I heard about magnesium and calcium supplements being recommended when working to improve your flexibility and high kicks. Researching on the subject I can see it recommended for martial arts and fighting sports in general.
Is this correct? Why?
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Both calcium and magnesium are electrolytes which aid in the transmission of fluid in the body. Among other things, a deficiency in magnesium or calcium is linked to muscle twitches and/or cramps. Thus, if you have a deficiency, increased intake of electrolytes may aid flexibility by allowing your muscles to function properly, plus the calcium and magnesium will aid in bone development, the parts that said muscles are attached to.
That said, evidence has been scanty in actual studies that supplementation is useful. Magnesium is easily retained in the body, and the majority of people get sufficient amounts through their diet. Similarly, calcium is present in many foods and the body has little trouble keeping it there. More worrying, longterm studies in calcium supplementation in women with osteoporosis (I know, not what you're asking about, but it's the closest relevant study) have shown that non-dietary supplementation has little effect on calcium in the bones, and actually has a positive correlation with heart disorders. The reasons for this were debated the last time I checked, but the general consensus seems to be that the calcium in supplements may not be as bio-available, which means your body does not absorb it correctly (my pet theory is that it might also have to do with that many calcium supplements are made from cuttlefish, and there might be a link with minor seafood allergies and negative effects from constant low-level exposure, but I have no real good basis for that theory).
TL;DR If you have a deficiency, magnesium and calcium in your diet may be helpful for healthy bone and muscle development, which will help with stretching, but you probably do not need supplements.