After my bout of muscle cramping after my last epic mtn bike ride, I decided to look into what caused muscle cramping. I had assumed all along that it was primarily caused by the loss of sodium, calclium and potassium through perspiration during exercise. And because those two moleculres, in their ionic forms (Na+, K+, Ca+) are essential to the process of muscle contraction (especially Calcium).
After a bit of research, I found this article, which was written by someone with a vested interest in selling electrolyte replacement products, which lists three reasons:
1. Electrolyte depletion.
2. Dehydration
3. Energy depletion
His explanation for Dehydration as a cause of cramping makes sense- If the volume of water in which these ions exist is distorted greatly, it will have a direct impact on the absolute concentration of the ions in solution. When that happens, lots of unintented consequences may ensue, one of which is muscle cramping.
The energy depletion explanation also makes sense, and in retrospect, I should have suspected it. As I learned as an undergrad, the process of muscle fiber contraction consumes Adenosine Triphosphate (ATP) as an energy carrier produced by glycolysis (the Krebs cycle, etc.). What I had forgotten was the fact that both fiber contraction and fiber relaxation consume ATP. That is why rigor mordis affects dead people- no more ATP around to keep the muscle in a relaxed state. So if your muscles run out of ATP and the immediate prerequisites to ATP production like glucose and glycogen, you can also experience muscle cramping.
So what does this tell me about my next ride? I should probably eat a bit more during the ride to prevent energy depletion, and I should also consider adding an electrolyte replacement to the water I bring with me.