The Physiology of Running Performance

What determines how fast you can run — and how the pieces fit together

13 min read

Distance running performance is not determined by a single number. There is no one metric that tells you how fast you can race. Performance emerges from the interaction of a small number of physiological variables, each trainable, each mattering more or less depending on the individual.

Most runners have heard of VO₂max. Fewer have a clear picture of how it relates to lactate threshold, running economy, and durability — or why improving the wrong variable is a slow path to frustration. This article walks through each determinant, what drives it, and how they combine into a working model of distance running performance.

The goal is not to make you a physiologist. It is to give you a clear enough model of your own engine so that you can think intelligently about what to train and why.

VO₂max — The ceiling

VO₂max is the maximum rate at which your body can take in, transport, and use oxygen during exercise. It represents the upper boundary on aerobic energy production — the size of your engine.

What sets that ceiling is primarily a delivery problem. The biggest factor is cardiac output: how much blood your heart pumps per minute, driven mainly by stroke volume — blood ejected per beat. Endurance training produces a larger, more powerful left ventricle that pumps more blood per contraction. On the extraction side, mitochondrial density and capillarisation in the working muscles determine how effectively oxygen is pulled from the blood and used. Both adapt to training, but the cardiac adaptations tend to dominate.

Genetics sets a wide range for VO₂max. Training determines where within that range you sit. Most of the large, early improvements a newer runner experiences come from cardiac adaptations — a heart that pumps more blood per beat, serving muscles that are becoming better equipped to use it. VO₂max is notably responsive in the early years of training and becomes progressively harder to shift with training age. A runner in their first year of structured training might see substantial gains. A runner with a decade of consistent work has likely captured most of their genetic potential — and further improvement comes primarily from the other variables.

What it means for performance

VO₂max sets the ceiling, but most runners are not racing anywhere near it. Two athletes with identical VO₂max values can differ dramatically in race performance because of the other variables. VO₂max matters most as a limiter when it is low relative to the athlete's other qualities.

Training connection

The stimulus for VO₂max development is well established: accumulated time above roughly 90% of VO₂max. For less experienced runners, straightforward high-intensity interval formats — such as 4×4 minute efforts at 90–95% of maximum heart rate — are effective because the athlete can reach and sustain near-maximal oxygen uptake across each repetition. For well-trained athletes with already high VO₂max values, a different challenge emerges: their aerobic fitness handles moderate-duration intervals comfortably for the first minutes, so oxygen uptake doesn't reach the critical zone until late in each repetition. Research by Rønnestad and colleagues showed that short intervals with short recoveries — 30 seconds hard, 15 seconds easy — keep oxygen uptake elevated across the entire set, producing superior adaptations in well-trained and elite athletes. The principle is the same at all levels; the format that achieves it differs substantially — and this is an area where research genuinely guides format choice, not just the underlying principle.

Lactate threshold — The sustainable fraction

If VO₂max is the size of your engine, the lactate threshold determines how much of it you can actually use.

During easy running, the aerobic and glycolytic energy systems operate in balance, and blood lactate — a measurable byproduct of glycolysis — stays low and stable. As intensity rises, a point arrives where glycolysis begins to outpace what the aerobic system can process. Above this threshold, fatigue accumulates progressively and the effort becomes unsustainable. Below it, you can run in a metabolically stable state.

There are two key lactate landmarks — a first rise above baseline at moderate intensity, and a second, steeper inflection at higher intensity. In the Joyner performance model, "threshold" refers to the second — the upper boundary of sustainable intensity. The distinction between these two landmarks, and what each means for training and racing, is covered in The Lactate Curve and What It Means for Your Training.

A note on what we're actually measuring: lactate itself is not a waste product and is not what causes fatigue. It is a normal metabolic intermediate — produced during glycolysis and used as fuel by mitochondria and other tissues. We measure blood lactate because it is practical to measure and serves as an indirect marker of the balance between glycolytic and aerobic energy production. It is the broader metabolic disruption associated with this imbalance — not lactate itself — that limits sustainable intensity.

What it means for performance

The threshold governs your fractional utilisation of VO₂max — the percentage of your ceiling available for sustained racing. A runner who reaches threshold at 88% of their VO₂max will substantially outperform one who reaches it at 78%, even if their ceilings are identical.

For many amateur distance runners, the threshold is a more consequential limiter than VO₂max. An experienced runner whose VO₂max stopped improving years ago can still get meaningfully faster by raising their threshold. The relevant adaptations — mitochondrial function, capillary density, fibre recruitment patterns — continue to develop over months and years of training, long after VO₂max has plateaued.

Training connection

The threshold responds to sustained or repeated work near threshold intensity — the rationale behind tempo runs and threshold interval sessions. The Norwegian double-threshold model targets this zone with high frequency, accumulating large volumes of sub-threshold work to drive the adaptations that raise it.

Running economy — The conversion rate

VO₂max and the threshold together determine the rate of aerobic energy production you can sustain during racing. Running economy determines how much speed you get from that energy.

Running economy is the oxygen cost of running at a given pace — how many millilitres of oxygen per kilogram per kilometre. Better economy means less oxygen consumed at the same pace, or equivalently, faster running at the same metabolic cost. Among runners with similar VO₂max and threshold values, economy is often what separates performances.

What determines it

This is where honest uncertainty is appropriate. Running economy is influenced by biomechanical factors (ground contact time, vertical oscillation, limb mechanics), neuromuscular qualities (motor unit recruitment efficiency, tendon stiffness and elastic energy return), and metabolic efficiency. But the relative contribution of each is less well understood than for VO₂max or the threshold. Joyner and Coyle noted this gap in their 2008 review; it remains partly open. Notably, there is little evidence that consciously changing biomechanical variables — altering your stride, for instance — reliably improves economy. The body appears to self-optimise its movement patterns over time, and deliberate interference often makes things worse rather than better.

What we can say with confidence is that two things improve economy: years of consistent running, and heavy strength training. Accumulated mileage appears to drive neuromuscular and connective tissue adaptations that make each stride more efficient — part of why experienced runners often perform better than their lab numbers predict. And meta-analyses consistently show that heavy strength training improves economy, likely through increased tendon stiffness and neuromuscular efficiency.

Training connection

Heavy strength training is the most actionable intervention for economy — the evidence is clear and the timescale is months, not years. The mileage-driven improvement is real but slow; it is the compound interest of consistent training over years, not something that will shift race times in an 8-week block. This is an area where the science provides clear direction on what helps but precise prescription — how much strength work, what kind, how to integrate it — remains more a matter of coaching judgment than settled protocol.

Durability — Performance under fatigue

The three variables above are typically measured in a laboratory, on fresh legs. These rested values are real and informative. But they don't tell the whole story, particularly for longer races.

Durability is the rate at which VO₂max, threshold, and economy degrade during prolonged exercise. A runner's physiology at kilometre 5 of a marathon is not the same as at kilometre 35. Economy worsens. The threshold drops. The sustainable fraction of VO₂max decreases. Two runners with identical fresh physiology can perform very differently over 42km if one's values hold up and the other's collapse.

This is why some runners are "better marathoners than their 10k time would predict" — and others the opposite. Durability is particularly relevant at half-marathon distance and beyond, where races last long enough for meaningful physiological degradation.

Epistemic honesty

Durability is the newest of the determinants discussed here, and the scientific understanding is shallower than for the other three. Part of this is simply that the concept is newer. But part of it is that durability is inherently harder to study — it requires assessment under prolonged load, not a single laboratory test, which makes controlled research more expensive and logistically difficult. Fundamental questions remain open: what physiological mechanisms determine it, how to measure it in a standardised way, and how to train it specifically. What we can say is that it appears to matter, it is at least partially distinct from the other three variables, and the research will deepen over the coming years.

Training connection

Long runs, marathon-specific sustained efforts, and fuelling practice are the primary tools associated with durability development. But the specifics of optimal prescription are less settled than for the other determinants — an area where coaching experience and attention to the individual carries particular weight.

The system — How the pieces combine

The four determinants do not operate in isolation. Performance is a product of their interaction.

VO₂max sets the ceiling on aerobic energy production. The lactate threshold determines what fraction of that ceiling is sustainable — your performance VO₂. Running economy converts that oxygen consumption into speed. Durability determines how well all three hold up over the race distance.

This has a practical consequence that matters enormously: improvement comes from identifying which variable is the current limiter for the specific athlete.

A newer runner will often improve fastest by building VO₂max — the ceiling is low and responsive to training. An experienced runner whose VO₂max has plateaued often finds the gains in threshold development — using more of a stable ceiling. A runner with strong short-distance performances who fades in the marathon may have a durability gap. A runner with good aerobic numbers but pedestrian race times may have an economy problem that strength training or years of accumulated mileage can address.

A training plan that targets the wrong variable is not useless — any training produces some adaptation. But it is inefficient. The runner who hammers VO₂max intervals when their limiter is threshold fitness is working hard without addressing the constraint that actually governs their performance. Good coaching identifies the limiter and directs training toward it. This is where the physiological model meets coaching: the science tells us what the variables are and how they interact; identifying which one to target for a specific person, and designing the training that shifts it, is where informed judgment takes over.

This is the physiological model that underpins how Runaid's coaching team thinks about training. The science here is among the more settled terrain in exercise physiology. What is less settled, and where coaching judgment becomes essential, is how to apply this model to a specific athlete: identifying their individual limiter, selecting the training that addresses it, and integrating those stimuli into a sustainable structure.

The previous article examined that challenge. The next, The Lactate Curve and What It Means for Your Training, goes deeper into the intensity spectrum — the two key thresholds, the training zones they define, and how different race distances map onto the curve.

References and Further Reading

Physiological Determinants of Performance

Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. Journal of Physiology. 2008;586(1):35–44. — The foundational review on how VO₂max, lactate threshold, and running economy interact to determine endurance performance.
Bassett DR, Howley ET. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Medicine & Science in Sports & Exercise. 2000;32(1):70–84. — A comprehensive treatment of the physiological ceilings on aerobic performance.

VO₂max Training

Seiler S, Jøranson K, Olesen BV, Hetlelid KJ. Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. Scandinavian Journal of Medicine & Science in Sports. 2013;23(1):74–83. — The study comparing 4×4, 4×8, and 4×16 minute intervals, finding 4×8 produced the greatest VO₂max improvements.
Rønnestad BR, Hansen J, Vegge G, Tønnessen E, Slettaløkken G. Short intervals induce superior training adaptations compared with long intervals in cyclists — an effort-matched approach. Scandinavian Journal of Medicine & Science in Sports. 2015;25(2):143–151. — Evidence that 30/15 intervals outperform traditional long intervals in well-trained cyclists.
Rønnestad BR, Hansen J, Nygaard H, Lundby C. Superior performance improvements in elite cyclists following short-interval vs effort-matched long-interval training. Medicine & Science in Sports & Exercise. 2020;52(12):2534–2544. — Extension of the short interval findings to elite cyclists.

Lactate Threshold

Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports Medicine. 2009;39(6):469–490. — A critical review of threshold concepts and their validity for performance prediction.

Running Economy

Balsalobre-Fernández C, Santos-Concejero J, Grivas GV. Effects of strength training on running economy in highly trained runners: a systematic review with meta-analysis of controlled trials. Journal of Strength and Conditioning Research. 2016;30(8):2361–2368. — Meta-analysis demonstrating a large beneficial effect of strength training on running economy.
Barnes KR, Kilding AE. Running economy: measurement, norms, and determining factors. Sports Medicine Open. 2015;1:8. — A comprehensive review of what is and isn't known about the determinants of running economy.

Durability

Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The importance of 'durability' in the physiological profiling of endurance athletes. Sports Medicine. 2021;51(8):1619–1628. — The paper that formalised durability as a distinct performance determinant.

Individual Variation

Bouchard C, Rankinen T. Individual differences in response to regular physical activity. Medicine & Science in Sports & Exercise. 2001;33(6 Suppl):S446–S451. — The landmark paper on inter-individual variation in training response.
Mann TN, Lamberts RP, Lambert MI. High responders and low responders: factors associated with individual variation in response to standardized training. Sports Medicine. 2014;44(8):1113–1124. — Why the same training produces different results in different people.

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