Yes surely no doubt reduced magnecium levels may be asociated with the symptoms such as muscle cramps & it may require magnecium suppliment. But most common cause of the muscle cramps is calcium deficency which should be investgated first rather than jumping directly on magnesium. As you are 76 yr women & quite healthy thre may be the possibility of the low calcium levels. So firstly you should go for sr calcium level check ups which will give idea whether we should go for , further testing of sr magnecium level. As old age females have defficiency of calcium commonly, That should be the first deffrential diagnosis & you should be started upon calcium first. So my opinion is get your calcium level checked & visit your physician with the repors who may properly guide you after going through same. Mean while you can start with oral calcium & vit d3 suppliments as per your physicians consent.