If you have spent any time hanging around open water swimmers you may have heard the term “after-drop”. If you’ve done any swimming in cool water, you may have experienced it. For the uninitiated, after-drop refers to the decline in your core body temperature after you have got out of the water.
When you swim in cool water the body cleverly tries to protect vital organs by reducing blood flow to the skin and limbs. Thus the core stays warm while the skin, arms and legs cool down. The process is known as peripheral vasoconstriction.
Shortly after you exit the water, peripheral vasoconstriction ends. Cold blood from your limbs and skin returns to your core where it mixes with warmer blood thereby causing your deep body temperature to drop, even if you’re warmly dressed and move into a warm environment. This is why you often only start shivering 10 to 15 minutes after leaving the water.
It’s a good theory, but can it really be true that your core temperature keeps falling for quite some time after finishing swimming?
Last week I had the chance to find out when I took part in a study at the Extreme Environments Laboratory at Portsmouth University. For the experiment I had to swim in cool water (16 degrees and 18 degrees) for two hours while the researchers monitored (among other things) my deep body temperature. After two hours at 18 degrees my body temperature had dropped by about half a degree. I towelled off, dressed, put on a coat and hat and drank a hot tea. I was then able to watch my temperature fall to just over 36 degrees before it stabilised and then started climbing back up. The same thing happened at 16 degrees but the effect was greater, the minimum temperature lower and the time taken to stabilise longer. After-drop is real. While your average body temperature may be increasing, your core will be cooling.
What to do about it:
- Get dressed quickly and warmly. Immediately after swimming you may feel great as the cooled blood has not yet returned to your core. Best to wrap up warmly before it does. It’s much harder to dress when you’re shivering.
- Don’t take a hot shower as this will increase the rate at which cooled blood returns to the core and makes the drop faster and deeper. Cold water swimmers have been known to faint in hot showers. Wait until you’ve warmed up again before showering.
- Don’t attempt to drive or ride a bike until your core temperature has recovered. Driving and shivering is not a good combination. If your core temperature drops too much and you become hypothermic it can also affect your cognitive abilities. Again, not good for driving.
- Drink something hot and eat something. Shivering is a highly energy consumptive bodily function. You need to fuel it.
- Keep an eye on your fellow swimmers. Someone who appears completely fine getting out of the water may be in trouble 10 minutes later and may need your help.
- Get out of the water before you get too cold as you will continue to get colder after swimming – give your body a margin of safety.