If you’re a serious golfer, you might get asked, “How far can you drive it?”

If you’re a baseball pitcher, it’s, “How hard can you throw it?”

If you’re a runner, a similar question might be, “What is your VO₂ max?”

The longest drive and fastest fastball don’t always win, and the same is true for VO₂ max. Instead of being the bellwether of performance, VO₂ max is one of many variables that go into making a complete trail runner. Here is a primer on what you need to know about VO₂ max.

What is VO₂ max?

VO₂ max is the maximum amount of oxygen a person can use, expressed in milliliters of oxygen per kilogram of bodyweight per minute. In basic terms, the more oxygen you can take in, the faster you can run.

VO₂ max generally corresponds to an effort an athlete can sustain for seven to 11 minutes (with variance due to physiology and how VO₂ max is being measured). Typically, VO₂ max is measured in a lab, on a treadmill, with an athlete pushing herself until she heaves up her Froot Loops. The point at which measured oxygen consumption plateaus is VO₂ max.

 Many of the best athletes are famous for otherworldly VO₂ maxes. While most people have VO₂ maxes around 30-55 ml/kg, Kilian Jornet has an astounding 89.5 ml/kg VO₂ max. Matt Carpenter, who currently holds the record for Pikes Peak Ascent, was at 92 ml/kg in his peak fitness. A typical Iditarod sled dog tops the charts at 200 ml/kg.

What is VO2 max not?

While many top athletes have high VO₂ maxes (mostly due to genetic luck), this alone does not predict much. Variables like lactate threshold, aerobic threshold and running economy also play a major role. To put it another way, a good VO₂ max will get you into the club, but it won’t teach you how to dance.

The VO₂ max paradox centers on the fact that VO₂ max doesn’t improve much with training in experienced runners. One study in the Journal of Sports Medicine and Physical Fitness looked at 33 elite runners who improved on average one percent over the course of three years. However, those athletes’ VO₂ maxes didn’t improve at all.

Another case study in the International Journal of Sports Science and Coaching looked at marathon world-record holder Paula Radcliffe and found that Radcliffe’s VO₂ max remained steady from 1992 to 2003, despite a massive increase in training and performance.

The runners that benefit most from VO₂ max training are less-trained athletes, who might see large jumps in VO₂ max when they first start out. For example, a study in the journal Clinical Science put six healthy recreational athletes on a program involving increasing weekly running volume. Over the course of 36 weeks, the athletes went from from 20K to 73K per week, with gradually increasing intensity. At the end of the study, the athletes’ VO₂ maxes had increased an average of 13.7 percent. However, all of the changes were measured in the first 24 weeks, without any increase in VO₂ max in the ensuing 12 weeks.

In sum, there is strong evidence that while VO₂ max is determined mostly by genetics, it can be improved by five to 20 percent early on in training. Past a certain point, however, it stabilizes (and reduces with age). However, VO₂ max stabilization does not correlate with stabilization in performance gains. Even after VO₂ max has plateaued, athletes can have major leaps in performance.

Why is focusing on the VO2 max number in training sub-optimal?

If VO₂ max doesn’t change significantly in well-trained athletes, hammering out VO₂ max intervals probably isn’t going to make that number budge. Moreover, as described by top coach and exercise scientist Steve Magness in an article titled “The Fallacy of VO₂ Max,” using VO₂ max to determine training intensities might not yield the hoped-for results, since “each individual will have a wide range of adaptation, even if training at the same percentage of VO₂ max.”

For example, one athlete running at lactate threshold (around their one-hour effort) might be at 85 percent of his/her VO₂ max, while another runner at that same effort level might be at 91 percent of his/her VO₂ max. The same workout could provide a very different stimulus for each of them. VO₂ max could be used in a sophisticated training program in conjunction with other variables (such as Lactate Threshold to VO₂ max ratio, or velocity at VO₂ max, which incorporates running economy) to determine optimal training response, but VO₂ max alone does not tell the whole story.

So should we do VO₂ max intervals at all?

While you shouldn’t sweat your specific VO₂ max number, and you shouldn’t train with the main focus of improving it, traditional VO₂ max intervals can provide lots of benefit.

When you train at traditional VO₂ max intensities, you’re also improving your running economy, aerobic endurance, lactate threshold and muscle-fiber strength and composition, among other physiological variables.

For trail runners, VO₂ max intervals can play a major role in improving running economy (how much energy it takes to go a certain pace) and climbing economy (how much energy it takes to climb a certain amount of vertical feet in a given amount of time). They can also have positive impacts on muscle tension, speed and even pain tolerance.

Plus, getting faster and stronger at high levels of exertion is always beneficial, especially for newer runners who will see quick improvement in VO₂ max. Here are some examples (always start with an easy 20-minute warm up and end with an easy 20-minute cool down).

Running economy VO₂ max intervals: 8 x 2 minutes fast/2 minutes easy. These intervals essentially let you practice running at faster paces that will make all paces—even slower ones—easier later on.

Climbing economy VO₂ max intervals: 5 x 3 minute hills hard (run down recovery). These hills improve your power output, letting you use that efficiency built earlier in training to become a faster climber.

Race-prep VO₂ max intervals: 12 x 1 minute steep hills hard about a week before a race (run down recovery). These hills hurt, and sometimes being comfortable with a bit of hurt goes a long way on race day.

What’s the takeaway?

Don’t sweat your VO₂ max. You are not that number, and it’s not going to change much anyway. Instead, if you are working toward trail runs longer than 5K, focus on running lots of easy, steady miles to improve aerobic endurance; strides and short intervals (like VO₂ max intervals) to improve running economy and speed; and longer intervals (like lactate threshold intervals) to improve a more trainable metric of performance.

David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play