Muscle cramps can be caused by overexertion of the calf muscles while exercising, insufficient stretching before exercise, muscle fatigue and dehydration.
Source: Muscle spasms and cramps @ WebMD
- Swim Cramps: What, Why & How To Deal With Them @ triathlete europe
Excerpt: When people come to me late into a swim practice with a cramp and ask how to reduce the incidence of cramping we usually talk about diet and hydration. Most tell me they are well up on these areas being keen fit triathletes. A few admit to possibly not drinking enough during the day especially after a lunchtime spin session that may have left them dehydrated ahead of the evening swim. Stretching, nutrition, hydration and better technique seem to be the traditional points to look at to help minimise the chances of cramp. It’s also important consider the fact that most triathletes bring tired legs to the pool from cycling and running.