A Guide To Improving Muscle Endurance In Crossfit (Updated 2025)

A Comprehensive Guide To Improving Muscle Endurance For The Crossfit Athlete

An ask: If you liked this piece, I’d be grateful if you’d consider tapping the “heart” ❤️ in the header above. It helps me understand which pieces you like most and supports this newsletter’s growth. Thank you!

---

🧬 Going Beyond VO2max To Improve Crossfit Performance

In recent months I’ve spoke to a large number of competitive Crossfit athletes dedicating a significant amount of time to increasing their VO2max, including two former games medalists. Of these athletes i’ve spoken to, a subset have meaningful increased their VO2max (+ >5ml/kg/min), only to find that their sport-specific performance in metcons hasn’t improved. To me, this shows a fundamental misunderstanding about the relationship between VO2max and performance in mixed, and cyclical, sports. My general stance on this topic is as follows:

Having a high VO2max is necessary, but not sufficient for elite level endurance performance.

In other words, an individual is unlikely to be an elite endurance athlete if they do not have a high VO2max, but having a high VO2max alone is not enough to compete at the highest levels of sport. Additionally, once someone is already at an elite level in an endurance sport, improving their VO2max further may not improve their performance and can even be counter productive. This same concept broadly applies for other work capacity works, like Crossfit.

The broader the demands of a given sport, the more factors that are necessary, but not sufficient, for to perform at a high level. For example, being able to complete thirty ring muscle-ups in under four minutes is necessary, but not sufficient to be an high level Crossfit competitor. As is being able to snatch 125kg and row a 2k meters in under 6:20, among many other metrics. This concept reminds me of a quote by Marilyn Strathern where she states, "When a measure becomes a target, it ceases to be a good measure.” If sufficient rewards are attached to some measure, people will find ways to increase their scores on that measure one way or another, and in doing so will undercut the value of the measure in assessing what it was originally intended to assess.

In order to be a competitive Crossfit athlete we know there are some minimum strength metrics that need to be achieved, certain paces an individual need to be able to sustain for a given duration on the rower, and some ballpark estimates they should be able to hit on workout such as thirty muscle-ups for time, one hundred strict handstand push ups for time, and so forth. However, being able to achieve all of these milestones does notnecessitate that an individual will be a great Crossfit athlete. It’s a matter of necessity versus sufficiency. Being able to complete these metrics is the equivalent of being accepted to study at a university. Just because a student has been admitted through the doors does not mean they are automatically eligible to graduate, or that they’ll do particularly well in their classes.

For these reasons I’m typically hesitant to put athletes I work with through generic sport specific testing batteries, like those traditionally advocated for by or like Opex (formerly OPT) — anyone who was part of that community in the early days of Crossfit as a sport knows exactly what i’m talking about. At best, these types of assessments allow me to check a few boxes and see where a given athletes stack up relative to the field, which is helpful for determining what they need to prioritize in their training. However, improving on specific skills, or benchmark workouts, in isolation won’t necessarily translate to improved sports performance, broadly speaking. Furthermore, it can create the illusion that improving on the test metrics is the goal in and of itself. As a result, I take a different approach and approach sport specific assessments from a bottom-up standpoint.

Conceptually, I think about Crossfit, or any other work-capacity based event, as a cyclical endurance sport. Like any cyclical endurance sport, the goal for a Crossfit athlete is to move continuously, in effect turning a metcon into a cyclic activity.

When an athlete is incapable of performing a metcon in a cyclic manner I'll investigate why that is the case. After identifying their rate-limiting factor I will train them to overcome that limitation. For example, instead of having an athlete perform a classic test such as thirty ring muscle ups to see how they stack up against their competition, I'll assess whether or not they can perform ring muscle ups in a cyclic fashion during a sport specific event. If not, my aim is to understand why. For example, a beginner athlete may lack the requisite strength to perform consecutive muscle ups. An intermediate athlete may struggle with their breathing mechanics and coordination under fatigue. Finally, an advanced athlete may be limited by their ability to supply oxygenated blood to the working muscle at a fast enough rate.

---

🧬 Science Backed Wisdom From A World Class Coach

Prior to the 2016 Crossfit games Training Think Tank hosted an athlete camp with a handful of top competitors. One of the competition simulations at the camp included a high volume of kettlebell snatches, box jump overs, and rope climbs. During the event the head coach, Max El-Hag, made a comment that one of the athletes was able to complete the metcon as if it was a cyclical event. It wasn’t intended to be an analytical statement. Rather, he was acknowledging the fact that the athlete did not stop moving for more than a split seconds whereas a lot of the other competitors were breaking up their kettlebell snatches, using more time for their transitions, and generally looked like they were approaching the metcon as a circuit with defined work and rest periods.

Max’s observation really struck a chord with me because it matched my observations from conducting physiological tests on a LOT of Crossfit competitors ranging from the beginner to games level. In my observations, the best Crossfit athletes can turn the majority of metcons into cyclical workouts whereas the rest of the pack cannot. For example, the top athletes have steady blood flow to the working muscles, a nearly linear rate of oxygen utilization from start to finish, and their VO2 kinetics look similar to what you’d expect during a two thousand meter row versus a circuit style workout. This is demonstrated in figure below, which depicts two crossfit games athlete’s muscle oxygenation (SmO2) trends during a metcon that includes thrusters, burpees, and rowing.

At the time of the aforementioned workout the athlete whose data is on bottom, in the image above, was a top ten individual games competitor, and the athlete whose data is on top was a sanctional level competitor who later qualified for the games as an individual. Note that the athlete on bottom has a near linear oxygen desaturation trend across the workout without any major dips or peaks. They were able to move through the workout unbroken with minimal rests and transitions, thus allowing for a very high rate of energy turnover. The athlete on top, on the other hand, had less steady blood flow to the working muscles and as a result they were forced to stop, rest, and complete the workout in small chunks of intervals interspersed with rests and long transitions. Interestingly, the total amount of work time for the athlete on top is actually slightly less than the athlete on bottom, indicating a faster rep speed, but it was so broken up so much that they ended up taking over two minutes longer to finish the workout.

This raises the question, Why can some athletes turn metcons into cyclical work but others cannot? The rest of this newsletter will be used to answer this question, as well as to provide some practical takeaways for how we can get an athlete to make metcons more cyclical in nature based on their individual sport specific limiters.

---

🧬 Understanding Local Muscle Fatigue In Crossfit

Among hybrid athletes like Crossfit competitors, and mixed martial artists, local muscle endurance is a commonly cited exercise limiter. However, the cause of local muscle fatigue is poorly understood, and as a result training interventions intended to improve local muscle endurance have mixed results.

One of the leading causes of local muscle fatigue is a restriction of muscle blood flow due to high intramuscular mechanical pressure. Under ordinary circumstances there are two different mechanisms by which muscle blood flow increases. During muscle contraction muscle blood flow is diminished, and during muscle relaxation blood flow increases. This process is known as active hyperemia, and it regulates blood flow on a contraction by contraction basis. Across many muscle contractions there is another process called auto-regulation that increases blood flow in response to muscle deoxygenation. Interestingly, devices like the NNOXX wearable can clues athlete’s into this process by monitoring both muscle oxygenation (SmO2) and nitric oxide (NO), the latter of which regulates local muscle blood flow and oxygen delivery.

An athlete’s muscle oxygenation (SmO2) and nitric oxide (NO) levels during a 2-mile Echo Bike time trial.

Both of the processes mentioned above occur simultaneously and their combined effects determine the net change in blood flow moment to moment. However, there are cases where both of these responses are blunted, thus decreasing muscle blood flow and oxygen availability. For example, when individuals employ high threshold movement strategies, contract their muscles with excessive force, or have elongated muscle relaxation times they will impede muscle blood flow, which will quickly lead to local muscle fatigue limitations and an inability to sustain work-output.

The aforementioned phenomenon was observed in a study titled, Assessment of lower-back muscle fatigue using electromyography, mechanomyography, and near-infrared spectroscopy , where the investigators observed mechanical pressure decreasing muscle blood flow during muscle contraction due to a compression of the blood vessels. At the start of muscle contraction the capillaries in the muscle are compressed, thus driving muscle blood volume down. Then upon the cessation of contract muscle blood volume returns back to baseline. When the capillaries in a muscle are compressed the muscle will deoxygenate as oxygen utilization supersedes oxygen delivery. This will manifest as local muscle fatigue, which is exacerbated during high density bouts of exercise where the muscle cannot fully reoxygenate between repeated contractions. Now you know the underlying cause of local muscle fatigue. However, that still leaves the question of how an individual can improve their local muscle endurance and performance.

🧬 Identifying Individual Limitations For Local Muscle Endurance (+ Training To Improve It)