Heart Rate Variability in the Water
Using wearable fitness trackers has been common practice among runners and cyclists for over a decade, but recently we’ve seen its popularity rise among our clients in the form of Fitbits, Apple Watches, and the like. With a new awareness of heart rate, calories burned, and other health statistics, clients often ask us about the accuracy of these devices in the pool and frequently inquire as to why their heart rate is lower than expected. (HINT: it’s not because pool workouts aren’t as effective!)
In today’s blog, we will explore some reasons why your heart rate is, indeed, lower in the water. We’ll also give you a formula to calculate your target heart rate during your water workout. This is a bit “science-y” so we’ve tried to break it down into digestible bits that, if nothing else, will give you a newfound appreciation for hydrostatic pressure!
Exercising in water is (obviously) different than on land. The benefits of buoyancy are well-known, but many people aren’t aware of the other properties of water & how these properties contribute to some rather amazing physiological effects in your body when you’re immersed.
Your heart is able to work more efficiently in water due to the effects of hydrostatic pressure. Hydrostatic pressure is the pressure of the water against your body, and it increases in proportion to depth measured from the surface. If you’re upright in a pool, there’s more pressure at your toes than at your hips, and the more your body is immersed, the greater the pressure will be. Because of this pressure gradient, hydrostatic pressure assists with pushing blood from the extremities back to the heart and lungs… just like compression stockings!
To understand how this change in circulation helps your heart, let’s review a couple terms:
HEART RATE: the number of times your heart beats per minute (aka your pulse)
STROKE VOLUME: the amount of blood pumped with each contraction
CARDIAC OUTPUT: liters of blood pumped per minute
Your body is able to change its cardiac output by changing your heart rate and/or stroke volume. If your body needs more oxygen, as when exercising, the heart responds by increasing your cardiac output & sending more oxygenated blood to your cells. Typically, monitoring your heart rate allows you to determine how hard you’re working and ensures you stay in an aerobic (or anaerobic) zone. It is also sometimes an indicator of heart health.
When you’re in the pool, hydrostatic pressure increases blood flow to the heart. The healthy heart responds to this increased central blood volume (which “stretches” the heart) by increasing the force of its contraction and pushing out more blood with each beat (again, that’s the stroke volume!). Average stroke volume increases 35% with immersion to the neck when you’re at rest; this increases exponentially with exercise!
Still following? Let’s do some simple math:
CARDIAC OUTPUT = STROKE VOLUME x HEART RATE
If you’re exercising at the same intensity on land as in the water, your cardiac output demands are the same. But since your stroke volume is much greater in the water, your heart rate will be lower. The lower heart rate doesn’t mean you’re not working as hard… the water is assisting your heart to work more efficiently.
This is the same reason why conditioned athletes tend to have lower resting heart rates: their stroke volume is greater. When the heart can pump more blood, it doesn’t need to contract as often to maintain the same output.
If you’re determined to use your Fitbit to monitor your performance, here’s how you can determine your “Kruel Aquatic Heart Rate Deduction” -
1. Determine your heart rate on deck. Stand out of water for 3 minutes before checking your pulse.
2. Determine your heart rate in water. Stand in armpit-depth water with as little movement as possible for 3 minutes. Check your pulse.
3. AQUATIC HEART RATE DEDUCTION = Deck pulse - water pulse
To find out your most accurate target aquatic heart rate, use your Heart Rate Deduction in the Karvonen Formula:
[(220 – Your Age – Resting HR on land – Aquatic HR Deduction) x Desired Intensity Percentage] + Resting HR on land
For example, a client is 50 years old. Her resting heart rate is 72. She finds her aquatic HR deduction to be 10. She wants to work at about 60% of her max heart rate.
[(220 - 50 - 72 - 10) x .60] + 72 = 125 bpm for her target heart rate
Since most of us aren’t high-level athletes or cardiac rehab patients, diving into this level of specificity is likely unnecessary. But if you are using a wearable fitness tracker, hopefully you now understand why your heart rate may seem lower than expected given how hard you’re working. We’ll talk about other methods for measuring your exercise intensity, like the BORG scale and the “Talk Test” in our next blog (Benefits of HIIT).
Lastly, keep in mind that there may be a few other reasons why your heart rate may be low or it may be hard for you to get your heart rate up during your workouts. Some of these reasons include medical conditions (like hypothyroidism or cardiac diagnoses) and certain medications (beta-blockers, narcotics, and anti-depressants, to name a few).
Still have questions? What’s your favorite wearable fitness tracker to use in the water? Have you ever noticed that your heart rate seems lower in the pool? What are your favorite exercises, classes, or intervals to work on your cardiovascular conditioning? We would love to hear from you!
Content provided on this website is for informational and educational purposes only; it does not constitute providing medical advice or professional services. Always seek the advice of your physician regarding your health conditions and prior to initiating an exercise program.
