Trained runners gain durability, not VO2max
Medium confidence
If you already run regularly, a bigger week won't move your top-end fitness much. That engine is largely built. What grows instead is durability: how well your muscles, tendons, and legs hold up to the extra miles. Studies point this way, though the evidence is still thin. For a trained runner, base building works on the frame, not the engine.
Why it works
Trained runners already sit close to their aerobic ceiling (VO2max, the most oxygen the body can use), so extra volume has little central room to grow. Instead the change happens locally, in the muscles and tissues. They build more oxidative capacity and capillaries, tendons tolerate more, and the legs resist fatigue longer. The body adapts where there's still headroom.
What it means in practice
This shapes the established base-building plan, which builds structure from week one. If you're ramping from 25 to 35 miles a week, the real work is handling more volume and toughening your legs, not "remembering how to run."
The evidence
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Over 16-plus weeks of first-marathon training, previously non-athletic adults raised skeletal-muscle oxygen consumption by 48% (p=0.004) while cardiopulmonary peak VO2 stayed flat (0% change, p=0.97). The muscle adapted substantially even though the whole-body aerobic ceiling did not budge. Much of what lets legs absorb more running is this peripheral gain, not a rising central ceiling. With no control group, read it as a within-cohort observation in young adults.
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Untrained men ran faster after 20 weeks of endurance training: 1500 m speed rose 14% and speed at 85% HRmax rose 9.6%. Oxidative enzyme activity (citrate synthase, COX) and biogenesis markers (PGC-1a, TFAM) climbed too, yet gastrocnemius mitochondrial volume density did not change. The authors flag prior work from Lundby where six weeks raised mitochondrial volume density ~55% while VO2max moved only ~7%. Both results separate mitochondrial volume from functional and performance capacity.
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Three well-trained runners absorbed a dramatic jump in running volume without paying a central-fitness cost: absolute VO2max held flat (4.6->4.6 L/min) while relative VO2max rose (61.7->66.7 ml/kg/min) and running economy improved. The improvement reflects peripheral change, not a bigger central one. Rodrigues dos Santos flags that such long, heavy blocks call for complementary strength work to blunt loss of muscle mass and strength. With n=3 and no control group, read this as a case study rather than a general rule.
n=3
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Resilience, the ability to preserve running economy and neuromuscular function late in prolonged exercise, is a trainable performance attribute distinct from VO2max, lactate threshold, or fresh-state economy. Runners matched on VO2max and 10K performance still differ in how well they hold economy deep into a run: those who regularly train long runs over 90 minutes show only a 3.1% rise in oxygen cost across a 90-minute effort at ~80% VO2max, versus 6.0% in less-accustomed runners. Higher training volume, regular long runs, and strength work all track with better durability. Specificity applies too, so building resilience at race pace likely requires training at race pace, including long aerobic intervals such as the 15 × 1000m sets elite marathoners run; as a narrative review of a recent concept, its specific prescriptions still run ahead of the RCT evidence.
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Trained middle-distance runners had a shorter, stiffer free Achilles tendon with a larger cross-sectional area than healthy controls, plus longer T2* relaxation time. Total tendon volume did not differ: the larger CSA offset the shorter length. That geometry may let a runner transfer high muscle forces more rapidly, and it likely reflects years of running load reshaping the tendon. The design is cross-sectional, so it cannot pin down when or how that remodeling happened.
Why we call confidence medium
A few strands of evidence point the same way. Jones 2017 found muscle oxygen use rose 48% while central capacity didn't move at all (0%). Zoladz reports that trained muscle gains content rather than raw capacity, and a single-runner case (Santos) tracked the same pattern as volume climbed. Jones 2025 ties higher volume and long runs to durability that is distinct from VO2max. But the base is thin: one cohort, one review, one case study. The resilience link is adjacent rather than a direct test of volume tolerance, so confidence stays medium.
Where it applies
Adult runners who are already trained or well-trained and adding volume on top of an existing base.
Does not apply to: beginners and returning runners building from a low base (their gains are more central).
Last reviewed Jun 29, 2026. See how we score.