Water and Heart Rate

Question:

I searched the Research section of the site on "heart rate monitor in the pool" or "in water aerobics." I did see an article by Sally Edwards that said HR would be 14 beats lower in the water than when running, but I would like to know why this is the case, and how do I explain this to my class participants?

Answer:

A heart rate monitor in water aerobics is not a good measure of intensity in the water for many reasons. While it does gauge intensity levels of the participants quite accurately, the output figure on the monitor is influenced or “tricked” by many responses within the body. Sally Edwards’s article about lower heart rate in the water is accurate, and heart rates in water do vary from 10 to 17 beats lower, depending on the activity level and also the depth and temperature of the water.

While there is not a lot of research out there that specifically deals with heart rate training in aqua aerobics, there has been a lot of research done both with humans and animals on their heart rate reaction to exercise in water. This research focused on divers and their differences to heart rate, respiration, blood pressure blood flow and the core temperature of the body. These variables were then compared at different depths as well as at different water temperatures. Other studies looked at the effect of the water on different organs of the body (heart and lungs and peripheral limbs).

So how do we explain this and simplify it for our class participants?

We need to go back to basic laws of physics and apply these to our physiology. The properties of water used in aqua are as follows:

• Drag
• Resistance
• Gravity
• Buoyancy
• Changes in direction
• Surface area

All of these relate to our density in water and how our body reacts to these forces when air and water pressure are applied to our organs, primarily the heart and lungs. This pressure and temperature affects how hard our heart is working under these variables which are a different force applied to the organs on land.

Research on submersion in cold water shows the following reactions within the body.

1. All mammals have a condition called Mammalian Diving Reflex, which puts the body into energy saving mode ensuring that it can stay under water longer. This is found primarily in sea mammals and occurs more in cold water, but research says that this is carried over into our bodies and is clearly reflected in situations of hypothermia.
2. This leads to a condition called Bradycardia, a slowing of the heart rate of up to 50 percent in humans.
3. Vasoconstriction then occurs, which takes blood only to the organs where it is needed away from extremities to the heart and the brain. Our respiration slows down and so too does our heart rate.

Physics of air pressure around the body, air surface flow and gravity relate to how our body physiology reacts in these conditions.

Now let’s look at specific things that will affect our heart rate most.

1. 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.
   • 50 percent heat lost from the head and neck alone due to the blood vessels close to surface of the skull
   • Remember children lose heat more quickly because of ratio of body mass to skin surface
2. 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. (One function of the heart is to keep the body cool during sustained exercise.)
3. Compression - 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.
4. Hydrostatic Pressure - The pressure that the water exerts on the body while submerged assists in blood flow and improves the exchange of oxygen into the blood.
5. Dive Reflex - 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 doesn’t even need to be in the water for the dive reflex to occur. Some people experience it when standing in chest deep water.

So in summary to all of the research above dealing with different factors relating to heart rate in water, the easiest way to answer this for your clients is that water temperature affects the cooling and heating process of the body. This also varies with water depth and how much of the body is submersed.

Water pressure and the surface area in contact with the water affects the heart’s blood and lungs, slowing down respiration and blood flow. Even though we are breathing heavily, the load on our body is somewhat lessened due to core temperature being lower from the water. The skin surface also feels cooler and requires less effort to exercise due to buoyancy and less downward pressure. (Of course, this varies when we are working in a depth where we are unable to touch the bottom).

The most effective way of monitoring intensity in the water is the RPE scale in conjunction with how they are feeling and breathing.

References

1. Baldwin, Ken. “Add water to the mix.” IDEA PT Magazine, March 2007
2. Bandy, William D. & Sanders, Barbara.  Therapeutic Exercise for Physical Therapist Assistants
3. International Fitness Association, Aqua Aerobics Training Heart Rates.
4. Triche, Teresa, M.ED. “Moving to the Pool with Deep Water Running.” AEA Public Pages Athletic Injuries, Aquatic Exercise Association.