By Dr. Michael Wald - submitted for publication Swimming is an entirely parasympathetic nervous system effort; meaning, that this part of the autonomic nervous system causes dilation of arterioles to the digestive track in response to foods (eating). The dilation of blood vessels allows large amounts of blood go to the organs of digestion. When the parasympathetic nervous system is handling digestive processes, it is not optimizing blood flow to muscles and several other organs that are not "working" at the time. Exercises like swimming, require the sympathetic portion of the autonomic nervous system to kick in increasing muscle power, reflexes and muscle stamina. If both parts of the autonomic nervous system are activated at the same time neither portion works optimally. Simply put, if blood is flowing to the guts for digestion there will be less circulation to the muscles involved in any other non-digestive activity such as swimming. The increased demand of blood flow to the muscles during swimming may not be met in certain people, and depending upon the size and/or type and timing of the meal, resulting in painful cramping of muscles.