Possibly one of the strangest discomforts that a runner may experience during his or her run is the sensation of of extremely itchy legs, so itchy that you may even need to stop and scratch them, but never to any avail. While this can occasionally be explained away by a lack of humidity, dry skin, or irritating detergents, it can still happen when using the mildest of detergents, in 100% humidity and slathered in the best moisturizing lotion on the market.
When itchy legs occur under such conditions, it is generally in runners who have taken a break from running for some time and are just getting back into it. As the heart rate starts to increase and the blood starts to flow more rapidly, the capillaries and arteries that carry blood through your body expand and contract. In a fit, in-shape person who is used to doing cardio exercise, the capillaries (particularly in the legs, which are the most active when running and therefore receiving the greatest amount of blood flow), would stay open to allow the blood to flow with maximum ease and velocity, therefore bringing oxygen to the legs in the most efficient way. If you have fallen off of your regular exercise regimen, however, the capillaries allow themselves to close, hindering the flow of blood. When they are forced to expand, the nerves around them read the sensation as itchiness, and so your brain tells you that your legs are itchy. Thus, the only way to prevent this phantom itch is to keep up your regular exercise routine.
In extreme cases, the itch can sometimes be accompanied by welts or a rash. These symptoms should subside once you are back into the swing of things and exercising on a regular basis again, but if you experience welts, a rash or hives for longer than a week you should speak to your doctor.