Monitoring your heart rate helps increase your performance and improve your training to get to your goals faster. Just about every endurance activity you participate in is better measured by heart rate monitoring. It is actually one of the most efficient ways to guarantee you get the maximum cardiovascular benefits.

When it comes to exercising in the water, heart rates are notoriously lower than any on-the-ground exercise. When you’re swimming, your heart rate is usually 15 to 20 BPM lower than when you’re running. To be able to understand why, you first need to know about the 6 main factors that influence and lower the aquatic heart rate, then understand the concept of aquatic deduction.

Factors that affect the heart rate:

  • Temperature: Water temperature affects heart rate while swimming. The hotter the water, the more similar your heart rate is to the same effort while running. A runner’s heart rate also increases when it gets hotter.
  • Compression: the water acts like a compressor on all body parts, thus the heart has to work less to return the blood from the limbs.
  • Partial Pressure: In running your body position is vertical and your blood pressure and the assistance of gravity are different than when you are horizontal while swimming, where your blood pressure is lower and there is no gravity assisting the blood to be pumped out of the heart and down to the lower extremities. The swimming position as well as water pressure allows more blood flow to the heart.
  • The dive reflex: this is the body’s physiological response to submersion in cold water and includes selectively shutting down parts of the body in order to conserve energy for survival.
  • Reduced body mass.

So how do you take all these factors into account to calculate your heart rate in the water?

This difference in heart rates can be evaluated with the concept of aquatic deduction. In the past, an acceptable heart rate deduction when exercising in water was just reducing your BPM by 17. Nowadays, a percentage deduction is more advised as a straight calculation might risk skewing the numbers at the upper and lower end of the target HR range. There are 2 different options to calculate an aquatic HR, derived from 2 different studies, both agreeing that the approach varies with each individual.

Option 1: British study:

This study assumes that the heart rate in dry land needs to be reduced by an aquatic deduction factor of 13%:

Option 2: Brazilian study:

This study uses the new Kruel Aquatic Individual HR Deduction to find the heart rate that corresponds to the required intensity:

[(HRmax – Resting HR – Aquatic Deduction) x Desired Intensity Percentage] + Resting HR = Target Intensity with the aquatic deduction.

HRmax = 220 – age OR

HRmax = 206.9 – (0.67 x age)

At Instabeat, we use your maximum and resting heart rate in order to determine your aquatic heart rate zones that correspond to the real-time color-coded display on your device.



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