muscle cell becomes stronger is not a physiological mystery; we know how it happens. During the resistance stage, your muscle cells take up new proteins and synthesize them into the microfilaments needed to cause muscle contraction and into other protein structures in the muscle. At the end of this process, which may take as many as three to four days (depending on the level of the damage), you will have more microfilaments pulling on tendons, which creates more force and increases strength. So the next time thrusters are on the whiteboard, you’ll be better able to meet that physical demand.


The Exhaustion Stage

We all need the alarm and resistance stages if we are to become stronger, faster, fitter and more powerful. However, if we don’t allow sufficient time for the body to complete the repair process, we run the risk of moving into the next stage — exhaustion. In the exhaustion stage, cells are not fully repaired before again encountering a demand they cannot meet. Once again, damage occurs, and the alarm process begins. Over time, insufficient repair time, stemming from training sessions that are too intense, too frequent or too frequently intense, leads to overtraining and breakdown. 

The first marker of overtraining and insufficient recovery is poor physical performance: WOD times get worse, and it can be harder to lift loads you used to manage easily. Taking the general adaptation syndrome into account, three things become clear for those wanting to understand how CrossFit can and does improve athletic capabilities. 

First, how much time the body spends in the resistance stage is key. Therefore, rest truly is part of the adaptation picture. Rest is not laziness; it’s required for improvement. Second, nutrition is an important part of the picture. In particular, sufficient protein intake ensures that muscles in the repair process have adequate circulating amino acids to synthesize new microfilaments and increase muscle force. Third, herein lies the beauty of CrossFit’s mantra, which specifies that the stressor must be “constantly changing.” Show us someone who enters a gym every day and does the same thing, and we will show you someone who is merely going through the motions, whose body adapted a while ago and is no longer making improvements. 

By virtue of its very structure, CrossFit does not allow such stagnation to occur. This sport gives your muscles and cardiorespiratory system something different every day, something to which they must adapt. That creates an environment in which muscles and energy systems regularly encounter an unaccustomed stress and have no choice but to respond.

Body Fat and Breath

We haven’t done any formal research, but we’d be willing to bet a significant number of burpees that one of the primary reasons people start CrossFitting is to lose weight. Fortunately, it assists quite handily in that pursuit. As with muscle mass, CrossFit will burn just enough fat to yield optimal CrossFit performance. A physique that is too lean doesn’t provide adequate energy stores for a long WOD or repeated bouts of performance, and, even though it’s common to feel stronger when you’re packing a few extra pounds, a body that carries too much fat will struggle when cardiorespiratory endurance is called on. 

These two physiological variables are intrinsically tied. Cardiorespiratory endurance is measured by assessing how much oxygen an athlete can use during intense exercise, and this indicator of aerobic capacity is termed VO2 max (maximum oxygen uptake). VO2 max is expressed in milliliters of oxygen per kilogram of body mass per minute (ml/kg/min), and because body mass is part of the measurement, your weight affects your ability to carry and use oxygen during endurance exercise. 

Here’s what happens as you progress at CrossFit: During a WOD, your body supplies energy to the working muscles through various means. The more aerobic the exercise, the more energy comes from oxidative mechanisms (aka burning glucose and/or fat). Over time, two things happen to make you more aerobically fit and better able to withstand a long WOD. First, your working cells become able to extract more oxygen from the bloodstream, thereby providing a way for those cells to attain more ATP (the body’s source for energy during exercise) for muscular contraction. Second, your body’s oxygen-utilization capacity increases. That is, you become more efficient at accessing oxygen and using it for energy production during a workout. 

Athletes often work hard to improve their VO2 max, but what many don’t realize is that one way to increase VO2 is to drop a few pounds. Less bodyweight means less work for your body to do — less mass to carry during “Murph,” so to speak. Right there you can see how CrossFit benefits the body. It optimizes body mass so that muscular work can be performed with sufficient energy stores, and it simultaneously burns fat (thereby reducing total bodyweight) so that aerobic capacity can be optimized, as well.


On the Outside

Now that you know what goes on inside your body when, say, “Fran” or “Helen” or “Cindy” (or any of their friends) come calling, it’s easier to outline the more visible results of CrossFitting: what it does to your musculature.


When muscle grows as a result of CrossFitting, it does so as a function of the stresses placed on it. Those stresses are specific to CrossFit, just as stresses from other sports are particular to them. For instance, by doing multiple sets of eight to 12 reps and employing short rest periods, bodybuilders achieve a large amount of muscle size without the same level of force development, or strength. 

Likewise, powerlifters work with low-speed movements at weights of five to eight RM and achieve increases in force production without the same level of muscle growth. In the same way, a CrossFitter’s muscle size adaptations are particular to him or her. While we know what is going on in the muscle (increased protein synthesis as a function of the “alarm” phase), we also see that the wide variety of demands placed on the muscle causes it to adapt in many ways — not only with some growth but also with low- and high-speed strength. (This is important in strengthening the associated tendons, as well.) 

So the CrossFitter develops the muscle size specific to his or her needs. Microfilaments are increased and muscles get bigger, but more important, they become functional. That is, they become better at meeting a specific physical demand. This means the CrossFitter has only the amount of muscle size necessary to achieve performance objectives. But that muscle is also efficient, and the nervous system calling it into action is primed through all those many hours of practice.


CrossFitters need muscles that are strong through full ranges of motion and at high speed. If mobility is compromised because of muscle bulk, that full range of motion cannot be realized. Ever seen an athlete with large shoulders and biceps try to rack a bar at the top of a clean? His muscle tissue gets in the way, and he ends up holding the bar off his shoulders. 

So does that mean CrossFit automatically limits muscle size? No. What it means is that CrossFit will induce adaptations such that muscle size is enhanced for optimal performance yet not at the expense of range of motion. In other words, CrossFit creates a balance of musculature. It’s very common for athletes to have training experiences in which they focus on certain lifts, emerging with compromised ranges of motion because of muscle imbalance. Not only does this limit mobility, but it also places the athlete at increased risk of injury.

 CrossFit doesn’t let such an imbalance develop. Muscles on all sides of joints are taxed in WODs and full ranges of motion are maintained accordingly. If you stay with CrossFit, you will experience the opposite of what many experience in standard gyms; instead of becoming less mobile as your muscle tissue grows, you will become more mobile.



It seems to be a fairly common belief among athletes who transition into CrossFit from other athletic pursuits that they’ll somehow become weaker in exercises at which they previously excelled. This is rarely the case, and in fact, strength even in exercises not commonly performed in CrossFit boxes either remains steady or increases. What is happening physiologically is a phenomenon called “muscle memory.” Muscles can retain (and regain after injury) their previous strength with far less activation.

Exercise scientists still don’t quite understand the mechanism behind this, but they suspect that some function of the nervous system and/or hormonal activation triggers a kind of “memory” in the muscle. Further, because of the variety of movements intrinsic to CrossFit, it’s very common for athletes to encounter exercises that include the same biomechanical movements as the ones they’ve stopped doing. The muscles involved will still have been trained (and hard), just in a different way, further reducing the risk of atrophy.

The Shape of CrossFit

Movement molds the body, and athletes who specialize in one sport have adaptations skewed to that sport. This is why Rich Froning and Camille Leblanc-Bazinet have the bodies they do. They, and other high-level or long-term CrossFitters, have the optimal physical structure and muscular adaptations for functional performance in CrossFit. In other words, that’s what a CrossFitter should look like. And further, those changes are what enables a body to be great at CrossFit. Because so many muscles are trained in a single WOD, it’s actually quite difficult to identify or highlight one or more muscle groups that “define” a CrossFitter. However, two specific muscle groups tend to become remarkably well-developed in dedicated CrossFitters.


In CrossFit, your quads have no choice but to adapt. From thrusters to cleans to squats, quads are stressed so much and so often that the only way they will survive is to become stronger. Some of that strength will include enlarging the muscle tissue; most will include increasing myofilaments within the muscle cells. But what will also occur with training, especially if caloric intake does not increase dramatically, is that (a) intramuscular fat in the quads will be reduced, and (b) the quads will grow to realize the fullness of the muscle. Yes, that can mean that the quads will get bigger (depending on how much intramuscular fat was lost relative to the amount of muscle gained), but their overall shape will also change.


We challenge you to find another sport (or even training program) that incorporates more overhead movements than CrossFit. And when the pressing and jerking and handstand push-ups stop, there’s always the isometric overhead work, such as overhead squats and overhead walking lunges. All that overhead work is why CrossFitters experience shoulder muscle (deltoid) development. Think of the deltoid as a round cap to the shoulder joint. In its pushing, pressing and pulling movements, CrossFit calls on all three aspects (groups of fibers) of the deltoid: the anterior, lateral and posterior portions. Regular CrossFitters simply can’t avoid training any part of the deltoid, and that creates balance that gives the shoulders a full, round look.