The Fourth Kilometre Problem: Lactate's Second Career as a Cellular Messenger
Every Saturday morning, thousands of runners meet a familiar stretch of discomfort somewhere around the fourth kilometre. Lactate is usually blamed. The biology is more interesting: lactate is a fuel, a shuttle and a signal, moving between muscle fibres, the liver and the brain. This explainer follows its journey and asks what the parkrun 'wall' can teach students about metabolic communication.