Understanding VO2 Max & Its Importance in CrossFit
If you’ve been in and around a CrossFit gym lately, you’ve probably heard the term “VO2 max” tossed around. As a measure of fitness, athletes tend to place it in high regard. But what is your VO2 max? And does it really matter for CrossFit performance? Let’s find out.
WHAT IS YOUR VO2 MAX?
VO2 Max, as its name would indicate, refers to the maximum volume of oxygen (O2) you can use during a workout. During aerobic exercise, our bodies take oxygen from the air, transport it through the bloodstream, and utilize it during cellular respiration. At the cellular level, that oxygen combines with the glucose you get from food to produce a ton of ATP (energy). It’s a slow process, but it can fuel certain levels of intensities for a long time. The more work you do, the more oxygen you take in. However, like everything else, it has its limits.
Eventually, whether it’s through increased intensity or duration, we all reach a point where our oxygen intake can no longer keep up with the high demands. That limit is your personal VO2 max. Oxygen consumption starts to plateau, and, if you continue to train past this point, your performance will likely drop soon after. Even if exercise intensity continues to increase, your mitochondria physically cannot process any more oxygen. Fortunately, we can rely on anaerobic metabolism for more fuel, in theory. But in reality, most people reach their lactate threshold way before this point. More on that later.
HOW DO YOU MEASURE YOUR VO2 MAX?
To measure VO2 max, we use something called the Fick equation, which is as follows:
VO2 = Q x (a - vO2)
- Where VO2 refers to the volume of oxygen, measured in liters per minute,
- Q refers to cardiac output (heart rate times stroke volume)
- ...and a-vO2 is the difference in oxygen content between arterial and venous blood, measured in ml of oxygen per 100ml.
When VO2 reaches its maximum, your heart can’t beat any faster (heart rate), pump any more blood per beat (stroke volume), and your muscles are extracting maximum amounts of oxygen from your arteries (a-vO2). But how do you know when you’ve reached this point in a CrossFit metcon?
Within the lab, exercise scientists and trained professionals will set up athletes on a piece of stationary exercise equipment, such as a bike or treadmill. Using a specially-designed assessment tool, athletes place a heart rate monitor around their chest and a mask fully around their mouth and nose. These devices connect to a machine to collect data - both heart rate, oxygen intake, and carbon dioxide exhalation. After a period of calibration, the athlete will begin exercise at a low level of intensity, gradually increasing intensity at regular intervals (usually every 1-3 minutes). The measured VO2 rises accordingly until the athlete reaches their max, upon which the machine will indicate a plateau.
While this is all well and good, I've yet to see Dave Castro put "masked breathing run hooked up to a machine for distance" in the Open. So can it really serve as a test for CrossFit?
Other, more practical methods do exist. For example, you could estimate VO2 max using a treadmill or bike. The Bruce Treadmill method involves seven stages of increased running speed and incline every three minutes. The test continues until exhaustion, and an estimate of VO2 max is determined accordingly. That protocol can be found here. You can also use a Wattbike to do the same for cycling, which could be important for those of us who frequent the Assault Bike. That protocol can be found here.
Regardless, CrossFit is so much more than running or biking. By definition, it requires fitness across multiple codes - anaerobic endurance, aerobic endurance, strength, power, speed, flexibility, and skill. While scoring well on the above VO2 max assessments indicates strong aerobic endurance, it tells you nothing about your ability to snatch 225lb or climb a rope. However, there is one other hidden measure within these tests that does correlate with success in CrossFit - the lactate threshold.
THE LACTATE THRESHOLD
At some point on our journey towards maximum oxygen use, we reach another milestone. Lactic acid, a natural byproduct of glucose metabolism, builds within the blood in response to exercise intensity. This is especially the case as oxygen utilization reaches its peak. Lactic acid rapidly breaks down into lactate and hydrogen ions. Those free, positively-charged ions compete with calcium for binding sites, interrupting muscle contraction. Therefore, power is significantly reduced, performance drops, and you’ll feel pretty gassed. At some point, it becomes too much and sort of overwhelms the system. That point is called your lactate threshold.
Endurance training can increase mitochondrial density, adding more of a "clean-up crew" to remove lactate from the blood and recycle it as pyruvate. Training at the edge of your abilities (with adequate recovery, of course) can, therefore, extend your lactate threshold. Even with equivalent VO2 max scores, an athlete with a higher lactate threshold can go longer at greater intensities before seeing a decrease in performance. This is especially critical in a sport like CrossFit, where it's a race to see who can complete grueling tasks the fastest.
THE RELATIONSHIP BETWEEN CROSSFIT, AEROBIC FITNESS, AND ANAEROBIC THRESHOLD
So, will increasing your VO2 max and lactate threshold make you better at CrossFit?
The frustrating answer: it depends.
Plenty of Ultra marathon runners with VO2 maxes through the roof would struggle to complete Grace in under six minutes. Your lactate threshold is a better indicator of CrossFit performance, in theory, but it's impossible to determine. That's because CrossFit, for all of its amazing benefits, varies so much from workout to workout, box to box.
According to its founder, Greg Glassman, CrossFit measures fitness according to 10 physical skills: “cardiovascular/respiratory endurance, stamina, strength, flexibility, power, speed, coordination, agility, balance, and accuracy". Moreover, he continues to insist on competency in all three of the metabolic pathways - phosphagen, glycolytic (lactate), and oxidative (aerobic).
By definition, training to increase your VO2 max benefits the lactate and aerobic systems. But let’s look at the 2019 Games. Elite CrossFitters were asked to do everything from swimming 2000m and rucking 6000m to clean 385lbs and handstand walk 132ft. VO2 max has everything to do with the former, nothing with the latter.
Meanwhile, this sprint couplet is 100% a test of anaerobic capacity, involving a highly-technical skill:
- 172-ft. sled push
- 15 | 18 bar muscle-ups
- 172-ft. sled push
In short, improving your VO2 max and lactate threshold will increase CrossFit performance ONLY if the skills required to compete aren’t ignored. That means focusing on basic strength, maximum power, and gymnastic skills with equal fervor.
ASSESSING AND DEVELOPING VO2 MAX FOR CROSSFIT
If you can get access to a facility with VO2 max testing equipment, take advantage of it. That's the most reliable measure. It's also the only way to truly test your lactate threshold (although The Journal of Strength and Conditioning Research has shown a new wearable device to have promise). Unfortunately, a large majority of the CrossFit community won’t have this opportunity. As such, look to one of the treadmill or cycling tests mentioned previously to get a gauge. Once you know where you’re at, you can start working to get better.
IMPROVING YOUR VO2 MAX
If you’ll remember the Fick equation from above, in order to increase VO2 max, you have to improve one of the corresponding variables. Maximum heart rate is mostly a function of age, but stroke volume, oxygen use, and lactate clearance can be trained. Research shows that both resistance training and high-intensity interval training can increase stroke volume. Further research confirms that elite endurance athletes can increase their stroke volume in response to incremental exercise. Other studies suggest that a 2:1 interval training protocol (with work at levels nearing VO2 max) elicits improved endurance performance and oxygen use. Finally, research shows that endurance training can increase time to peak blood lactate, essentially elevating your lactate clearance capacity. This training includes high volume, maximal steady-state, and interval training.
WHAT DOES THAT LOOK LIKE FOR CROSSFIT?
Train aerobic capacity through intervals and pacing, and every so often test your lactate threshold. If anything, riding the “pain train” every few weeks helps overcome mental blocks to pushing through a workout. For example, use anywhere from 2:1 to 4:1 intervals during an AMRAP - pushing to hit unbroken reps for 20 seconds, resting for 5-10, then going again.
When designing workouts, coaches should consider utilizing timed intervals with rest periods as well. This approach yields consistent, clean reps nearing one’s maximum capacity. For example, ask athletes to complete X amount of reps under a minute, rest for 30 seconds, and repeat. Pepper in endurance days as well. Not only will this aid recovery, but it’ll teach pacing and develop the aerobic capacity needed to increase VO2 max. Examples include 5000m rows, Murph, or 30-40 minute metcons. Smart athletes will aim to keep a consistent “rep per minute” rate on these workouts.
Finally - and arguably most importantly - focus on movement economy. CrossFit requires an elite level of skill. No other sport asks you to be a gymnast, weightlifter, Strongman competitor, and triathlete all-in-one. A ring muscle-up, burpee box jump-over, sandbag carry, and clean all involve specific, coordinated muscle activity. Any displacement from the absolute necessary amount required is a waste of energy. AKA the more skilled you are at a movement, the more energy you’ll have left in the tank. While CrossFit often champions pushing through a metcon, practicing poor form every day can lead to wasted energy. Make everything crisp and consistent. That way, as your aerobic and anaerobic capacity improve, you can actually apply them towards performance. And if you ask me, that’s all an elite athlete can ask for.