HR Pacing Simulator
Cardiac drift is the gradual rise in heart rate that occurs during prolonged exercise at a constant pace. Over a marathon, heart rate can drift 10–20 beats above the starting value due to dehydration (reduced plasma volume), rising core temperature, and decreased stroke volume. This tool simulates the three components of cardiac drift — baseline fatigue, thermal stress, and hydration state — to help you plan realistic heart-rate targets and aid station timing for race day.
Frequently asked questions
What causes cardiac drift during a marathon?
Cardiac drift has three main components: (1) progressive dehydration reduces blood plasma volume, forcing the heart to beat faster to maintain cardiac output; (2) rising core temperature increases heart rate to shunt blood to the skin for cooling; (3) accumulated muscular fatigue reduces stroke volume. Drift of <5% indicates excellent aerobic conditioning; >10% suggests insufficient base fitness or hydration issues.
How should I pace by heart rate in a marathon?
Start at 70–75% of heart rate reserve (HRR) for the first half, allowing drift up to 80–85% HRR in the second half. This approach budgets cardiac drift into your race plan rather than fighting it. Trying to hold a fixed heart rate forces pace to drop progressively.
How much fluid should I drink during a marathon?
Replace 60–80% of sweat losses, typically 400–800 mL/hr depending on conditions. Overdrinking risks hyponatremia (dangerously low blood sodium). Weigh yourself before and after long training runs to estimate your personal sweat rate.
Sources
- Coyle & González-Alonso (2001). Cardiovascular drift during prolonged exercise. Sports Medicine.
- Wingo et al. (2012). Cardiovascular drift during heat stress. Exercise and Sport Sciences Reviews.
- Montain & Coyle (1992). Influence of graded dehydration on hyperthermia and cardiovascular drift. Journal of Applied Physiology.
Related glossary terms
- Cardiac Drift
- Aerobic Decoupling
- HRR (Heart Rate Reserve)
- Hyponatremia
- Plasma Na+