Inside The Body As It CrossFits

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


On the Inside


The Alarm Stage


The Resistance Stage




The Exhaustion Stage





Body Fat and Breath






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.