Heart rates when swimming are usually significantly less than when undertaking other sports on dry land. You may have noticed you feel to be working really hard in the pool but if you measure your heart rate it is much lower than you expect if you are used to monitoring your heart rate when running for example.
This is thought to be a result of several factors, first, the body is in a horizontal position so your heart does not have to work as hard to pump blood around the body against the effects of gravity. Second, the cooling effect of the water reduces body temperature and decreases stress on the circulation. Thirdly, the dive reflex is known to lower heart rate and blood pressure which is a neurological response to immersion in water.
So how does this affect you?
The change between aquatic heart rate levels and dry land levels can be estimated. McArdle et al. 1971, suggested aquatic heart rate was 13% lower than dry land, Sova 1991, suggested a 17 beat per minute deduction was appropriate. Recent research from Brazil (Kruel et al) suggest a comparision test you can undertake easily.
Stand still at the side of the pool for 3 minutes then take your heart rate. Get in the pool and stand in the pool at armpit depth for 3 minutes then take your heart rate again. The aquatic heart rate deduction is determined by subtracting these two values.
If we use the commonly known formula of “220 – your age” to determine maximum heart rate. The maximum heart rate in the pool will be “220 – your age – aquatic deduction”.
e.g. for a 40 year old
Maximum heart rate on dry land = 220 – age = 180 bpm
Heart rate when standing out of pool = 101 bpm
Heart rate when armpit deep in pool = 90 bpm
Aquatic deduction = 101 – 90 = 11 bpm
Maximum heart rate in the pool = 220 – age – aquatic deduction = 220 – 40 – 11 = 169 bpm