Muscle cramps can be caused by many things, but the most pronounced during training and events are:
- Over-hydration: consuming too many ounces of fluid an hour that results in a flushing effect.
Further Reading: The 10 Biggest Mistakes Endurance Athletes Make
- Electrolyte imbalances: can occur from too much sodium or not enough. Further Reading: Electrolyte Replenishment - Why It's So Important and How to Do It Right
- Calorie replacement from simple sugar sources: electrolytes are used to breakdown sugar instead of replacing critical nutrients needed for muscle expansion and contraction. This creates a demand for not only more fluid, but more electrolytes to remain in balance.
Further Reading: How should a person replace electrolytes and fluids during endurance efforts?