Why your VO2 Max Doesn’t Matter for Ultramarathons…

If you've spent any time around endurance training content, you'll know VO2 max gets treated like the be-all and end-all. It's the number everyone wants to see go up, the one your watch loves to tell you about, the one that gets thrown around as shorthand for "how fit are you really." And for shorter efforts, that focus makes sense.

But if you're training for something long — a 50k, a 100 miler, a big day out in the mountains — VO2 max isn’t actually the thing that decides how your race goes. For most people covering long distances, the race isn't decided by how much oxygen they can take in and use. It's decided by how long their legs can keep producing force.

That's the bit that tends to get missed, so I want to walk through it properly.

The fourth element: muscular fatigue

Most endurance frameworks talk about three pillars — aerobic capacity, threshold, and economy. What I think deserves a seat at that table is muscular fatigue, because in anything long, it's often the actual limiter.

Something that isn’t well understood by most runners is that as your muscles fatigue locally over the course of a long race, your heart rate often drifts downward, not upward. That sounds backwards if you're thinking purely in cardiovascular terms, but it makes sense once you understand what's happening. As the muscles fatigue, they actually need less oxygen, not more, because they're producing less force and doing less work. The decline in heart rate you see late in an ultra is a direct reflection of the decline in cardiac output needed to supply those working muscles with oxygen. So your heart isn't struggling to keep up — your muscles have simply stopped being able to ask as much of it.

Why does this happen?

There are two kinds of fatigue going on, and they're worth separating out.

The first is central fatigue. This is happening in your brain and nervous system — quite literally, your ability to fire your muscles as hard as you could when you were fresh starts to decline. Your brain is, in a sense, protecting you by dialling down the signal.

The second is peripheral fatigue, and this is happening in the muscle itself. Glycogen is running low, metabolites are building up, and the muscle fibres themselves are getting tired at a local level.

The important thing to understand is that these two aren't separate problems running in parallel. They feed each other. Peripheral fatigue in the muscle sends signals back up that contribute to central fatigue, and central fatigue changes how hard you're able to drive the muscle, which changes the peripheral picture. Round and round it goes for the rest of the race.

Your main limiter is not your lungs

This is where I'll point to Renato Canova, a coach who's had a huge influence on distance running and has long argued that for prolonged efforts, the thing that actually limits performance is muscular fatigue, not aerobic capacity.

I think that's a really important reframe. If muscular fatigue resistance is genuinely one of the main things standing between you and a good result in a long race, then it shouldn't be something you train "eventually," once you've ticked every other box. It should be one of the main things you're training for from fairly early on.

A caveat: VO2 max still matters, but...

I don't want this to read as "VO2 max is a waste of time," because it isn't. It still decides how much aerobic capacity you've got to draw on in the first place — it's the ceiling for your engine. But here's the practical issue: once you're reasonably well trained, that ceiling is genuinely hard to move much further. You're often looking at low single-digit percentage gains from here, even with focused work.

Fatigue resistance is a different story. The ceiling there is much higher — you can be looking at gains of up to 20%, because for most people it's a far less developed quality than their aerobic system is. That's not a small difference, and it's the reason I think this deserves more attention than it usually gets.

Where to start: build the aerobic base first

None of this means you should skip straight to muscular endurance work. If you're aerobically deficient, that needs to be sorted first. It's the biggest lever you can pull in your overall fitness, and everything else — including the fatigue resistance work — works better once that base is in place.

Once your aerobic base is solid, the next layer is building your threshold volume. The idea here is to increase your total volume of work done at or around threshold intensity, so you get more efficient at using lactate for fuel rather than getting overwhelmed by it. The better you get at this, the less fatigue accumulates from that particular pathway, which buys you more capacity for everything else. These first two will get you a long way to building fatigue resistance, especially if you can fine tune your threshold training to be more ultra specific. You might not even need to do the next bit of specific muscular endurance, especially for flatter 50km to 100km courses - the difference is when elevation is going to play a role, and how your muscles are going to respond to being placed under increased time of contraction as you either climb, or absorb eccentric forces from continuous descending. If you have a lot of elevation coming up in your event…

Then: add a muscular endurance block

Once the base and threshold work are in place, this is where a dedicated muscular endurance block comes in. I'd typically look at 6 to 12 weeks, with one or two weekly sessions specifically designed to keep certain muscle fibres more aerobic for longer.

The logic behind this: when you run, your slow twitch fibres are doing the majority of the workload. They're built for exactly this kind of sustained, aerobic effort. Muscular endurance sessions — something like a weighted incline treadmill hike — are about trying to keep some of your fast twitch fibres more aerobic for longer too, so you've got more of your total muscle mass contributing efficiently later into a race, rather than relying so heavily on the same fibres the whole way.

Final tips

A few things worth keeping in mind if you want to actually apply this. Don't get too carried away with it. Going from zero muscular endurance sessions to two a week is a big jump, and it can easily leave you too fatigued to recover properly from everything else you're doing. Start with one session and see how your body responds before adding more.

If you're doing weighted sessions, don't go above about 10% of extra bodyweight. And pay attention to what's actually limiting you in these sessions — it shouldn't be your heart rate. The real limiter should be how long you can keep tension in the muscle mass while you're climbing. Start easy, and build from there.