How does it work?

Your blood vessels have little circular muscles around them that contract(constrict) to reduce blood flow or relax(dilate) to increase blood flow to certain areas of the body. When you add cold to an area, the muscles around the vessels contract, therefore constricting the vessels and reducing blood flow. When you add heat to an area, the opposite happens and the muscles around the vessels relax and dilate the vessels, therefore increasing blood flow. Using moist heat vs dry heat tends to penetrate deeper into the body because water is an excellent conductor of heat and we are about 70% water as well.

How can this affect my tight muscles?

When tension builds up in your muscles, there is reduced blood flow in that area from the compression on the blood vessels. This sometimes causes  chemicals that tell your muscles to contract as well as many pain markers, etc., to get locked in place, thus creating this vicious cycle of tension and pain. Adding heat to this area will cause the blood vessels to dilate, allowing more blood to pass through, thus increasing blood flow, flushing the area out and sequentially reducing tension. 

To take it to another level, you can even "pump" your circulatory system by alternating from hot to cold to hot again. Sometimes adding heat just isn't enough, so you add cold after to constrict the vessels again. This will "flush out" the blood you just introduced, taking with it all the pain markers and chemicals it no longer needs, so you can add heat again, and introduce fresh blood with all the healing compounds your sore muscles do need. This method is called Contrasting, I may use it locally with hot/cold stones or with your whole hand/foot in an immersion bath.

How can this help my acute injury?

If you have recently injured yourself, are experiencing a muscle spasm or extreme pain - use COLD first (it doesn't have to be extreme). Too many times I have had people come and see me after using heat on a fresh muscle spasm or painful muscles and they have just made things so much more worse. When there is an injury to an area, an inflammatory process starts which is marked by pain to let you know something is wrong, followed by an increase in circulation to that area so your body can heal that area. This is a good process of course, but sometimes can feel a little overwhelming. As much as you can, try and just let your body run it's course, but if the pain is a little too much, applying ice or a cold compress can be helpful. This will constrict the blood vessels, flush the pain and inflammation from the area and slightly numb the area for a while. You do not want to over due this, as your body does need the inflammation to heal the area, but a little flush out every couple hours will clean it out, calm it down and make room for fresh blood to come in. I may use this technique if you come in with an injury or after working really deep in the treatment sometimes.