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.

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