actually AFAIK it is more usually calcium deficiency that leads to muscle weakness.
1) Painful muscle spasm reversed by magnesium sulphate. A case report.
Clinton CW, Braude BM, James MF.
A 25-year-old paraplegic man who had sustained a T3/T4 vertebral compression fracture 3 years previously presented with severe, painful spasm of the left hamstring muscle group of 2 hours' duration. This spasm produced extreme knee flexion of a degree which held the left foot posterior to the right buttock. An intravenous injection of 2 g magnesium sulphate produced immediate relief. Possible mechanisms of action of magnesium are discussed.
2) Magnesium sulphate for treatment of severe tetanus: a randomised controlled trial
"Magnesium infusion ... does reduce the requirement for ... drugs to control muscle spasms and cardiovascular instability."
3) The multifaceted and widespread pathology of magnesium deficiency
Even though Mg is by far the least abundant serum electrolyte, it is extremely important for the metabolism of Ca, K, P, Zn, Cu, Fe, Na, Pb, Cd, HCl, acetylcholine, and nitric oxide (NO), for many enzymes, for the intracellular homeostasis and for activation of thiamine and therefore, for a very wide gamut of crucial body functions. Unfortunately, Mg absorption and elimination depend on a very large number of variables, at least one of which often goes awry, leading to a Mg deficiency that can present with many signs and symptoms. Mg absorption requires plenty of Mg in the diet, Se, parathyroid hormone (PTH) and vitamins B6 and D. Furthermore, it is hindered by excess fat. On the other hand, Mg levels are decreased by excess ethanol, salt, phosphoric acid (sodas) and coffee intake, by profuse sweating, by intense, prolonged stress, by excessive menstruation and vaginal flux, by diuretics and other drugs and by certain parasites (pinworms). The very small probability that all the variables affecting Mg levels will behave favorably, results in a high probability of a gradually intensifying Mg deficiency. It is highly regrettable that the deficiency of such an inexpensive, low-toxicity nutrient result in diseases that cause incalculable suffering and expense throughout the world. The range of pathologies associated with Mg deficiency is staggering: hypertension (cardiovascular disease, kidney and liver damage, etc.), peroxynitrite damage (migraine, multiple sclerosis, glaucoma, Alzheimers disease, etc.), recurrent bacterial infection due to low levels of nitric oxide in the cavities (sinuses, vagina, middle ear, lungs, throat, etc.), fungal infections due to a depressed immune system, thiamine deactivation (low gastric acid, behavioral disorders, etc.), premenstrual syndrome, Ca deficiency (osteoporosis, hypertension, mood swings, etc.), tooth cavities, hearing loss, diabetes type II, cramps, muscle weakness, impotence (lack of NO), aggression (lack of NO), fibromas, K deficiency (arrhythmia, hypertension, some forms of cancer), Fe accumulation, etc. Finally, because there are so many variables involved in the Mg metabolism, evaluating the effect of Mg in many diseases has frustrated many researchers who have simply tried supplementation with Mg, without undertaking the task of ensuring its absorption and preventing excessive elimination, rendering the study of Mg deficiency much more difficult than for most other nutrients.
4) The effect of magnesium oral therapy on spasticity in a patient with multiple sclerosis
The effects of magnesium glycerophosphate oral therapy on spasticity was studied in a 35-year-old woman with severe spastic paraplegia resulting from multiple sclerosis (MS). We found significant improvement in the spasticity after only 1 week from the onset of the treatment on the modified Ashworth scale, an improvement of range of motion and in the measures of angles at resting position in lower limbs.
Magnesium is the natural physiological antagonist of calcium. For over one century it has been known to be a mulscular relaxant agent.
5) A longitudinal study of serum 25-hydroxyvitamin D and intact parathyroid hormone levels indicate the importance of vitamin D and calcium homeostasis regulation in multiple sclerosis
in this study you have to have full text access to read the table but the MS patient levels for magnesium averaged 0.85 mmol/L.
although that level is technically within the normal range, the normal range is too wide to be considered healthy. according to published peer-reviewed literature, patients should have levels of at least 0.90 mmol/L.