After filming the entire TTT Exercise Physiology course TWICE last week I’ve had a lot of time to think about some of the most misunderstood physiological concepts in training CrossFit athletes. It dawned on me while I was explaining the role of the cardiovascular system during exercise that so few people understand what is actually happening in the body during endurance training. Furthermore most coaches have no way to identify what training adaptations they actually need to target in their athletes to improve their metcon performance. As a community we are reductionists: we tend to look at an athlete's endurance capacity simplistically…someone is either “good at long-endurance metcons” or they are not.
There are however two major players (there are others….) that interact to determine an athlete’s ability to perform for longer durations: the local aerobic capacity of the working skeletal muscle tissue AND the capacity of the oxygen delivery system (cardiopulmonary/cardiovascular systems). While in single-modality sports like running or cycling these two systems adapt in parallel - CrossFit™ on the other hand, a sport that is defined by mixing modes of exercise, coaches can easily develop a mismatch in the capability of the body to deliver versus consume oxygen.
The basic concept is pretty simple: the capacity of our skeletal muscle tissue to utilize oxygen in order to create energy (in the form of ATP) is determined mostly by the density of mitochondria in that tissue. The more mitochondria we have the more oxygen we can utilize, the more energy we can produce aerobically. The key here is that the primary way our bodies increase the mitochondrial density of a specific muscle is to subject that muscle to repeated fatiguing contractions (generally: endurance training). Rowing ONLY develops mitochondrial density in the muscles used to row, running ONLY develops mitochondrial density in the muscles used to run, double-unders ONLY develop mitochondrial density in the muscles used to hop in place (you probably understand my point now).
Now, in most endurance athletes USING oxygen in the muscles is NOT the limiting factor, mostly because runners run, rowers row, swimmers swim, etc. The training they do on a daily basis develops the oxygen utilization capacity (AKA aerobic capacity) in the exact muscles they need to be increasing endurance in. For these athletes the limiting factor is their ability to deliver oxygen to the working muscles NOT utilization. As we undertake a structured endurance training program we have a number of “central” adaptations that occur allowing us to deliver blood (and therefore oxygen) more effectively to exercising muscles. The chambers of our heart grow to allow more blood to be pumped with each contraction, our blood volume increases making our hearts fill more efficiently, and our bodies make more red-blood cells - the oxygen carrying component of our blood. These central cardiovascular adaptations occur generally in parallel with improvements in muscular aerobic capacity but inevitably the oxygen carrying capacity of the blood will become the ceiling limiting further improvements in endurance performance.
Enter the CrossFit™ athlete. CrossFit™ athletes do lots of “mixed modal” training in order to become more well-rounded and prepared for any task. The problem with this is that the sport targets SO MANY muscle groups that we rarely perform enough training volume in any one single modality to increase local muscular aerobic capacity. HOWEVER all of the mixed high-intensity training does stimulate central adaptations, making the cardiovascular system of elite CrossFitters as efficient as those of elite single-modality endurance athletes. You see, our hearts don’t really know WHAT kind of activity we’re doing, they just react to the increased demand for oxygen of the working tissues by delivering more oxygen. So in essence we have these athletes with amazing oxygen delivery systems...but they cannot use all of the oxygen delivered in order to make energy aerobically. The result is an athlete who is limited by “local muscular endurance” not breathing.
As a coach I’ve had many athletes explain a similar phenomena to me: at some point in their training they reach a point where they can’t “get their breathing up” the way that they did when they first started. In other words, they feel like they aren’t working that hard from a cardiovascular / pulmonary standpoint (which is probably true) but they can’t go any faster because their legs/arms/shoulders were too fatigued. As a coach this is a really good indicator that it is time to shift your athletes focus from cardiovascular fitness toward developing muscular endurance. The protocols for developing local-muscular endurance for the sport of CrossFit™ have not been refined to the point that we can make specific recommendations. Traditionally this has been done with high-volume cyclical endurance training but this doesn’t really address the upper-body gymnastics capacity needs of CrossFit™ athletes. Max and I have been working to develop a system that incorporates using isometrics, higher time-under tensions, and forced cyclical recovery work to develop this in our athletes.
The interesting thing is that many of these athletes initially describe their primary limiting factor during their initial consults with me as “breathing” and their worst workouts are “heavy breathing workouts”. So many athletes associate high-respiration rates with fatigue and therefore when they start breathing hard they feel the need to slow down or stop to breathe. Part of this is driven by the fact that it takes energy to breathe. In fact research shows that the “energy-cost” of breathing can be upward of 30% of the total ATP production in athletes working at near-maximal effort levels for extended periods of time. Developing a very efficient cardiovascular/pulmonary system allows athletes to (1) be less limited by “breathing” during mixed-modal metcons (2) reduce the energy cost of breathing making the body more efficient at supplying energy for working muscles instead of breathing muscles.
I think the tests in the 2015 Open were a really good indicator for most athletes that developing both high levels of cardiovascular capacity as well as improving their local muscular endurance is critical to success in this sport. Without targeting these adaptations sequentially in a planned progression throughout the training year coaches risk ensuring that their athletes are optimally prepared for the demands of our sport.