As a side note, magnesium is a critical nutrient since it is involved in many functions. Studies have pointed to the role of magnesium in the role of Chronic-Fatigue-Syndrome and many other diseases. Any stressor including physical activity will reduce magnesium. You can read Dr. Mildred Seelig's research on this.
Vitamin D levels should be checked since it is synergistic to magnesium. Secondly checking the other minerals by way of hair analysis will help as well.