Muscles that become easily fatigued are a sign of essential fatty acid deficiency and / or Vitamin B1 (thiamine). Most people take their B vitamins in a vitamin B complex. Dr. Royal Lee considered the B complex to be two distinct complexes, with different physiological actions. According to Dr. Lee, Vitamon B was thiamine based and contained the other B vitamins that were soluble in alcohol including B12, B5, and B4. Vitamin G, on the other hand, was riboflavin based and contained the B vitamins that were not soluble in alcohol.
People who have muscles that are easily fatigued should be assessed for essential fatty acid deficiency, and the B complex they should be on is a naturally occuring thiamine. This symptoms could also be due to gallbladder dysfunction and lack of tissue oxygenation requiring supplemental vitamin E.
Reference: Weatherby, D. (2004). Signs and Symptoms Analysis from a Functional Perspective. Jacksonville, OR: Bear Mountain Publishing.