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=Cold Induced Vasodilatation=
Cold initially reduces blood flow, but after about 20 minutes the body increases blood flow, possibly to prevent skin damage. This is called Cold Induced Vasodilatation (CIVD) and has been known about since 1930<ref name="Lewis-1930"/><ref name="O'Brien-2005"/>). It's important to stay warm overall while icing a muscle, as reduced body temperature prevents CIVD<ref name="Daanen-2003"/><ref name="Daanen-1999"/>. This alternating reduction in inflammation and increased blood flow is believed to act as a 'pump', speeding up heeling. This cycle typically takes a few minutes, as shown below.
[[File:CIVD.jpg|center|thumb|300px|]]
=The (Lack) of Science=
There is remarkably little science produced on Cold Induced Vasodilation. A 2004 analysis of the available research at the time stated "Currently, no authors have assessed the efficacy of ice in the treatment of muscle contusions or strains. Considering that most injuries are muscle strains and contusions, this is a large void in the literature." A 2008 study stated in its conclusion "There is insufficient evidence to suggest that cryotherapy improves clinical outcome in the management of soft tissue injuries"<ref name="Hubbard-2004"/><ref name="Collins-2008"/">. We'll therefore look at some anecdotal, real world experiences and recommendations.
=Common Recommendations=
The general recommendation for ice is to apply it for 20 minutes, then remove it for 20-60 minutes, repeating this cycle several times<ref name="Born2005"/>. The general advice is to avoid applying ice for too long as it can damage the skin. I have found while this approach does help a little, it is not as effective as leaving the ice in place for a much longer period.
=The Danger of Gel Packs, Ice Blocks, and Frozen Vegetables=
I use ice cubes in a hefty Ziploc bag rather than gel packs as Gel packs start off too cold, and then warm up too quickly. If you use ice, the temperature will remain constant around freezing until all the ice is melted<ref name="Merrick-2003"/>. You should avoid large blocks of ice, as the ice will be at freezer temperature which is cold enough to damage the skin. The goal is to have a mixture of ice and water, which will be around freezing point. Likewise, frozen vegetables can remain too cold and do not result in a layer of water.
=Monitoring Skin Temperature=
You should monitor your skin temperature closely, and if it gets too cold you should stop. Generally, I find my skin temperature doesn't drop below 2c/35.6f and is typically warmer than that. I've used several different technologies for skin temperature monitoring, each with their own advantages.
* A thermocouple is a temperature sensor on the end of a wire, which allows you to continuously monitor your skin temperature, and I have one that supports four sensors at the same time and is only <jfs id="B01CNLX9M4" noreferb="true"/>. This is probably the most cost effective approach.<br/>[[File:Temperature Monitor.jpg|center|thumb|300px|This is the temperature monitor I use for checking skin temperatures.]]<br/>[[File:Skin Temperature Probes.jpg|center|thumb|300px|I tape the probes over the area I'm icing to check the temperature. Depending on the location, I may spread them out rather more than this.]]
* A [[Thermal Camera]] is great as it allows you to see not only the temperature at one spot, but the pattern of temperature changes. The downsides are that you have to take the ice off to check the temperature, and the cameras are expensive. I'd use this with the above temperature sensor rather than instead of it. I recommend the FLIR ONE <jfs id="B00VILVV62" noreferb="true"/>, but check out [[Thermal Camera]] for more details. <br/>[[File:Cryolipolysis Thermal Camera.jpg|center|thumb|300px|An example image from my [[Thermal Camera]] showing the skin temperature.]]
=Typical Skin Temperatures=
I've made a number of tests, monitoring my skin temperature for up to three hours. The graph below shows three tests where I took temperature readings every 15 minutes, and it's fairly typical of what I see. The skin temperature initially drops fairly quickly, dropping to around 5-15c/40-60f within five minutes, and stabilizing around 5-10c/40-50f within 15-20 minutes. After that, the temperature tends to be reasonably stable, and most of the variation is down to how much the ice water is moved. If the bag of ice water is kept very still, the skin temperature will rise somewhat, but if it's actively agitated, then the skin temperature can drop. It takes some effort for me to get the skin temperature below 2c/36f, and obviously, the skin temperature never drops below freezing, as the ice water isn't that cold.
[[File:Skin Temp Graph.jpg|center|thumb|300px|Skin temperature against time while icing.]]
Here are some other tests I performed, showing some of the variation you might experience, with temperatures rising and falling.
[[File:Skin Temp Graph2.jpg|center|thumb|300px|Various other tests of icing.]]
=Ice, Inflammation and Healing=
A 2010 study<ref name="Lu-2011"/> has shown that inflammation is necessary for healing, something that has been known for some time<ref name="Alaseirlis-2005"/>. The study did not look at the use of ice for healing, only the role of inflammation itself, though it's conclusions were widely misinterpreted<ref name="xTelegraph"/>.
