Running economy describes the oxygen cost of running at a given speed and typically deteriorates over the course of a marathon. Factors causing this drift include substrate depletion, neuromuscular and metabolic fatigue, biomechanical changes, and environmental stressors. Evidence from exercise physiology links these mechanisms to observable performance loss and greater injury risk as the race progresses.
Physiological drivers
Muscle glycogen availability is central. Edward F. Coyle at the University of Texas at Austin demonstrated that lower glycogen stores reduce muscular work efficiency and impair prolonged endurance performance. As glycogen falls, runners rely more on slower fat oxidation and experience higher relative oxygen cost for the same pace. Neuromuscular fatigue and muscle damage increase stride variability and reduce force production, raising the metabolic cost per step. Tim Noakes at the University of Cape Town emphasized that both peripheral muscle failure and central fatigue contribute to pacing changes and perceived effort late in endurance events. Thermoregulation and dehydration further increase cardiovascular strain and oxygen demand, so heat and humidity accelerate economic decline.
Environmental, cultural, and tactical context
External conditions and cultural practices shape how economy changes in real races. Asker Jeukendrup at Maastricht University has shown that timely carbohydrate intake during exercise can blunt declines in performance by sustaining blood glucose and delaying severe glycogen depletion. Elite runners from high-altitude regions often display different muscle and cardiovascular adaptations that influence how economy evolves under race stress, and local climate and course profile alter the balance between metabolic and mechanical causes of economy loss. Cultural pacing norms—such as aggressive early surges—can precipitate earlier and steeper declines in economy, especially among less-experienced runners.
Consequences of decreased running economy are practical: slower splits, increased perception of effort, and a higher probability of hitting a performance wall or sustaining musculoskeletal injury. Training interventions that improve efficiency and resilience—such as specific strength work, long runs with fueling practice, heat-acclimation protocols, and race-pace neuromuscular training—aim to reduce the magnitude of economic drift. Individual responses vary, so combining physiological insight, nutritional strategy, and course-aware pacing gives the best chance of preserving running economy through 42.195 kilometers.