It is pretty common for your heart rate to gradually increase with time during steady-state workouts. This increase is known as Cardiac Drift which refers to the increase in heart rate that occurs during prolonged endurance exercise with little or no change in workload.
During steady-state aerobic exercise, you might expect your heart rate to reflect the intensity of the work being performed. However, it is common to see heart rate "drift" upward during the workout.
This drift is mostly caused by increased body core temperature and can be enhanced by the decrease in plasma volume that is caused by water loss during exercise.
Cardiac Drift can be blunted by ensuring proper hydration and controlling your core temperature. You may need to check the balance of your hydration to electrolytes.
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