Lifting, Running, Walking & HIIT

I.1The engine: aerobic fitness

Cardiorespiratory fitness is how much oxygen your body can use at its limit (VO₂max), often expressed in METs, multiples of resting metabolism. It depends on how much blood the heart pumps per beat and how much oxygen the muscles' mitochondria can burn.

A Cleveland Clinic cohort followed 122,007 adults for a median 8.4 years after a treadmill test. Mortality fell with every step up in fitness and never levelled off. Compared with the elite group (top 2.3% for age and sex), the lowest quarter had five times the death rate (HR 5.04). Being below rather than above average carried an HR of 1.41, the same as smoking (1.41) and diabetes (1.40), and more than having coronary disease (1.29).¹ A meta-analysis of 33 cohorts put it per unit: each extra MET of fitness, about 1 km/h faster running pace, was linked to 13% lower all-cause mortality.²

Strongas a predictor: large, consistent, measured rather than self-reported. Inferredthat raising your own VO₂max buys the same drop in risk. Part of fitness is genetic, and sick people test lower.

I.2The chassis: strength and muscle

In the PURE study, grip strength measured once in 139,691 people across 17 countries predicted death over four years: 16% higher mortality per 5 kg less grip, for both heart and non-heart deaths. It predicted all-cause and cardiovascular death better than systolic blood pressure.³ In the Aerobics Center cohort (8,762 men, 19 years), men in the top two thirds of leg-press-plus-bench-press strength had about 25% lower mortality than the weakest third. That held for all-cause and cancer deaths after adjusting for treadmill fitness, so strength isn't just a proxy for the engine.⁴

Muscle also matters as a place to put glucose. Skeletal muscle takes up most of the glucose from a meal. In NHANES III (13,644 people), each 10% increase in muscle mass relative to body weight went with 11% lower insulin resistance and 12% lower odds of prediabetes, after adjusting for obesity.⁵

Strongas a predictor; Moderatefor muscle mass and insulin resistance, which is cross-sectional.

II.1Walking

Steps. A pooled analysis of 15 cohorts (47,471 adults, step counts from devices rather than memory) found mortality fell steeply from the lowest quarter (median 3,553 steps) to the highest (10,901), with an HR of 0.47. The curve flattened at 6,000–8,000 steps a day in people over 60 and 8,000–10,000 under 60. Once total steps were accounted for, how fast people walked mattered less.⁶ A harmonised accelerometer analysis found the same shape: light activity alone was linked to much lower mortality, and the most sedentary quarter had 2.6 times the death rate of the least.⁷

Pace. In 50,225 British walkers, those who said they walked at an average or brisk pace had 20–24% lower all-cause mortality than slow walkers. The link was clearest in people over 50 and in those who did no vigorous exercise, where brisk walking is the most intense thing they do.⁸

After meals. In a New Zealand crossover trial in 41 people with type 2 diabetes, walking 10 minutes after each main meal lowered post-meal glucose 12% more than walking 30 minutes at one time of day, and 22% more after dinner.⁹ A meta-analysis of seven sitting studies found that short light walking breaks cut post-meal glucose and insulin, and beat standing breaks.¹⁰ The working muscle pulls glucose in without needing insulin, so the timing matters more than the distance.

What it doesn't do. For anyone already fit, walking is too easy to raise VO₂max much, and it doesn't load muscle or bone enough to build them.

Strongfor steps and mortality (device-measured, consistent); Moderatefor post-meal walking (short crossover trials, glucose rather than outcomes).

II.1bWhat walking does besides steps

Step counts and glucose are the best-measured effects of walking. Four others matter day to day, and none of them needs a hard pace.

Lymph. Fluid that leaks out of capillaries into tissue returns to the blood through lymph vessels. Lymph has no heart of its own. The vessels have one-way valves, and the fluid moves when the surrounding muscle squeezes them and when breathing changes the pressure in the chest. In eight men with a labelled protein injected into the thigh muscle, ten minutes of easy repeated contractions cleared it three to six times faster than rest, and it cleared fastest when the muscle could shorten fully.³⁹ Walking works the calf and thigh through that kind of rhythmic, full-range contraction for as long as you keep going, and sitting leaves the pump idle. Lymph also carries immune cells and whatever they've picked up to the lymph nodes, so flow is part of how the immune system surveys tissue. There are no human studies showing that walking improves immune outcomes through lymph flow.

Stress, when it's outdoors. Where you walk changes the result. In a Stanford experiment, a 90-minute walk in a natural setting lowered rumination (repetitive negative self-focused thought) and activity in the subgenual prefrontal cortex, a region tied to it. The same walk along a busy road did neither.⁴⁰ Across 24 forests in Japan, 280 young adults walked for about 16 minutes in a forest one day and a city the other. The forest days gave lower salivary cortisol, a lower pulse and more parasympathetic (rest-and-digest) nerve activity.⁴¹ In 36 city dwellers followed for eight weeks, time in a natural spot lowered salivary cortisol 21% an hour beyond its normal daytime decline, and 20 to 30 minutes gave the most per minute.⁴² One small study took 12 men on a three-day forest trip and a city trip with the same amount of walking: natural killer cell number and activity rose after the forest trip and stayed up for more than a week, and didn't change after the city trip.⁴³ Walking outside in the day also gets daylight into the eyes, which sets the body clock (Sunlight).

Mood. In 15 cohorts covering 191,130 adults, people doing the equivalent of 2.5 hours a week of brisk walking had 25% lower risk of depression than inactive people, and half that amount still gave 18% lower. The curve was steepest at the low end, so the first hour a week buys the most.⁴⁴

Digestion. A slow walk after eating moves food out of the stomach faster. In 10 healthy men who ate a 576-calorie meal, walking at 2.5 mph (4 km/h) cut the time for the stomach to half-empty from 123 minutes to 107.⁴⁵ In 94 adults with functional bloating, a 10–15 minute walk after each meal for four weeks improved belching, gas and abdominal discomfort as much as a prokinetic drug did, and improved post-meal fullness more.⁴⁶ For constipation, nine randomised trials (680 people, mostly walking and qigong) found exercise about doubled the chance of improvement, though the trials were poorly controlled.⁴⁷ This is the same after-meal walk that flattens the glucose spike in II.1, so one habit covers both.

It's the easiest one to do. Walking needs no equipment, skill or recovery, and it can replace a drive. Among 263,450 UK commuters, those who walked to work had 27% lower cardiovascular disease and 36% lower cardiovascular death than those who went by car or public transport. Walking commuters showed no difference in all-cause mortality over the five years of follow-up; cycling commuters did.⁴⁸

Moderatefor lymph flow, cortisol, depression risk and post-meal digestion (small experiments, or large cohorts that can't prove cause); Inferredfor immune benefit from lymph flow (physiology only) and from forest walks (12 men, one lab).

II.2Running

In 55,137 adults followed for 15 years at the Cooper Clinic, runners had 30% lower all-cause and 45% lower cardiovascular mortality than non-runners, worth about three years of life expectancy. The benefit was about the same in every quintile of time, distance, frequency and speed. Less than 51 minutes a week, or running slower than 6 mph, was enough.¹² A meta-analysis of six cohorts (232,149 people) agreed: any running was linked to 27% lower all-cause mortality, and there was no dose-response for frequency, duration, pace or total volume.¹³ For comparison, the Taiwanese cohort that asked how little exercise counts found 15 minutes a day of moderate activity bought 14% lower mortality, with each extra 15 minutes adding about 4%.¹¹

Knees. Running doesn't wear out healthy joints at recreational doses. Across 17 studies (114,829 people), hip or knee arthritis was found in 3.5% of recreational runners, 10.2% of non-running controls, and 13.3% of competitive runners.¹⁴

Strongthat some running is linked to lower mortality; Moderatefor the knee finding (observational; prior injury not controlled).

II.2bIs there too much?

Three findings get cited as evidence that heavy endurance training harms the heart.

• The Copenhagen joggers. Light joggers had the lowest mortality (HR 0.22) and "strenuous" joggers had an HR of 1.97, no different from sedentary people.¹⁵ But the strenuous group's confidence interval ran from 0.48 to 8.14. That's a handful of deaths, too few to say anything.
• Coronary calcium in masters athletes. In 152 lifelong endurance athletes (median 13 marathons) with low risk factors, male athletes had more coronary plaque than sedentary men (44% vs 22%), and years of training predicted it. But the athletes' plaques were mostly calcified, the stable kind, while the sedentary men's were mostly mixed, the kind more likely to rupture.¹⁶ Following 289 Dutch male athletes for six years, very vigorous exercise (≥9 METs, roughly running at 9 km/h or faster) was linked to faster calcium build-up, while vigorous exercise (6–9 METs) was linked to slower build-up. Total volume made no difference. These men averaged 41 MET-hours a week, about four or five hours of running.¹⁷
• Atrial fibrillation. Among 52,755 Vasaloppet skiers (a 90 km race), men who'd finished five or more races had 29% more AF and twice the rate of slow-heart-rhythm problems as one-time finishers.¹⁸

Against that, the Cleveland Clinic data found no upper limit to the benefit of fitness.¹ The fair reading is that a very high VO₂max is good, and that years of hard endurance training carry a modest arrhythmia cost and more calcified plaque, which is the more stable kind. Whether that plaque matters for heart attacks is unknown. The findings are from men doing several hours a week for decades. Mixing intensities, as in Part V, avoids piling up hours at hard-steady pace.

Contestedfor harm at extreme volume; Strongthat ordinary amounts of running are not harmful.

II.3HIIT

High-intensity interval training alternates hard bouts near maximum heart rate with easy recovery. The best-studied version is the Norwegian 4×4: four 4-minute efforts at 85–95% of maximum heart rate, separated by 3 minutes easy.

VO₂max. In a Trondheim trial in 40 moderately trained men, matched for total work, 4×4 running raised VO₂max 7.2% in 8 weeks, and 15-second intervals raised it 5.5%. Both beat long slow distance and threshold running of the same total work. Stroke volume, the blood pumped per beat, rose about 10% with intervals.¹⁹ In patients with heart disease, hypertension, metabolic syndrome or obesity, a meta-analysis of 10 trials found HIIT raised VO₂peak almost twice as much as moderate continuous training (an extra 3.0 mL/kg/min).²⁰

Time. At McMaster, sedentary men did either three 20-second all-out sprints (10 minutes a session, 1 minute of it hard) or 45 minutes of steady cycling, three times a week for 12 weeks. VO₂peak rose 19% in both groups, and insulin sensitivity and muscle mitochondrial content rose about the same, on a fifth of the time.²¹ In the UK Biobank, non-exercisers whose wrist devices recorded three 1–2 minute bursts of vigorous daily activity (stairs, hurrying, carrying shopping) had 38–40% lower all-cause mortality than those with none.²⁴

Hard outcomes. Two trials go further than VO₂max.

• Generation 100 randomised 1,567 Norwegians aged 70–77 to 4×4 HIIT twice a week, moderate training twice a week, or national guidelines, for five years. Mortality was 4.7% in the control group, 3.0% after HIIT (HR 0.63) and 5.9% after moderate training. None of these differences reached significance. The controls muddied it: many chose to do intervals on their own.²²
• CERT randomised 60 people with coronary stents to six months of supervised 4×4 HIIT twice a week or usual care. Intravascular ultrasound showed coronary plaque shrank in the HIIT group (percent atheroma volume −1.4% vs control). Both groups were mostly on statins, so this is exercise on top of the drug, not instead of it.²³ See Statins, VII.3.

Strongthat intervals raise VO₂max more per minute; Moderatefor plaque regression (one small trial); Contestedfor mortality (Generation 100 trended the right way but missed significance).

II.4Lifting

Mortality. A meta-analysis of 16 cohorts found muscle-strengthening activity linked to 10–17% lower risk of death, cardiovascular disease, cancer and diabetes, independent of aerobic activity. For mortality the curve was J-shaped: the most benefit was at 30–60 minutes a week, and the advantage faded at higher volumes. For diabetes, the curve dropped steeply up to 60 minutes a week and stayed low. People who did both lifting and aerobic exercise did best.²⁵

Glucose. In Copenhagen, people with type 2 diabetes trained one leg with weights three times a week for 6 weeks, 30 minutes a session, and left the other leg untrained. The trained leg cleared more glucose under insulin, and its muscle had more GLUT4 (the glucose transporter), more insulin receptor and more glycogen synthase. That's a local effect in the muscle that was worked.²⁶ In two diabetes trials, lifting alone and aerobic training alone each helped HbA1c a little, but the combination did best. In the Canadian DARE trial, combined training lowered HbA1c a further 0.46 points beyond aerobic alone.²⁷ In the US HART-D trial, only the combined group improved significantly against control.²⁸

Bone. LIFTMOR randomised 101 postmenopausal women with low bone density to 8 months of supervised heavy lifting and jumping (5 sets of 5 at over 85% of max, twice a week, 30 minutes) or home-based light exercise. Lumbar spine density rose 2.9% with heavy lifting and fell 1.2% with light exercise, and femoral neck density held while it fell in the light group. There was one adverse event, a minor back spasm.²⁹ Walking, cycling and swimming don't load bone this way.

Strongfor muscle, strength and glucose uptake; Moderatefor bone (one well-run trial) and mortality (observational).

II.5Side by side

The post (@siimland, October 2026): "High-intensity interval training (HIIT) outperformed resistance training for mitochondrial adaptations in this 12-week study. HIIT significantly increased: mitochondrial size and abundance, mitochondrial respiration, mitochondrial fusion. Lifting alone didn't result in significant changes in these measures, but it produced greater gains in muscle mass. Different adaptations, but you should do both, obviously." It gives the protocols and cites PMID 37616334, and links his YouTube video on mitochondrial health.

The study is Ruegsegger et al. 2023 from K. Sreekumaran Nair's lab at the Mayo Clinic.³⁰ It's a follow-up analysis of the same 12-week trial published in Cell Metabolism in 2017.³¹ Healthy sedentary adults, some young and some older, were randomised to HIIT (n=13), lifting (n=14) or combined training (n=11). The authors declare no conflicts.

Exercise works by stressing the muscle, which then adapts. Several common drugs, supplements and habits interfere with the stress signal or the adaptation.

IV.1Statins

In a Missouri trial, overweight adults with metabolic syndrome did 12 weeks of aerobic training with or without simvastatin 40 mg. Without the statin, fitness rose 10% and muscle mitochondrial content (citrate synthase) rose 13%. With it, fitness rose 1.5% and mitochondrial content fell 4.5%.³³ The Statins paper covers the mevalonate pathway, which also makes CoQ10 for the mitochondria. Don't stop a prescribed statin over this. Raise it with your doctor if you're training and not improving.

Moderateone randomised trial, 37 completers, one statin.

IV.2Metformin

In older adults doing 12 weeks of aerobic training, metformin blunted the gains in insulin sensitivity and VO₂max and wiped out the rise in muscle mitochondrial respiration.³⁴ In the MASTERS trial, adults over 65 doing 14 weeks of lifting gained less lean mass and thigh muscle on 1,700 mg metformin than on placebo.³⁵ This matters most for people taking metformin "for longevity" without diabetes.

Moderatetwo double-blind trials, consistent direction.

IV.3High-dose vitamins C and E

In young men doing 4 weeks of training, 1,000 mg vitamin C plus 400 IU vitamin E a day blocked the improvement in insulin sensitivity and the rise in the muscle's own antioxidant enzymes and PGC-1α, the master switch for building mitochondria.³⁶ The oxidants that exercise produces are part of the signal. Food amounts are fine. See Vitamin C and Vitamin E.

Moderateone clamp-based trial in 39 men.

IV.4Ice baths right after lifting

Men who sat in cold water for 10 minutes after each of 24 lifting sessions gained less strength and muscle than men who did an easy cool-down. Their type II fibres didn't grow, and satellite-cell and mTOR-pathway signalling were lower.³⁷ Cold exposure has its own benefits (Cold Exposure). Do it on other days, or several hours away from lifting.

Moderate21 men in the training study.

IV.5Running on lifting days

A meta-analysis of 21 studies found that adding endurance training to lifting reduced muscle growth, strength and power. Running caused the interference; cycling didn't, and longer and more frequent endurance sessions caused more.³² If muscle is a priority, do intervals on a bike or rower, and keep running away from leg days.

Moderatemeta-analysis of small trials.

IV.6Why exercise rarely melts fat on its own

Doubly labelled water measurements in 332 adults across five countries found that total daily energy use rises with activity at low levels but plateaus at higher ones. The body compensates by spending less elsewhere.³⁸ That's why exercise programs alone produce less weight loss than the calorie math predicts. Exercise changes where energy goes (muscle versus fat, mitochondria, insulin sensitivity) more than how much is burned. See Energy Metabolism.

Moderatewell-measured, cross-sectional.

V.1Ranked by what each one buys

V.2The week

1. Walk every day: 7,000–10,000 steps (6,000–8,000 is enough over 60). Make some of it brisk, and put 10 minutes after the biggest meal, usually dinner.^6,9 Walk outside in daylight when you can, somewhere green for 20–30 minutes if there's one nearby, and walk the short trips you'd otherwise drive.^42,48
2. Lift twice a week, 30–45 minutes. Compound lifts (squat or leg press, deadlift or hinge, press, row, pull-up), working up to heavy sets of 5–8. Two sessions is enough for most of the observational benefit.²⁵ Creatine (1 level tsp, about 5 g, a day) adds to strength and lean-mass gains.
3. Intervals once or twice a week, on a different day from leg lifting if possible:
   • 4×4: 10 minutes easy warm-up, then four 4-minute efforts at 85–95% of max heart rate (hard enough that you can only say a few words), with 3 minutes easy between. About 35 minutes.¹⁹
   • Short version: three 20-second all-out sprints with 2 minutes easy between. About 10 minutes.²¹
   • A bike, rower or hill walk is easier on the joints than flat sprinting, and interferes less with lifting.³²
4. Running is optional. If you like it, an hour or two a week at an easy pace is where the mortality curve sits. More isn't clearly better.^12,13
5. Protect the adaptation: no ice bath within a few hours after lifting, no megadoses of vitamins C and E around training. If you're on a statin or metformin and not improving, raise it with your doctor rather than stopping.