Can we maybe stop posting Running Writings? Terrible site with AI slop images and a rushed out book just to release before sirpoc.
Agree on the AI slop but otherwise you're off base. The calculators are very useful, and Davis has been writing books and long training articles for 10+ years. One of the best resources out there.
Ultimately treadmill running is another ‘skill’ for ppl to learn if they arent used to using it — it will take a little time to adapt to running on the belt. Eventually itll be 1:1 as far as where youre intensity is on a spectrum, but at first it will very likely be slightly harder and harder for you to go as deep as you could running outside
Just like with cycling, it takes time for your body to adapt and not only will you not be able to go as deep at first, but your HR zones won’t really line up at first, so if you have lactate available it would be a better ‘bridge’ from an intensity standpoint until things start lining up bpm wise as you gain those motor patterns/efficiency
This may seem stupid, but I sometimes feel I am not running at the speed of the belt. if I increase my cadence, I feel I am running faster than the belt, slow down cadence and it works the other way.
But if HR is in the right place it shouldn’t matter much, as pace will also vary outside.
This post was edited 1 minute after it was posted.
Unless you're depleted of glycogen, following or low carbohydrate, high fat diet, or exercising for a long time while only drinking water, you're always burning a mix of carbohydrate and fat during exercise, predominantly the former, not the latter.
Unless you're depleted of glycogen, following or low carbohydrate, high fat diet, or exercising for a long time while only drinking water, you're always burning a mix of carbohydrate and fat during exercise, predominantly the former, not the latter.
More importantly, you don't have to exercise at lower intensities to increase capillarization, improve mitochondrial respiratory capacity, or "teach" your body to burn fat "efficiently". In fact, these adaptations are maximized by training at higher intensities.
I'm not sure if its possible to have 5% MHR to be honest, which is why it makes absolutely no sense for me to use % of MHR. Isn't Heart rate reserves (HRR), where its actually possible to have 0% HRR more accurate to use? Where did this 70% of MHR come from? Because its definitely not 70% of my heart's capacity, if we assume that my HR can't be lower than my resting heart rate unless I'm dead?
Looking at my easy pace, I average 72% of MHR which is equivalent of 64% of HRR. Not that it matters as my view on easy is as slow are required to recover and be ready for the rest of the week. But I am not sure I like the 65 of 70% of MHR being prescribed for easy runs in this thread.
Above roughly 70–75% HRmax, your body starts to cross the line from primarily aerobic metabolism (fat oxidation) into a zone where lactate production rises and you start burning more carbohydrate for fuel. That’s fine in small doses, but if most of your training time is spent there, you never fully develop the low-intensity adaptations that make you efficient and durable over time.
Running below 70% HRmax targets your Type I muscle fibers, builds mitochondria, expands capillary networks, and teaches your body to burn fat efficiently, all without creating significant fatigue or hormonal stress. It’s the only way you can safely accumulate the volume of running needed to improve long term.
That’s the secret: the intensity itself isn’t “magic”, it’s that by keeping the effort low enough, you can run more often and recover faster, which compounds into massive aerobic gains.
When you creep up into 75–80% HRmax, you’re in what many call the “gray zone.” It feels productive, but it’s too hard to fully recover from and too easy to drive a real adaptation. Elite and recreational runners alike fall into this trap - training moderately hard every day, never fresh enough to go truly hard or easy enough to build their aerobic base.
The best evidence and coaching experience from Lydiard, Maffetone, Seiler, Daniels, and the OG double threshold Norwegians all converge on the same point:
“Most of your running should feel gentle. If you think you’re going too slow, you’re probably doing it right.”
So it’s not that 72% or 80% are “bad.” They’re just not as efficient for building the aerobic foundation that supports all the faster running you want to do later. Below 70% HRmax is where your body quietly gets stronger every day.
As an older runner (51), since I've been doing this method I've had a few moments here or there where maybe I pushed a bit too hard on a sub-t rep and felt a bit drained afterwards. My immediate thought is that I won't be ready to repeat the effort in 48 hours. Even a few hours later I start to wonder if I will be ready for the next one.
The next day comes and I run at around 60-65% MHR and feel really good and whatever doubts I had the previous day are gone, especially as the day unfolds. Then the next workout comes and I feel fresh and ready to go. Rinse and repeat.
I certainly won't question the effectiveness of running at such a low heart rate and don't really see much point in trying to push the envelope there. If I'm going to make any mistakes at all with being a bit too aggressive, I'd much rather do it on the sub-t side. That's my philosophy at least.
You’re absolutely right that we’re always burning a mix of carbohydrate and fat. It’s never purely one or the other. The real question is which system predominates and what adaptations you’re targeting.
Below roughly 70% HRmax, around 65% VO₂max, fat oxidation peaks and Type I fibers do most of the work. Holloszy’s classic study in Journal of Biological Chemistry (1967) showed that prolonged submaximal exercise increases mitochondrial enzyme activity and capillary density, which are the foundations of aerobic efficiency. Later research by Achten and Jeukendrup (Sports Medicine, 2004) confirmed that fat oxidation reaches its maximum at moderate intensities before dropping sharply as intensity rises.
High-intensity sessions also stimulate mitochondrial growth through AMPK and PGC-1a pathways, but they cannot be performed frequently enough to build the long-term adaptations that come from consistent volume. That is why coaches such as Arthur Lydiard, Stephen Seiler, Phil Maffetone, and the Norwegian systems all emphasize doing most training at low intensity. It allows recovery, high mileage, and steady aerobic development.
As Seiler wrote in Scandinavian Journal of Medicine & Science in Sports (2009):
“The distribution of training intensity in successful endurance athletes is typically characterized by about 80% of sessions performed at low intensity.”
And Holloszy observed back in 1967:
“Prolonged, low-intensity exercise causes the same mitochondrial adaptations as repeated intense exercise, but without the fatigue that limits total work.”
So yes, high-intensity work can build mitochondria and capillaries, but low-intensity training is what lets you do enough total work to make those adaptations truly accumulate. That is the practical reason elites and well-trained amateurs spend most of their running time below about 70% HRmax. It is the only way to train a lot, recover well, and keep improving year after year
I've noticed sirpoc did 3x4k today. Anybody have the scoop on pace and recovery length for this session? The 3ks I remember being from 25k-30k race pace, it stands to reason these would be even slower.
I know I'm preaching to the choir for the most part on here rather than vs the Reddit or Strava group, but for those who sincerely believe that they are a unique snowflake and any given heart rate rule doesn't work for them, scheduling a test with either a metabolic cart, lactate, or both is pretty accessible in most places nowadays. Should be a quick thing to either confirm that you are special and tell you where you should be training, or break your little heart.
Or you can do an aerobic decoupling test (like this: ).
I know I'm preaching to the choir for the most part on here rather than vs the Reddit or Strava group, but for those who sincerely believe that they are a unique snowflake and any given heart rate rule doesn't work for them, scheduling a test with either a metabolic cart, lactate, or both is pretty accessible in most places nowadays. Should be a quick thing to either confirm that you are special and tell you where you should be training, or break your little heart.
Or you can do an aerobic decoupling test (like this: ).
Something the HR folks should understand is that if you’re not significantly aerobically deficient, you should reach VO2max well before max hr.
My individual case is about 93% of max hr is where I hit VO2max. I verified this by doing four ramp tests of different slopes during a week of training. Whether I quit at 6-8 minutes in a steep ramp, or at 45 minutes in a shallow ramp, the VO2ax was the same within 2-3 ml/min/kg.
I honestly don’t recommend getting any VO2max test unless you can do it on the track, or have some control over the testing protocol. Generic protocols are not great at getting quality data.
