Muscle flexing is the result of Sodium (Na) and Potassium (K) and some other chemicals being pumped out and coming back into the muscle cell. Often called the sodium/potassium pump. When we get muscle spasms it is often because we are short of potassium in the cells. Does that help?

Lack of water and/or potassium in the system.

Muscle spasms and cramps occur when a muscle contracts involuntarily and painfully. There are a number of causes of muscle spasms, ranging from nutritional deficiency to serious nervous system conditions. The process of moving a muscle is actually quite complex, although you may not be aware of it. It involves sending a signal from the brain through the nervous system to the muscle in question. An ornate series of chemical reactions and communications will ultimately result in a contraction of the muscle, causing it to move. Millions of involuntary muscle movements keep your blood circulating and your lungs moving, even as you sleep. If this process is disrupted, the result can be a muscle spasm. On the brain's end of the deal, muscle spasms can be caused by neurological disorders or conditions such as strokes. Damage to the nervous system may also cause muscle spasms; many toxins, for example, attack the nervous system. Metabolic disorders can also be responsible for muscle spasms, by disrupting the metabolic communications which cause muscles to contract. In all of these cases, medical attention may be required to resolve the situation. A deficiency in salts and electrolytes can also be responsible for muscle spasms. Dehydration is a common cause for deficiency, and more rarely you can disturb the balance of salts in your body through water intoxication. It is also possible to eat a diet which does not satisfy the needs of your body, especially if you exercise heavily, thus depleting the salts you need to control your muscles. Exercise can also contribute to muscle spasms if you work out too hard, thereby exhausting your muscles.