Exercise Intensity Measurement For Swimming

Aquatic exercise (also known as water aerobics) is increasing in popularity every day! Its true that it is a wonderful medium for people with injuries, with arthritis or other joint disorders, for seniors, and for expectant mothers. But more and more, it is becoming a means of cross-training and an alternative to land-based exercise for fit participants and even professional athletes.

Something special happens when people enter the water to exercise. They have fun! And, they do not experience the hot and sweaty feeling often associated with land exercise. This does not mean that the workout is not as effective. Rather, the water helps to keep the body cool and comfortable.

Studies have shown that a training VO2 can be achieved in deep and shallow water exercise. But, how can instructors and trainers ensure that participants are exercising at an intensity to achieve cardiovascular benefits? And, people exercising in the water on their own as well? Let’s dive into intensity measurement in the pool. And, I’ll share my own experience.

According to the Aquatic Exercise Association’s Aquatic Fitness Professional Manual, aquatic heart rates may be lower than heart rates achieved during comparable land exercise. Several theories to explain this are:

Temperature:Water cools the body with less effort than air. This reduced effort means less work for the heart, resulting in a lower heart rate.

Gravity:Water reduces the effect of gravity on the body. Blood flows from below the heart back up to it with less effort, resulting in a lowered heart rate.

Compression: The water is thought to act like a compressor on all body systems, including the vascular system, causing a smaller venous load to the heart than equivalent land exercise.

Partial Pressure: A gas enters a liquid more readily under pressure. In water exercise, the gas is oxygen and the liquid is the blood. So, more efficient gas transfer due to water pressure may reduce the workload of the heart.

Dive Reflex: This is a primitive reflex associated with a nerve found in the nasal area. When the face is submerged in water, this reflex lowers heart rate and blood pressure. This reflex is stronger in some individuals than in others. Some research suggests that the face doesnt even need to be in the water for the dive reflex to occur. Some people experience its effect when standing in chest deep water. And that is the depth at which many aqua aerobics participants exercise.

In aerobic workouts on land, exercisers generally take a 10 second pulse count at the radial pulse on the wrist or the carotid artery at the side of the throat. The 10 second count is then multiplied by 6, to calculate the heart rate in beats per minute (BPM). However, AEA recommends that water exercisers take a 6 second heart rate count, and multiply by 10. A 10 second pulse count may not be as accurate due to how fast the water can cool the body.

Even using the 6 second pulse count, a 13% or 17 beats per minute deduction should be taken from the exerciser’s land-based minimum and maximum training thresholds, according to AEA, due to the effects of water I have described.

I have found that a heart rate monitor is a superior tool for accurately measuring aqua exercise intensity. My aqua personal training clients wear a heart rate monitor. I take the 17 BPM deduction from their land training heart rate target ranges, calculated using Karvonen’s formula, and advise them of their "aqua target range". I lend them a monitor to use, and some have later purchased their own monitor, becoming excited about the ease of accurately gauging their workout intensity.

To accurately calculate their land training target rates per Karvonen, my clients sleep while wearing a monitor for 3 consecutive nights, and record their resting heart rate upon awakening, before rising, on the 3 mornings. We then take an average of the 3 morning measurements, for the land-based Karvonen calculation.

Karvonen Formula

To review the Karvonen calculation:

220 - Age= Predicted maximum heart rate- Resting heart rate (average of 3 mornings)= Heart Rate Reserve

Heart Rate Reserve x .50( )+ Resting heart rate= Minimum Training Threshold

Heart Rate Reserve x .85 ( )+ Resting Heart Rate= Maximum Training Threshold

Example: 220- 30 years old= 190 Predicted maximum heart rate- 60 Resting heart rate average of 3 mornings= 130 Heart Rate Reserve

130 x .50= 65+ 60= 125 Minimum Training Threshold (Land)

130x .85= 111+ 60= 171 Maximum Training Threshold (Land)

Subtracting 17 BPM, the aqua minimum and maximum training heart range is 108-154 BPM.

Some of my aqua class participants also wear heart rate monitors. We have experienced excellent performance from the monitors while submerged in the pool. However, I caution clients and class participants that monitors are not to be used in salt water.

Another practical consideration -- female exercisers with underwires in their swimsuits have had erroneous or absent readings. I believe it is due to interference from the metal underwires near the chest strap of the monitor. Wearing a non-underwire swimsuit with a jog bra beneath it for additional support, if needed, solves that.

The foremost reason why I encourage the use of heart rate monitors is that I find that heart rate measurement using the 6 second method is troublesome for aqua exercisers, and may lead to inaccurate counts. Because the cooling effect of the water accelerates their heart rate recovery, by the time they locate their pulse, the 6 second count may no longer be worthwhile. Also, due to the acoustics in an indoor pool, with music playing during class or our personal training session, they may be counting the beats of the music, rather than their pulse.

Many report a heart rate that curiously matches the beats per minute of the music! Another problem with a pulse count is that aqua exercisers may be wearing webbed gloves to increase drag resistance, or be using hand buoys or other equipment. By the time they take off or put down this gear to take their heart rate, heart rate recovery has begun.

So, for people not wearing monitors, I feel that measuring intensity via perceived exertion is the next best choice. I tell them that they should feel as if they are working "somewhat hard" to "hard" during the aerobic component of class or the training session. This corresponds to ratings of 12 to 16 on Borg’s Rating of Perceived Exertion scale. This is also AEA’s intensity recommendation.

I also advise them to use the "talk test", in which they can tell that they are working above their maximum threshold if they cannot talk while exercising. They should be able to breathe comfortably and rhythmically. This method is conservative, but very safe.

Water exercise challenges the body, reduces stress, and most of all, is a wonderful, fun way to help achieve a healthy lifestyle. I hope these tips help instructors, trainers, and exercisers to make the most of it!