In general, muscle cramps in the feet and legs more commonly occur at rest, especially at night. But some athletes are prone to muscle cramps during competition, despite getting advice and help from skilled personal trainers. People who have flat feet or hyper flexible knees are reported to experience more cramping.
Dehydration with low intake of sodium and potassium can definitely cause muscle cramps. If you are rehydrating with plain water, you might want to try a sports drink instead. People who have low blood levels of calcium or magnesium can experience cramps. But taking extra dietary calcium or magnesium if your blood levels are normal probably won't help.
When we breathe rapidly, we blow off more than the usual amount of carbon dioxide. This causes our bodies to transiently become more alkaline, which lowers the amount of free calcium in our blood. This is why people cramp when they hyperventilate. If you breathe heavily during exercise, this might contribute to cramping.
Your own discovery, bending your toes back with slowly increasing force when you feel a cramp come on, is what I recommend. Other preventives include taking plenty of fluids all day long, doing a longer warm up before you exercise, and stretching after you have fully warmed up.
