Lactic Acid

• Lactic Acid occurs after bouts of intense exercise. According to MedLine Plus, lactic acid builds up mostly in the muscle cells and red blood cells. It's a byproduct of exercise.
  
  

Lactic Acidosis

• Lactic Acidosis is a condition that happens when the body can't keep up with the production of lactic acid. MedLine Plus notes that lactic acid may build up faster than the red blood cells can remove it.
  
  

Exercise burn

• Exercise is one of the most common causes of lactic acid and lactic acidosis. However, serious diseases may cause it since lactic acidosis indicates a low level of oxygen. Those diseases include: AIDS, cancer, kidney failure, respiratory failure and sepsis.
  
  

Meralgia paresthetica

• A burning sensation in the outer thigh might be meralgia paresthetica, according to the College of Medicine at the University of Florida. It occurs when the nerve that controls the sensation in the skin surface of the upper leg is compressed, according to the Mayo Clinic.
  
  

Medication

• A final possible cause for burning-muscle sensation is the medicine Metformin, which is used to treat diabetes.