Physiology

Why heart rate rises during steady cardio

A rising pulse at the same pace is usually normal cardiovascular drift, not sudden lost fitness.

During a long run, ride, or other steady session, heart rate can climb even when speed and power stay unchanged. This is called cardiovascular drift. As body temperature rises, more blood is directed toward the skin for cooling, while sweating gradually reduces plasma volume.

These changes can lower stroke volume—the amount of blood pumped with each heartbeat. To maintain cardiac output and deliver enough oxygen, the heart compensates by beating faster. Heat and dehydration amplify the effect, but some drift can occur even when you are well hydrated.

A small rise is normal; a large rise paired with unusual breathlessness, dizziness, or falling performance means the session is becoming more demanding than planned. On long aerobic workouts, use heart rate alongside pace and perceived effort, drink according to conditions, and slow down if your internal effort keeps climbing.

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