How Effective are Water Cooling Boots for Horses?
Dr David Marlin
There are many different options for cooling legs after exercise: clays, gels, ice boots, water boots, pumped coolant systems, spas. I decided to look at how effective simple water soaked cooling boots are.
I purchased three pairs of water cooled boots – the ones that you soak in water and then place around the legs without putting in the fridge or freezer.
The boots were tested by placing them on a metal cylinder of a similar size to a horses leg rather than on a live horse as using horses it’s difficult to control and standardise conditions. The cylinder allows the same conditions to be reproduced for each boot. Each prepared according to the manufacturers instructions using water at 16°C. The boots were then placed on the cylinder which was at 46°C to simulate a high skin temperature after exercise. A fan was used to create airflow over the boot and the rate of decrease of temperature was followed for 10min. This testing setup has been found to be highly reproducible. The rate of cooling of the cylinder was then recorded for 10 minutes.
The ambient temperature in the lab was 22°C and the humidity was 60%. The cylinder without any boots on cooled at 6.5°C per minute. Two of the boots actually slowed down the rate of cooling (5.3°C/min and 5.7°C/min) whilst the third boot appeared to slightly increase the rate of cooling (7.1°C/min).
Cold water boots cool the leg by two different mechanisms. The first is conduction. When the boot is placed on the leg, where there is contact between the boot and leg, heat will move from hotter to colder areas until both are the same temperature; this is known as conductive heat transfer. This is the most effective way to transfer heat but depends on a number of factors.
Conductive heat transfer is greatest when:
There is a large area of contact
There is a large difference between the temperature of the two surfaces or “bodies” (i.e. boot and leg)
The colder body is large
So in terms of how effective a boot will be, the larger it is, the thicker it is, the colder it is and the higher the area of contact then the greater the conductive cooling. The one problem with conductive cooling is that as the hot (skin) and cold (boot) surfaces transfer heat, the rate of cooling slows down.
The second mechanism of cooling is by evaporation. Once heat has been transferred to the boot so that they are the same temperature (which will happen within a few minutes), any further cooling must take place by the process of evaporation. Cooling by evaporation is also much slower than by conduction.
The speed of evaporative cooling depends on:
The area of the boot
The relative humidity of the air
The temperature of the air
The thickness of the boot
The larger the boot, the greater the area available for evaporation but the larger and thicker the boot the more it will insulate (inhibit heat loss). The greatest rate of evaporation will be when the air temperature is high and the humidity is low and there is a breeze. Airflow stops the air around the boot becoming saturated which slows evaporation. If the humidity is 100% (e.g. as on a cold day in the winter) then there will be no evaporative cooling.
The speed of cooling of three commercial water cooling boots under good conditions (large temperature difference, good contact, constant airflow, moderate ambient temperature and humidity) was either worse than or only slightly better than doing nothing. Cold water hosing or use of ice boots is likely to be a better option for cooling legs after exercise.