Human Power Output and CrossFit Metcon Workouts Part-II

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“CrossFit Journal Article wrote by Tony Leyland and published in July 2008. We are republishing it in his own words”.


Muscle cell metabolism


In my article “Rest-Recovery During Interval-Based Exercise” I reviewed the three systems whereby a human can produce the energy required to do physical work (CrossFit Journal 56, April 2007. These systems were also discussed in CrossFit Journal 10, June 2003). I will not repeat a detailed review of those systems here, as I believe that most readers of the CrossFit Journal understand that energy for high force/power outputs is supplied by ATP/CP (high-energy phosphates stored in the muscle) and that as exercise intensity is reduced, more energy can be obtained by the glycolytic system and eventually, as exercise intensities are further reduced to levels that can be sustained for several minutes to hours, the oxidative prevails. As an example, 99 percent of the energy expended during a marathon is provided by the oxidative system.


Due to aerobic training and interval work, the oxidative metabolism of Type I fibers can be enhanced by increases in oxidative enzyme concentration and in the size and number of mitochondria (the site of oxidative metabolism). Similarly, anaerobic training will result in increased concentrations of anaerobic enzymes that enhance the anaerobic ability of Type II fibers.




Muscle tissue capillarization, blood volume, blood composition, and cardiac output are irrelevant for the highest exercise intensities. Single Olympic lifts, maximal throws, and high jump, for example, do not depend on the delivery of oxygen and substrate to the muscles.


As you move to lower-intensity exercise that can be sustained for longer periods, the need increases for the circulatory system to deliver oxygen and fuel and to remove carbon dioxide (CO2 ), waste products, and lactate. As we can see from Figure 1, ongoing power outputs of around 40 to 70 percent of maximum rely heavily on the glycolytic system and the oxidative system, so the delivery of glycogen and oxygen and the removal of metabolites (mostly waste products) from the muscle become increasingly important. By the time exercise intensity is around 30 to 35 percent of maximum, the athlete is in his or her aerobic maximum range, and performance relies heavily on having optimal values for cardiac output, blood volume, muscle tissue capillarization, and hemoglobin concentration (which determines the oxygen-carrying capacity of the blood).


Conventional strength and aerobic training specialization


Most steady, sustained aerobic training occurs around 20 percent of maximal power output and, as discussed above, recruits Type I muscle fibers. Although there may be some small increase in the cross-sectional area of these fibers, this is less important to performance than increases in the oxidative metabolic capacities of the same fibers and increased delivery of oxygen to them. Hence it is not surprising that elite marathon runners and Tour de France cyclists exhibit spare musculature but high blood volume, hemoglobin concentrations, and cardiac output.


Weight lifting prescriptions can be classified as light (12 to 15 rep sets), medium (7 to 12 reps), heavy (3 to 6 reps) and maximal (1 or 2 rep maxes). While specific responses will differ depending on the repetition scheme, it is fair to say that one general adaptation to strength training is an increase in the size (cross-sectional area) of Type II muscle fibers. An important point to understand is that conventional strength training will not improve cardiac function or blood composition and volume. It is interesting to note that there is, however, a change in muscle capillarization. There is not an increase in the number of capillaries, but the size of Type II muscle fibers does increase, which results in the muscle capillaries being moved farther apart. This is called capillary dilution and it is part of the reason very strong athletes who have focused just on strength and power training do not do well at aerobic exercise and CrossFit metcon workouts. (Aerobic training, in contrast, does produce an increase in capillary density that increases the capillaryto-muscle-fiber ratio and improves the muscles’ ability to extract oxygen.) 


Most athletes who strength train and then do some separate aerobic work tend to focus on running or cycling for their aerobic bouts. While these athletes are slightly better equipped than either pure lifters or pure aerobic trainees to handle some CrossFit workouts, even workouts that emphasize strength and lifting—such as “ Linda,” for example—are still done for time and are grueling metabolic conditioning workouts (sometimes surprisingly so) that will punish those who specialize at either end of the energy spectrum. CrossFit metcon training requires intense but extended work of all muscle groups. This prevents the capillary diffusion that occurs with a predominant focus on low-rep strength training with long rest periods between sets. Traditional strength training does not challenge the body to deliver oxygen and other fuels and to remove metabolites in the way a metabolic conditioning workout like “Linda” does. 


Many athletes, and nearly all of the general public who actually exercise, work at either end of the power spectrum, as described above. Many do both, but, by keeping them separate, they are in effect still specializing—just in two training modalities instead of one. Only athletes who are involved in sports that require frequent recovery from high bursts of power output tend to work at all power levels. For example, an elite soccer player will cover approximately 12 km in the 90 minutes of a match. As a steady running pace this isn’t impressive, but the soccer player typically walks for 20 to 30 percent of the match, jogs for 30 to 40 percent, runs at pace for 15 to 25 percent, sprints for 10 to18 percent, and runs backward for 4 to 8 percent of the time. Pretty much every pace is included and we know that recovery from sprints is an excellent way to improve aerobic capacity while also stressing anaerobic systems and Type II fibers. In addition to a variety of running paces, soccer players will have to jump, tackle, brace against shoulder charges, and get up off the floor after being knocked over dozens of times during a match.


Soccer is just one example of a sport with a variety of demands, but in this case, unfortunately, the vast majority of the (still relatively low) strength demands are on the leg musculature. Hence soccer players who do not do any strength training (ideally CrossFit-style) do not display significant strength and power, particularly in the upper body. Rugby, though, requires incredibly varied power outputs from all the body’s musculature. Obviously, mixed martial arts and many other sports would similarly tax all muscles, all three energy systems, and all muscle fiber types. My point is that while all athletes will find metcon WODs challenging, athletes who work in anaerobic-based sports with relatively long match durations are better able to handle CrossFit programming at the outset than those coming from purely strength- or aerobic-based training programs.


Reference: http://library.crossfit.com/free/pdf/71_08_Human_Power_Output.pdf?_ga=2.175298402.1199360240.1651169709-469422278.1651009173

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