Legs give out before max HR

Hey there, curious training question for the hive mind. Doing vo2 max work, the goal is obviously to get as high a HR as possible and hold it. My problem is, my legs give out before I can get my heart rate all that high.

For reference, I’m 25, so max HR should be about 195 (I know this is imprecise, but not sure of a better way to calculate). FTP around 290. Doing all-out vo2 max intervals around 120% of FTP for 3-5 mins, I can only get my HR up to no higher than 175 (often not even over 170) before my legs give out and fatigue to the point where I can’t go any further.

I’m sure this is still helping me get faster, because I’m doing the work, but should I be able to get my heart rate higher and closer to max?

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Unless you know from experience, i.e. really pushing yourself hard, you don’t know what your max heart rate is. Sounds like you’re using the 220-your age formula to come up with 195. As you’ll read other places, the best way to find your max heart rate is in some sort of competition. This is because it is hard to push yourself to max heart rate outside that motivational stimulus. And competition can be as simple as really trying to best your mates up a good hill.

And welcome to the TR forum!


During your base training, fit in a day or two of leg work at the gym. You’ll benefit greatly.

If you’re like me and hate the gym or don’t have access, have an on bike session where you’re doing very low rpm above threshold work.

Example : 30 minute endurance ride, 5x4 minutes (with 4min rest in between) 315 watts at 45-55rpm
If 315 watts feels easy while doing 55rpm, lower rpm before increasing power. If 315 watts still easy at 45rpm, only then do it at a higher power.

Basically you’re getting your leg strength up. For me, it’s always cardio that gives out first during VO2max. Everyone’s built a little different. Also depends what RPM you’re selecting for your VO2Max ofcourse. I personally increase my preferred RPM a little bit to hit VO2Max numbers. I like 90rpm for threshold. I’ll usually go 100-105rpm for VO2Max etc.

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Unless this is the maximum you’ve actually measured for yourself, this might not be anywhere near your max. It’s like saying “You’re 6ft tall, so you should be 175lb” while completely ignoring individual differences of bone, muscle, etc. It might work on a population average level but not for individuals.

The best way to ‘calculate’ your max HR is to go out and do something like a super hard 1-2 minute effort or a ramp test. Every thing else is just a guess.

Under 170 would be unusually low but not impossible for VO2 efforts.

How does it feel when you get to that 175HR? All out? Are you breathing like a fish out of water?

If yes, then it’s probably fine and you might just have a lower max HR than average.

If no, then there might be some reasons why you can’t push yourself that hard.

  • You’re new and you just don’t have the aerobic capability to push that hard. So the supply to your legs can’t keep up with the demand
  • You’re fatigued. You need to be pretty well rested for intervals like this.
  • You’re dehydrated.
  • You’re underfueled. Your muscle glycogen could be low and your legs just don’t have enough fuel to push.
  • And other similar reasons.
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What cadence are you riding at for your intervals?

If it’s less than 100rpm, try doing them above that, and preferably above 110rpm.

That’ll help bias the load towards your heart and lungs and away from your legs.


In time as you get fitter but dont go by the formula as to what it is. I’m 48yo and max at 197bpm, my mate 47yo max’s at 202bpm but when I was in my mid 30s I used to hill reps with a mate in mid20s who had a max HR in the region of 175bpm and he would tear me apart max HR is very individual.

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Don’t worry what your max HR is - keep doing your workouts and your max HR will reveal itself over time. IME, usually while on an outside ride or event. Typically, I find I can push myself further into the red when motivated on a group ride.


A hill-top finish in a Zwift race is also great for flushing out a Max HR estimate.


This. Studies have also proven that higher RPM elicits higher o2 uptake. I think 70 vs 110 rpm was something like +30%? Do at least 110 rpm (or more) if you want maximum vo2 improvement. Vo2 intervals should not be wattage based.

You’ll also notice higher HR is achieved the more intervals you do, assuming you aren’t completely fatigued. See study below:

Slow component of VO2 kinetics: mechanistic bases and practical applications - PubMed (nih.gov)

Temperature also plays a role, as it’s much easier to hit maximum HR in hot weather vs cold weather. It’s the middle of winter in most states, and I find that HR generally is lower in winter.


For me, the diff is mostly the “etc.”. :joy:


Seems like the majority of people are constrained by their legs, not heart - so you’re not alone.

I wouldn’t worry too much about what you think your HR “should” be. Just do the VO2max workouts to your best ability, and you’ll get stronger over time.


Like others have mentioned, ignore the 220 - age = HR_max estimate. Also, many people don’t get anywhere close to HR_max during training. I consistently hit HR_max during races, but usually don’t go beyond 175 bpm during training (including VO2max workouts and ramp tests). That includes ramp test that had me lying on the floor for a few minutes, panting for air.

For the record, I am 43 years old and my HR_max is at least 184 bpm since this is what I hit in a race last year. So 220 - age underestimates my HR_max.

You noticed something very important: your legs may give out before your cardiovascular system. The opposite may happen, too. Like @Helvellyn mentioned, you can shift the loads on your muscles vs. your cardiovascular system by adjusting your cadence. So if your legs give up first, add 5–10 rpm to your self-selected cadence. If you can’t hold that cadence, you can alternate cadences, e. g. by shifting up or down a gear and hold that cadence for a minute each. I’d prioritize getting workouts done.


I’ve mentioned this on other threads, so sorry if I sound like a broken record.

220 - age is not your max HR.

It’s an estimate of the average HR for a given age.

It is the nature of averages that some people will be above the average, some will be below it. Some will come to a forum and ask, I exceeded my max HR by 20 beats, how am I still alive? Some will come to a forum and ask, I can’t even get to within 20 beats of my max HR, what do I do?

There are formulas that say they are better. Like maybe 206 - 0.88 * age. That’s fine, they are still not your max HR. All of these stem from regression estimates, and sure, the best estimate will produce a estimate that’s closer to the true average.

Again, there is no way to calculate your max HR. You can empirically test it. Your max HR in the ramp test is most likely close to your actual max, but I’m pretty sure it’s not the actual max.

OK, back on topic: it does seem like if the OP can hold 120-ish% for 5 minutes and their heart is racing, they’re probably doing the right thing.

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As other’s have said, it’s probably best to not pay too much attention to your heart rate, especially if you don’t know what your max heart rate actually is. But…

It might be easier to set up heart rate zones by finding your threshold heart rate. Even here, though, there isn’t a universal method. Here’s a GCN video that shows the method I used (more or less): https://youtu.be/vluJECwQOMk?si=xZBWnt6szsIGUt94

Also, in VO2 training, you don’t actually need to hit your max heartrate. From my understanding, you’ll probably stay at least a few beats lower. I’ve seen it said that VO2 Max is typically at/above 90% of max heart rate, but in my case, that’s on the low side, depending on how fatigued I am (though, I actually don’t know for certain what my max heart rate is, only the max heart rate that I’ve actually hit).

But, as others have said, start with power, and use HR as secondary, if at all.


Are you able to finish all intervals?
If not, the low heartrate is probably a sign of fatique.

Perhaps take a couple days of, eat well and do the workout again. If your heartrate is still low, then I would adjust my HR-zones (if you care about it)

When I was 25 (oh, so many years ago), my max HR was 175. I couldn’t have sniffed 195 no matter how hard I pushed myself.

As noted multiple times, the only way to know your max HR is by testing it. Formulas don’t work.

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Hasn’t science come around on the ‘time near 90% of HRmax’? I seem to recall the Kolie Moore series of podcasts on VO2max where he saying that this idea came from Astrand in the 1960s and stuck.

In this That Triathlon Show podcast, Michael Rosenblat addresses this. He did a meta analysis of effective intervals. His conclusion was that total time in zone was the most important factor.

By that logic, rather than doing VO2 intervals at 120% and failing, try them at 115% or 110% so that you can complete the total time in zone.

This podcast may also help the OP:

He talks about his method of intensive and extensive VO2 intervals. For a new athlete who may struggle with a 5 minute interval, he’ll give them 2 minute intervals at the higher end of the prescribed power range. That way the athlete can actually get used to producing those watts before they move on to the longer extensive intervals at the lower end of the range.

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Two things:

  1. If you don’t know your max heart rate, you don’t know whether your heart rate is low, high or just right. But even then, I don’t think you can hit max heart rate during training — nor should you.

  2. Low heart rate could be a sign of fatigue, but so could be unusually high heart rate.

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I find HRmax to be a bit ellusive. Is it the highest heart rate you’ve seen on the bike in the last year or two? Or, do you look at an average of maybe the top 10 HRs you’ve seen in the year (can find that in Garmin). Does it apply if I saw my max HR on a super hot and humid day in a race scenario but never saw that number again on a day with normal temperatures.

If I do a longer VO2max interval I hit around 160-162bpm half way through the interval. I saw 187bpm on that hot and humid ride. That puts my VO2max HR at 86%. There is zero chance I could ride at 168bpm for more than a short time. So the idea of collecting minutes, as Seiler would say, at 90% of HRmax just doesn’t seem to work for me.


Along these lines…can someone shed some light on where heart rate actually SHOULD fall for training? Reason I’m asking is I’ve been setting power targets by feel and keeping track of heart rate during efforts, and have stopped doing any kind of ftp tests at all.

I know from training/racing my max heart rate is 195-196. I’ve been doing 20’ sweet spot intervals, with heart rate sitting in the 160s, creeping into the low 170s sometimes for the 2nd or third interval. Is this more or less normal?