"If your legs or feet fall asleep or begin to hurt during sitting meditation so that your concentration becomes disturbed, feel free to adjust your position. If you do this slowly and attentively, following your breathing and each movement of your body, you will not lose a single moment of concentration. If the pain is severe, stand up, walk slowly and mindfully, and when you are ready, sit down again.
In some meditation centers, practitioners are not permitted to move during periods of sitting meditation. They often have to endure great discomfort. To me this seems unnatural. When a part of our body is numb or in pain, it is telling us something, and we should listen to it. We sit in meditation to help us cultivate peace, joy and nonviolence, not to endure physical strain or to injure our bodies."
(Source: Peace Is Every Step)