Everyone Did the Same Fitness Training. The Results Weren't Even Close

LONGEVITY LATEST ISSUE 22 COMPANION · 5 AUGUST 2026

LONGEVITY LATEST · DEEP DIVE

Everyone Did the Same Training. The Results Weren't Even Close.

Four hundred and eighty-one people, one supervised twenty-week programme, and a spread in results that launched thirty years of argument about whether "non-responder" is a real thing.

By Christian Thomsen · Companion to Issue 22 · 5 August 2026 · ~6-minute read

You've read the newsletter, so you know where the fitness evidence landed: an A for cardiorespiratory fitness as a target, a B for the 4×4 interval, a B− for Zone 2 once you strip the mythology off it, and a Hype Check on the chamber that sells you an oxygen problem you don't have.

All of which assumes the training works on you. This is the article about what happens when it doesn't.

Here's the study that raised the question. Between 1992 and 1997, the HERITAGE Family Study put 481 sedentary white adults from 98 two-generation families through twenty weeks of supervised cycling. Not self-reported, not "aim for three sessions" — supervised, standardised, with the intensity set from each person's own baseline test and VO₂ max measured twice before and twice after to keep the noise down. The average participant gained about 400 millilitres of oxygen per minute.

Then you look inside the average. Some gained more than a full litre. Some gained essentially nothing at all. Same programme, same weeks, same supervision.

The part nobody expected

The obvious guess is that the already-fit had less room to improve and the unfit had more. That guess was wrong. Baseline fitness and the size of the training response turned out to be effectively unrelated — how fit you were on day one told you almost nothing about how much you'd gain by week twenty. They behave like two separate traits.

What did predict the response was your family. Variance in the training response was about two and a half times greater between families than within them, and the analysis put the maximal heritability of the response itself near 47%.

Read that carefully, because it's the sentence most often mangled. It does not say your fitness is half genetic. It says your responsiveness to a given training programme has a substantial familial component — a different, weirder claim. Your ceiling and your rate of climb are inherited more or less independently.

If you have ever trained beside someone on the same plan and watched them pull away, this is the first honest explanation anyone has offered you.

Then the methodologists arrived

Here is where I have to complicate my own article, because the "non-responder" idea got popular fast and the popularity outran the statistics.

The problem is deceptively simple. When you measure a person's VO₂ max twice, you don't get the same number, because biological variation and test-day noise are real. Now measure everyone before and after training. Some people will appear to gain a lot and some almost nothing purely from that noise, even in a world where training affects everyone identically. To claim the spread is genuine, you need a comparison group that didn't train — the yardstick that tells you how much apparent change happens anyway.

Most of the famous non-responder studies didn't have one.

In 2024, Renwick and colleagues published the first systematic review and meta-analysis to insist on that yardstick, pooling aerobic training interventions with non-exercising comparator groups. Their conclusion was blunt: across single interventions, there isn't strong evidence for genuine individual differences in VO₂ max trainability. Much of the apparent spread may be noise wearing a lab coat.

A caveat worth noting: "not strong evidence for" is not "evidence against." HERITAGE's familial clustering is hard to explain by measurement error, since noise doesn't run in families. The honest position is that true response differences are probably real but considerably smaller than the folklore suggests.

There's a third possibility that sits underneath both, and I think it's the most useful one. A lot of "non-response" may be response to the wrong dose. Prescribe a session at "70% of maximum heart rate" and you have set an identical number that lands in genuinely different physiological territory for different people — comfortably below one person's first threshold and well above another's. One of them is training. The other is commuting.

That question has now been asked properly. Meyler and colleagues pooled individual data from 1,544 people across 42 studies and found that anchoring intensity to a person's own physiological thresholds, rather than to a percentage of their maximum, produced substantially bigger average gains — and lifted the proportion of people clearing a meaningful improvement from 16% to 64%. Read that twice. Same broad activity, four times as many people getting somewhere.

What it did not do is shrink the spread of responses, which stayed almost identical between the two approaches. So the dose matters enormously, and it still isn't the whole story of why people differ.

What the research says to do next

If you have trained properly for two months and the number hasn't moved, the folk advice is to try harder. The evidence points somewhere else.

Bonafiglia and colleagues ran a randomised crossover in 21 recreationally active adults: a three-week block of endurance training and a three-week block of sprint intervals, separated by a three-month washout, with everyone doing both. Individual responses to each protocol varied a lot, as expected. But nobody turned out to be a non-responder to both. Non-response to one protocol largely disappeared when the same person was handed the other.

Twenty-one people and three-week blocks won't settle anything on its own, and I won't pretend otherwise. But it converges with the dose argument and with HERITAGE's own finding that responsiveness isn't one fixed trait — and it points at a course of action that costs nothing to try.

So the ladder, in order:

1. Check you measured properly. The newsletter's field test exists for this reason: one watch estimate before and one after is not a measurement. Take the test twice at each end, and treat anything inside that spread as no information at all.

2. Check the dose was real. Was the hard session genuinely hard? A great many "non-responses" are threshold sessions in interval clothing — and on Meyler's numbers, anchoring intensity to your own physiology rather than a percentage of maximum is the single highest-yield fix available to you.

3. Change the stimulus, not the effort. This is the counterintuitive one, and it follows directly from Bonafiglia. If eight weeks of intervals did nothing, the evidence-based response is not more intervals or harder intervals — it's a different shape entirely. Higher volume at lower intensity, or much shorter sprints. People who fail to respond to one stimulus routinely respond to another, and the folk instinct of doubling down on the one that failed is precisely the move the data argues against.

4. Give it more than eight weeks. Trainability studies run short because grant cycles are short. HERITAGE ran twenty weeks and still found people mid-climb.

What this means for you

The uncomfortable truth is that some people will do everything right and end up with a smaller engine than the person next to them who does less. That's the lottery, and no amount of protocol optimisation refunds the ticket.

But the practical consequence of the genetics is the opposite of the one people reach for. If your response is partly inherited, then the number you compare yourself to should never be someone else's — it should be your own, from eight weeks ago. That's the only comparison the biology supports. And every intervention in this issue that isn't training exists to sell you an escape from that comparison.

There's also a floor worth remembering, which the newsletter's Spotlight was really about. Even a modest response moves you along a mortality gradient whose steepest section sits at the bottom end. You do not need to gain a litre. Getting off the lowest rung is where the evidence is loudest, and the average response in these trials clears that bar comfortably — which is not a promise about you personally, but it is a much better bet than the alternative.

The plan itself is in this week's newsletter, and it hasn't changed: one hard session, two easy, eight weeks, a real field test at both ends. What this article changes is what you do at the end of those eight weeks if the line is flat. Not more effort. A different stimulus, an honest measurement, and more patience than a grant cycle allows.

If you run the eight weeks, reply and tell me the two numbers — the before and the after. I'll report what comes back, anonymised, in a future issue. There is no published dataset on what happens when ordinary readers run this properly, and I'd like one.

Sources and further reading

1. Bouchard C, An P, Rice T, et al. Familial aggregation of VO₂max response to exercise training: results from the HERITAGE Family Study. J Appl Physiol. 1999;87(3):1003–1008. PMID: 10484570 — n=481 sedentary white adults from 98 two-generation families; 20 weeks supervised cycling; mean gain ≈400 mL/min with responses ranging from ~0 to >1,000 mL/min; maximal heritability of the response ≈47%; between-family variance ≈2.5× within-family; baseline VO₂max was not a significant predictor of the response.

2. Renwick JRM, Preobrazenski N, Wu Z, et al. Standard deviation of individual response for VO₂max following exercise interventions: a systematic review and meta-analysis. Sports Med. 2024;54(12):3069–3080. PMID: 39160296 — the first such review to require non-exercising comparator groups; found no strong evidence for genuine inter-individual differences in VO₂max trainability across single interventions.

3. Bonafiglia JT, Rotundo MP, Whittall JP, Scribbans TD, Graham RB, Gurd BJ. Inter-individual variability in the adaptive responses to endurance and sprint interval training: a randomized crossover study. PLoS One. 2016;11(12):e0167790. PMID: 27936084 — n=21 recreationally active adults completed both a 3-week endurance block and a 3-week sprint interval block with a 3-month washout; no participant was a non-responder to both modalities.

4. Meyler SJR, Swinton PA, Bottoms L, et al. Changes in cardiorespiratory fitness following exercise training prescribed relative to traditional intensity anchors and physiological thresholds: a systematic review with meta-analysis of individual participant data. Sports Med. 2025;55(2):301–323. doi:10.1007/s40279-024-02125-x — n=1,544 across 42 studies; threshold-based prescription produced larger mean gains (4.1 vs 1.8 mL/kg/min in controlled studies) and raised the proportion achieving a ≥1-MET improvement from 16% to 64%, with no difference in the standard deviation of change.

5. Kodama S, Saito K, Tanaka S, et al. Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis. JAMA. 2009;301(19):2024–2035. PMID: 19454641 — 33 studies, >100,000 participants; each 1-MET higher fitness associated with ~13% lower all-cause mortality and ~15% lower risk of CHD/CVD events.

6. Mandsager K, Harb S, Cremer P, Phelan D, Nissen SE, Jaber W. Association of cardiorespiratory fitness with long-term mortality among adults undergoing exercise treadmill testing. JAMA Netw Open. 2018;1(6):e183605 — n=122,007, median follow-up 8.4 years; inverse association with all-cause mortality with no observed upper limit of benefit. A clinical referral cohort, which the authors flag as not representative of the general population.

7. Helgerud J, Høydal K, Wang E, et al. Aerobic high-intensity intervals improve VO₂max more than moderate training. Med Sci Sports Exerc. 2007;39(4):665–671. PMID: 17414804 — 40 young, moderately trained men, 8 weeks, four work-matched groups; the 4×4 protocol gained 7.2% while the threshold and 70%-HRmax groups did not improve VO₂max at all.

8. Stensvold D, Viken H, Steinshamn SL, et al. Effect of exercise training for five years on all cause mortality in older adults — the Generation 100 study: randomised controlled trial. BMJ. 2020;371:m3485. PMID: 33028588 — n=1,567 aged 70–77; HIIT vs control HR 0.63 (95% CI 0.33–1.20); overall mortality low and the trial underpowered for the endpoint.

© 2026 FrontWave Media Ltd · Longevity Latest

This article provides general educational information and is not medical advice. If you have known heart disease, uncontrolled blood pressure, a cardiac history, are pregnant, or have been inactive for a prolonged period, consult your physician before beginning high-intensity interval training or any new exercise programme, and do not stop prescribed treatment in favour of one.

© 2026 FrontWave Media Ltd · Longevity Latest 1