Coyle & González-Alonso 2001, Cardiovascular drift

Review of cardiovascular drift, the progressive rise in heart rate at a fixed workload during prolonged exercise, driven by rising core temperature, dehydration, a falling stroke volume and fatigue. The practical consequence is that holding a fixed heart-rate cap forces the runner to slow over a long effort, since the same pace costs more heartbeats as the run goes on. Strong (mechanism well established; full text paywalled).