Marathons are natural experiments, revealing patterns not detectable in labs, according to Marathon Handbook. Researchers study large groups of runners in varied environments. They found that performance depends on multiple factors.

Critical speed, the sustainable speed, predicts marathon performance. Faster marathoners run closer to their critical speed. This requires durability, the ability to maintain performance over long periods. Heart rate and speed decoupling indicate durability. Read more everyday miles coverage.

Environmental factors like heat and altitude affect performance. Marathon data show that even-paced runs yield better results. Training data reveal that most training occurs at low intensity, with less at moderate and high intensity. Related: Iron Levels Affect Endurance in Long Runs.

This original Runner World brief is based on reporting from Marathon Handbook. Visit the source for its complete reporting, context and any later corrections.