A bioRxiv preprint posted in late January and updated through the spring offers one of the most thorough recent looks at what happens to running economy when individual leg muscles are deliberately fatigued. The headline finding, from a thirty-two-subject treadmill protocol, is that submaximal running energetics, measured as oxygen cost at fixed sub-threshold paces, are largely preserved despite confirmed local strength loss in the plantar flexors and quadriceps groups. The result lands awkwardly against the long-standing coaching assumption that local muscle fatigue is the primary driver of late-race economy decay.
In the protocol, runners completed graded isolated-muscle fatigue tasks on a dynamometer between bouts of treadmill running at 70 per cent of velocity at VO2max. The authors used surface electromyography to confirm motor-unit recruitment shifts and indirect calorimetry to track oxygen cost across each run. Across both calf and quad fatigue conditions, mean submaximal oxygen cost moved by less than 1.4 per cent, well within day-to-day biological variability and below the typical effect size reported for advanced footwear technology in the same lab.
For coaches and runners, the implication is twofold. First, the data argue that the late-race deterioration observed in marathons and ultramarathons is dominated by central fatigue, glycogen depletion and thermoregulatory stress rather than localised force loss. Second, the result complicates how shoe-effect studies are read at distance: if local fatigue does not meaningfully degrade economy under the testing conditions used here, then late-race separations between footwear conditions may reflect the upper-stack and longitudinal-bending behaviour of the shoe at lower running forces, rather than a compensation for tired calves.
The preprint authors are cautious about generalising. The protocol used a controlled isolated-muscle fatigue stimulus rather than the diffuse whole-body load of a long run, and the test subjects ran at sub-threshold paces. Whether the same result would hold at marathon-pace efforts, or after multi-hour endurance loads with thermoregulatory drift, remains an open question. A follow-up by the same group is planned for the autumn with a four-hour treadmill protocol and a heat-stress arm.
Reaction in the coaching world has been mixed. Steve Magness, writing on his Substack on Wednesday, called the finding "a useful reminder that the easy story is rarely the right one", while Jared Ward suggested the result aligns with anecdotal observations from elite marathoners that pacing is the dominant late-race lever. The full preprint is open access on bioRxiv and has not yet been submitted for peer review. A separate Frontiers in Bioengineering carbon-plate geometry study published this week is being read alongside it.