Running physiology, explained

61 explainers covering the physiology behind marathon training and racing — what each concept means, the numbers that matter, where they come from, and what the research does not settle. Every page ends in the calculator that models it, so you can put your own numbers in rather than only reading about someone else's.

Fuelling and substrate

What you burn, what you can absorb, and why the marathon has a wall.

What is glycogen, and why does it run out? — Glycogen is the stored carbohydrate that fuels marathon running — roughly 400–600 g in muscle and 90–120 g in the liver, which is not quite enough to cover 42 km at race pace.
How many carbs per hour should you take in a marathon? — Start at 60 g per hour. Glucose alone is capped near 60 g/hr by intestinal transport; adding fructose lifts the ceiling to roughly 90–120 g/hr, but only if your gut is trained for it.
What is the SGLT1 limit on carbohydrate absorption? — SGLT1 is the intestinal transporter that carries glucose into the bloodstream. It saturates at roughly 1 gram per minute, which sets a hard ceiling of about 60 g/hr on glucose-only fuelling.
Why do sports drinks mix glucose and fructose? — Glucose and fructose cross the intestinal wall by different transporters, so taking both together roughly doubles the carbohydrate you can absorb per hour.
What is RER, and what does it tell you about fuel use? — The respiratory exchange ratio is carbon dioxide produced divided by oxygen consumed. It reads 0.70 when you are burning pure fat and 1.00 on pure carbohydrate, so it tells you what fuel you are actually on.
When do you burn fat, and when do you burn carbohydrate? — Fat dominates at easy intensities and carbohydrate takes over as you speed up. The crossover is a gradual shift, not a switch, and where it falls for you is trainable.
What is time to exhaustion (Tlim)? — Tlim is how long you can hold a given intensity before your glycogen runs out — a function of pace, your stored carbohydrate, and how much you take in along the way.
Why do runners hit the wall in a marathon? — The wall is muscle glycogen running out. Fat cannot supply energy fast enough to hold marathon pace, so pace collapses whether or not you are willing to keep pushing.
Can you train your gut to absorb more carbohydrate? — Yes. Intestinal transporter expression and gut tolerance both respond to habitual carbohydrate intake, so practising race fuelling in training raises the rate you can handle on race day.

Intensity and thresholds

Lactate thresholds, VO₂max, training zones, and how hard each run should actually be.

What is lactate threshold? — The intensity above which lactate accumulates in the blood faster than you can clear it. It is the single best physiological predictor of endurance race pace — better than VO₂max.
LT1 vs LT2: what is the difference? — LT1 is where blood lactate first lifts off baseline and marks the top of genuinely easy running. LT2 is where it accelerates away and marks the ceiling of sustainable hard running.
What is VO₂max? — The maximum rate at which you can consume oxygen during exercise, in millilitres per kilogram per minute. It sets the ceiling on aerobic performance — but not, on its own, your race times.
What is a good VO₂max for a runner? — There is no single good number — it depends on age and sex, and the range that supports enjoyable, successful marathon running is much wider than the elite figures suggest.
What is threshold pace, and how do you find yours? — Threshold pace is roughly the fastest pace you could race for an hour — comfortably hard, controlled, and the anchor for tempo and cruise-interval sessions.
What is marathon pace, and how does it relate to threshold? — Marathon pace is the effort you intend to race at — below threshold for everyone, but by very different margins depending on how long you will be running.
What is easy pace, and why do most runners run it too fast? — Easy pace sits below LT1 — conversational, sustainable, and for most runners noticeably slower than the pace they habitually run on easy days.
What is training intensity distribution? — How your weekly running time is divided across easy, moderate and hard zones. The shape of that distribution predicts adaptation better than total volume alone.
Polarized vs pyramidal training: which is better for a marathon? — Both put most volume at easy intensity. Polarized skips the middle zone; pyramidal keeps a moderate amount of it — which suits the marathon, because the race is run there.
What is the 80/20 rule in running? — The observation that successful endurance athletes spend roughly 80% of their training time at low intensity and 20% at moderate-to-high intensity.

Training load and recovery

Measuring what training costs you, and structuring a block so it adds up.

What is ACWR (acute:chronic workload ratio)? — The ratio of your last 7 days of training load to your 28-day average. Values between 0.8 and 1.3 are the commonly cited sweet spot; above 1.5 injury risk rises steeply.
What are CTL, ATL and TSB? — CTL is a 42-day average of training load standing in for fitness, ATL a 7-day average standing in for fatigue, and TSB is CTL minus ATL — a proxy for how fresh you are.
What is training stress score (TSS)? — A single number combining how long you trained and how hard, so that sessions of different types can be added together into a weekly load.
What is the Banister fitness-fatigue model? — A model that treats every training session as producing two effects at once — a long-lasting fitness gain and a short-lasting fatigue cost — with performance being the difference between them.
What is training monotony, and why does it matter? — Mean daily training load divided by its standard deviation across a week. Values above about 2.0 associate with overtraining, because they mean every day looks the same.
How long should a marathon taper be? — Two to three weeks, cutting volume by roughly 40–60% while keeping intensity and session frequency intact.
What is periodization in marathon training? — Organising training into phases — base, build, peak, taper — so that the stimulus becomes progressively more specific to the race as the race approaches.
How long do running adaptations actually take? — Days for glycogen storage and plasma volume, weeks for enzymes and heat acclimation, months for capillaries and mitochondria, years for tendon and bone.
What is neuromuscular load, and why does it matter separately from effort? — The mechanical, impact and eccentric cost of a session — separate from its metabolic cost, and a better predictor of injury than effort alone.
How many hard sessions should you run per week? — Two for most runners, three for experienced high-volume athletes, one when returning from a break. Below about three quality sessions a week in total, adaptation becomes unreliable.

Heart rate, heat and hydration

Cardiac drift, heat acclimation, and getting fluid strategy right in both directions.

What is cardiac drift? — The gradual rise in heart rate at a constant pace during prolonged running, driven by dehydration, rising core temperature and falling stroke volume.
What is aerobic decoupling, and what is a good value? — The percentage by which heart rate and pace drift apart over a long run. Under 5% suggests good aerobic durability for that pace; over 10% suggests the pace is beyond your current base.
What is heart rate reserve (HRR)? — The difference between your maximum and resting heart rate. Expressing intensity as a percentage of that reserve tracks oxygen uptake far better than a percentage of maximum heart rate.
How do you calculate running heart rate zones? — Anchor them to heart rate reserve and to your lactate thresholds rather than to a percentage of maximum, and treat the boundaries as soft.
What is heat acclimation, and how long does it take? — Repeated heat exposure that expands plasma volume, brings sweating on earlier, and lowers core temperature at a given effort. Adaptations begin after 4–5 sessions and plateau around 10–14.
Does sauna use improve running performance? — Post-run sauna produces heat-acclimation adaptations — chiefly plasma volume expansion — with reported endurance gains of roughly 2–7%. The evidence base is small.
What is plasma volume expansion, and why does it help runners? — An increase in the liquid portion of your blood, triggered by heat exposure or endurance training. More blood volume means higher stroke volume, lower heart rate and less cardiac drift.
How much should you drink during a marathon? — Enough to limit body-mass loss to a few percent, which for most runners means drinking to thirst. Sweat rates range roughly 0.8–1.5 L/hr and vary enormously between individuals.
What is exercise-associated hyponatremia? — Dangerously diluted blood sodium, usually below 135 mmol/L, caused by drinking more fluid than you are losing. It is rarer than dehydration but considerably more dangerous.

Running economy and biomechanics

The oxygen cost of running, and what actually changes it.

What is running economy? — The oxygen cost of running at a given speed. Two runners with identical VO₂max can differ by 30–40% in how much energy each kilometre costs them.
What is a good running economy? — Elite distance runners sit around 0.180 mL O₂/kg/m at a 16 km/h reference speed and moderately trained runners around 0.195. Recreational values run higher, and the ladder above 0.195 is interpolated rather than measured.
Do carbon-plate shoes actually make you faster? — On average yes — around 4% better running economy against a racing flat — but the individual response ranges from under 2% to over 7%, so the mean is not a promise.
What is the ideal running cadence? — There is no universal number. Most runners already sit within about 3% of their own optimum, and 180 steps per minute is a widely repeated figure with a weak basis.
What is vertical oscillation in running? — How far your centre of mass moves up and down with each stride. Excess vertical movement is energy spent going up rather than forward.
What is ground contact time, and does it matter? — How long each foot stays on the ground, typically 200–300 ms. Shorter contact times correlate with better economy, but they are largely a consequence of speed and structure rather than a lever.
Does running surface affect your pace? — Yes. Grass costs roughly 5% more oxygen than asphalt at matched speed, and trail more again — though the trail and track figures in most models are estimates rather than measurements.
How much does shoe weight affect running performance? — Roughly 1% metabolic penalty per 100 g added per shoe — which is why a typical trainer costs about 1% more than a racing flat.

Racing, pacing and terrain

Predicting a time, pacing a course, and what hills really cost.

What is grade-adjusted pace (GAP)? — The flat-ground pace that would cost the same effort as the pace you actually ran on a hill. It lets you compare a hilly run with a flat one honestly.
What is the Minetti gradient cost model? — A fifth-order polynomial fitted to measured oxygen cost across gradients from −45% to +45%. It is the basis for essentially every grade-adjusted pace calculation in use.
What is the Riegel formula for race prediction? — A power law that converts a race time at one distance into a predicted time at another: T₂ = T₁ × (D₂/D₁)^1.06. Roughly 80% accurate for general populations.
How do you predict your marathon time? — Convert a recent half marathon with a power-law model, sanity-check it against your weekly volume and long runs, then subtract some optimism.
What is age grading in running? — A percentage comparing your time with the world-best time for your age and sex, so runners of different ages can be compared fairly.
Should you run a marathon with negative splits? — A slightly negative or even split is the optimal race strategy on a flat course. On a hilly one, even effort matters more than even pace, and the split will follow the terrain.
Does a net-downhill course make you faster? — Less than you would expect. The metabolic saving from descending is much smaller than the cost of climbing, and sustained descent damages muscle in a way that shows up late.
How much does elevation gain slow you down in a marathon? — Enough that where the climbing sits matters more than how much there is. Grade-adjusted pacing gives a per-kilometre answer for your specific course.
How do you pace a hilly marathon? — Pace to effort, not to the watch. Give the climbs the time they cost, take the descents gently, and use a grade-adjusted split table so you know the numbers before you start.

Caffeine and ergogenic aids

Dose, timing, and honest effect sizes for the aids that have evidence behind them.

How much caffeine should you take before a race? — 3–6 mg per kilogram of body weight, taken about 45–60 minutes before the start, improves endurance performance by roughly 2–4%.
How long does caffeine last in your system? — Plasma caffeine peaks about 45 minutes after ingestion and has a half-life of roughly 5 hours, so a pre-race dose is still substantially present at the finish of a marathon.
Does daily coffee reduce caffeine's race-day benefit? — Habitual intake above roughly 300 mg per day attenuates the acute ergogenic response by around 30%. Whether a pre-race withdrawal helps is less clear than the folklore suggests.
Does caffeine cause stomach problems when running? — High doses, above roughly 400 mg, meaningfully raise the risk of gastrointestinal distress during a race — and running already stresses the gut on its own.
Does nicotine improve endurance performance? — The evidence says mostly no — 12 of 16 studies found no effect. It is highly addictive, and it has been on the WADA monitoring programme since 2012.
What is an ergogenic aid? — Anything that improves performance — substance, technique or equipment. The category spans well-evidenced interventions like caffeine and carbon-plate shoes and a great deal that does not survive scrutiny.