A Conversation With VO Health's Head of Human Performance, Sean Langan PhD, on the Physiology of Endurance Training

Ever signed up for a race and wondered what's actually happening inside your body between week one and race day? We sat down with Sean Langan, Head of Human Performance at VO Health, to find out. This is a lightly edited transcript of that conversation.

VO: Let's start with the basics. When someone goes from sedentary to training for an endurance event, what actually changes in the body?

Sean: The adaptations are pretty much the same whether you're untrained or already fit, it's really the magnitude that's different. On the cardiovascular side, you get an increase in blood volume and plasma volume, so more fluid circulating through your veins, arteries, and organs. Your heart also gets bigger. When blood returns to the heart after delivering oxygen to the muscles, it stretches the heart’s chambers, kind of like loading a slingshot, so it can pump that blood back out into circulation. Over time, that repeated stretch causes the chambers to dilate and enlarge, which lets you pump out more blood per beat. 

Your arteries also get bigger and more compliant, less stiff, which matters a lot for cardiovascular disease risk. And then there's capillary density: these are the tiny vessels that wrap around muscle and handle the exchange of oxygen and nutrients from blood into tissue. Training grows more of them, which shrinks the distance oxygen has to travel to reach the muscle. Think of it like a sprinkler versus a single hose: a sprinkler spreads water across the lawn more efficiently instead of pooling it in one spot.

And at the cellular level?

Every time you contract a muscle, it's a small energetic stressor. Cells sense things like energy status, oxygenation level, carbohydrate availability, and mechanical tension. These triggers send signals to the body to build better machinery to handle that stress next time. Two big things happen with mitochondria specifically: you get more of them (mitochondrial content), and the ones you have get better at their job (mitochondrial respiration, meaning more oxygen throughput and more energy produced). 

Metabolically, the improved mitochondrial content/function improves your ability to burn fat at higher intensities and uptake glucose from the bloodstream. That matters because your carb stores are limited, but fat stores are much larger, so being able to burn more fat is a kind of preservation strategy that keeps you from depleting your limited carb reserves as quickly. Some other cellular benefits of training can include:

• Upregulating proteins (GLUT4) that transport glucose from the blood into the muscle

• An enhanced capacity to store glycogen in muscle

• Improved glycolytic enzyme activity and muscle buffering capacity

Is VO₂ Max the best marker of how "trained" someone is?

Generally speaking, yes. It is a great marker describing the “size of your engine” and is the largest predictor of endurance performance. VO₂ Max is defined by how much air your lungs can take in, how much blood your heart can pump, and how much oxygen you can extract in the muscle. So it captures several layers of our physiology relevant to training status. But even with really hard training, most people see maybe a 20-30% increase in VO₂ Max, occasionally up to 40%. Mitochondria, on the other hand, can improve by 100-300%. That capacity is nearly unlimited by comparison.

That matters because mitochondrial adaptations are more important for endurance than for maximal capacity. If you're running a marathon or ultra-race, you need to sustain a submaximal pace for hours, and that depends much more on lactate threshold, which is driven largely by mitochondrial density and function, than on your VO₂ Max. However, a high VO2max gives you a higher ceiling to work with.

Depending on your event of choice, other factors also play important and potentially larger roles, like muscle strength, neuromuscular power, repeated sprint ability, technical proficiency, etc.

You mentioned lactate threshold. Can you break that down?

There are technically two. LT1 is the point where lactate first starts rising above resting levels. You're always producing some lactate, even sitting here or walking slowly, it's just being cleared as fast as it's produced. As intensity increases, eventually the workload overwhelms your aerobic system: the mitochondria and lactate transporters hit their capacity, and lactate starts spilling into the blood. LT1 is a strong proxy for your aerobic efficiency, basically how much work you can do before you tip out of predominantly aerobic metabolism.

LT2 happens later. This is where lactate stops rising gradually and starts increasing exponentially. Cross that point and you lose "steady state," meaning you can no longer hold that pace indefinitely; you're on a countdown to fatigue. LT2 approximates what's sometimes called critical power or maximal lactate steady state, and it's the fastest pace you can sustain while still being predominantly aerobic. In coaching language, this is often just called "threshold."

Are VO₂ Max and lactate threshold related?

Yes, they're intertwined but not identical. You want a high VO₂ Max because it raises your ceiling for how high you can push your lactate threshold. A recent study found that VO2 Max is the largest predictor of speed or power at lactate threshold. The average person might have a lactate threshold at 60% of their VO2max, and an elite athlete maybe 85%. So, let’s say your running pace at VO2max is currently 6:00/mile and you have a long term goal of running a 2:45 marathon (6:18/mile). Even with an exceptional lactate threshold of 85-90% VO2max (~6:40-7:00/mile), it’s going to be really difficult to run a 2:45 until you increase your VO2max or speed at VO2max, because 6:18/mile is too close to VO2max (or too far above threshold) to be sustainable for almost 3 hours.

But there are tradeoffs. In longer duration events, someone with a lower VO₂ Max but a very high lactate threshold can beat someone with a higher VO₂ Max but a lower threshold. For a marathon specifically, what you want is your lactate threshold to sit at a very high percentage of your VO₂ Max, which means you can sustain a high work rate for a long time.

What should a first-time racer actually track? Do they need to worry about lactate testing?

For a beginner, I wouldn't go straight to lactate testing, that's more useful once you've built a base and for those operating at the margins. I’ve personally been training consistently for many years with big improvements and have never done a lactate test. The simplest and most useful metric is heart rate. If you repeat a key workout regularly, like a zone two run, track your heart rate and pace on that same route or effort. Over time you should see one of two things: you can hold a faster pace at the same heart rate, or you can hold the same pace at a lower heart rate. Either one tells you your heart is getting more efficient, likely because your stroke volume has increased and there's less metabolic stress feeding back to your cardiovascular system.

Heart rate recovery is another useful one. After a hard interval, time how quickly your heart rate drops, at 30 seconds, 60 seconds, two minutes. As you get fitter, it comes down faster. That's a good window into vagal tone, the parasympathetic side of your nervous system, which also matters for long-term cardiovascular health.

At the same time, it is really important to get in tune with your body and monitor rating of perceived exertion, or RPE. Like heart rate, as you get more fit it will also come down at the same pace as you get more fit. It’s the most tried and true method and tracks really well with physiological signals, plus integrating the overall cognitive stress you might be under from life, work, etc. that might not be captured by heart rate or lactate alone.

What about someone training for a faster time, not just to finish?

Same heart rate tracking applies, but you can add lactate testing. One method is an incremental test: start easy, hold each stage for three or four minutes, take a lactate sample, then step up intensity and repeat until you either fatigue or see lactate rise exponentially. Do this at the start of a training block and then every 4-8 weeks or so. For longer distance events, what you're looking for is the curve shifting to the right, meaning at a given pace, your lactate is lower than it used to be. That tells you you're getting more aerobically efficient. 

You can also do lactate testing within workouts, which is becoming more common mostly with elite athletes during interval sessions such as 400m or 1km repeats. They'll check lactate every few reps and adjust pace up or down to stay in the intensity zone the session was designed for. It's a feedback tool for making sure you're actually training the system you meant to train.

Is there a point where more training stops helping and starts hurting?

This is really a question of health versus performance, and they're not the same thing. I'm training for a 10-mile race myself right now, and I wouldn’t say the training required to hit my goal time will make me healthier. General exercise guidelines suggest about 150 minutes a week, and I think that's probably a floor rather than a ceiling for most people. But somewhere beyond that, performance-focused training starts trading off against health in ways that are hard to pin down exactly.

A few known risks at the high end: "athlete's heart," where the same cardiac enlargement that helps performance can, in some people, contribute to arrhythmias like atrial fibrillation. Joint injury risk rises simply from exposure, more volume means more opportunity to get hurt. And overtraining can cause hormonal disruption, including loss of menstrual cycle in women who are under-fueling while training heavily. I've heard the threshold for elevated risk loosely estimated around 10-12 hours of training per week over many years, though the data isn't precise and depends on intensity as well. 12 hours of walking per week won’t harm you.

Interestingly, elite endurance athletes on average do tend to live a bit longer than the general population, by a couple of years in some studies comparing Olympic endurance competitors to the general public. But that's hard to attribute to training alone. Genetics, lifestyle, and lower chronic psychological stress likely all play a role.

Any research connecting something as simple as running a 5K a year to better outcomes?

There's data, including from biomarker companies that have looked at frequent runners across large panels of markers, showing better clinical outcomes in people who race regularly as part of their lifestyle. I don't know of a study that isolates the longevity effect of running one race a year specifically, but it fits within the broader evidence that exercise reduces mortality risk generally. There's also a social component worth mentioning, running clubs and race culture give people a reason to be around others regularly, and there's plenty of separate data connecting social connection to lifespan.

Last question. What should someone do after they finish the race they trained for?

Use it as information, not a finish line. Athletes who race often will sit down afterward and think through what worked and what didn't, then apply it to the next training block. Maybe you ran too many miles and were fatigued going in, so next time you try fewer miles at higher intensity. It's an iterative process.

What I'd caution against is treating the race as the peak of a story that ends there. I've seen people jump into a marathon within their first year of running, hit that goal, and then stop entirely for months, essentially back to zero. Exercise only keeps producing benefits if you keep doing it, the effects fade once you stop. That doesn't mean you have to keep running specifically. If you're sick of running after a big race, do something else for a while, tennis, dancing, cycling, whatever you'll actually keep doing. The goal is to stay in motion, not to defend one specific finish line forever.