Monitoring your heart rate is a great way for increasing your efficiency, your performance output or if you are overtraining and need to take a day or two off.  The heart rate monitor is a very useful tool to monitor just about every endurance activity. However when it comes to swimming, you will have to make some modifications when you are doing your heart rate zone calculations. 


Let’s say you calculate a number of heart rate zones for yourself using a formula (220-age, Karvorien, Maffetone’s MAF, Joe Friel, Hadd, etc.) and you have your “aerobic” and “anaerobic” zones. Whether you have 2 zones, 4 zones or 7 zones is irrelevant. Your goal is to perform at the highest level possible for your ability, be able to hold that indefinitely and in addition, have the ability to swim the later part of the swim faster than the first part (negative split). You want maximum output in the most comfortable zone possible. For longer endurance activities, that zone would be the upper level of your aerobic threshold. For most formulas and most people, that is at the high end of zone 2 to the low end of zone 3 or approximately 70 – 85% of your maximum heart rate.


I’ll use myself as an example. I like to use the MAF formula (180-age+5). I am 45 so my maximum aerobic heart rate is 140. My aerobic zone is 130-140. Once I go above 140 I am shifting from using primarily fat as a fuel source to glycogen. The byproduct of that is lactic acid. My goal is to get faster while remaining in this zone. If I were to use any other heart rate formula and calculate about 70 to 80 % of my maximum heart rate, it still falls in the 130-140ish range.


However, when you calculate these formulas, they are geared for running, not swimming. In running your body position is vertical and your blood pressure and the assistance of gravity are different than when you are horizontal, 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. Cycling falls in the middle where you have a horizontal upper body and a vertical lower body. 


So, running at a heart rate of 140 feels very easy for me. In fact, 140 is so low, that most people would find it difficult to run at this heart rate because their heart rates would shoot up too high, well over 140. In swimming, if my heart rate went to 140, it would feel like I was suffocating and although I am still technically “aerobic”, I would feel like I am anaerobic. 140 in the pool to me feels like 160 at the end of a 10k road race.


I don’t want you to think something is wrong with you if you do a swim session and a run session at the same heart rate and they feel totally opposite in intensity.


If you plan on using a heart rate monitor take an additional 10 to 15 heart beats off your calculations for each zone and that will compensate for the horizontal position.