I was told that muscle cramps are caused by a build-up of lactic acid in the muscles. Cramps most commonly occur after exercise if you push yourself too far too fast.

Muscles obtain their supply of oxygen from the blood and they use aerobic respiration to create energy. If a person is not used to a lot of exercise, then the heart and lungs might not be able to supply the necessary amount of oxygen, so the muscles use anaerobic respiration to produce lactic acid. This, however, is only as a last resort. The cramps occur about a day or so later.

I have found that the best way to rid oneself of muscle cramps is the gently strech the afflicted area until cramping subsides.

I am basing the explanation on isolated nuggets of information strung together with logic. Hopefully I am correct, but correction would be welcome. There is a previous article about muscle stiffness in the New Scientist book 'Does Anything Eat Wasps?' , called 'Delayed Reaction' if that would be of any interest to you.

Hope I was helpful :)

My left foot tends to cramp when I sit on the floor with my feet together to stretch my groin muscles. It cramps as I grab my feet to pull up while moving my knees down. It also tends to cramp if I am kneeling with the tops of my feet on the floor. Just the left though. The stretching one is during my warmup before I go for a run so it is extremely unlikely to be due to muscle fatigue. Putting the two situations together to me suggests either a nerve or circulation block that occurs due to the extreme range of movement. I do not otherwise suffer from muscle cramps despite being a long distance runner. If my salts get too low I get postural hypotension, but not cramps. Though my thighs did once cramp hard both together after a hot, humid half marathon which dumped me hard on my bum. I was trying to descend a steep grass bank at the time though.

To those who find instant relief from salt ingestion, it takes quite a few minutes for ingested salt to reach the bloodstream. I sense a placebo effect causing relaxation.