The answer is that it could be both. Muscle cramps are general caused by tired muscles, which is inevitable in distance running. They can also be caused by a sodium imbalance and dehydration. Staying hydrated is a tricky proposition because there are so many variables that impact how much fluid and sodium you sweat out during your run. Intensity, fitness, heat, humidity and altitude are some of the things that will impact how sweaty you get. How much any body sweats can vary between ten to eighty ounces per hour! That is a high amount of variability. The concentration of sodium in that sweat also varies greatly with an average concentration of about one-thousand milligrams per thirty-two ounces. In other words if you lose two pounds of sweat you may have also lose around one-thousand milligrams of sodium that needs to be replaced by drinking and eating sodium! Of course these numbers are highly variable with the environment and individuality. Determining your sweat rate can be a useful tool in bench-marking how much sodium …