Many medical conditions are associated with both pain in the legs and fatigue, and determining the cause of these symptoms requires a trip to the doctor. To make a diagnosis, your doctor needs a thorough understanding of your history, including what brings on your symptoms, what improves them, and what other symptoms occur.
Overtraining is a type of overexertion that can cause both fatigue and muscle aches. It is usually seen in competitive athletes, but according to the June 2003 edition of "The Physician and Sports Medicine," it can occur in ordinary fitness enthusiasts who exercise intensely over an extended period of time. Other symptoms of overtraining can include irritability, insomnia, anxiety and increased susceptibility to infection. It is more likely to affect the New Year's resolution-type of person—that is, someone who drastically increases regular exercise after a long period of inactivity.
Disorders that decrease circulation to the legs can cause aching and fatigue. An example of such a disorder is peripheral arterial disease, or PAD. PAD is caused by atherosclerotic plaques that block arteries in the extremities, similar to the plaques that block coronary arteries in coronary artery disease. PAD classically causes leg pain while walking or climbing stairs, and improves with rest. Frequently there is an overall feeling of fatigue due to decreased circulation. PAD typically affects older individuals with a long history of cigarette smoking and other cardiovascular disease risk factors, such as high cholesterol and diabetes. PAD is a potentially dangerous disorder, as it may also accompany a serious degree of coronary artery disease.
Autoimmune disorders occur when the immune system mistakenly recognizes part of the body as a foreign invader and attacks it. Autoimmune disorders typically cause pain in the area under immune attack as well as constitutional symptoms, such as fever and fatigue. Polymyalgia rheumatica is an example of an autoimmune disorder that can cause leg muscle aches and fatigue. Other symptoms include fever, malaise and unintended weight loss. Polymyalgia rheumatica typically affects older individuals and is more common in those of northern European ancestry. Like most autoimmune disorders, it affects women more often than men.
Fibromyalgia and Chronic Fatigue Syndrome
Fibromyalgia and chronic fatigue syndrome, or CFS, are diagnoses of exclusion. This means that they are not so much causes of leg pain and fatigue, but are diagnoses given when other causes can not be determined. Fatigue, headache, muscle aches and sleep disturbance are among the symptoms common to both syndromes. Fibromyalgia symptoms are more closely associates with increased pain perception and CFS symptoms more with fatigue and flu-like complaints; given their common features, however, these differences may represent a spectrum of outcomes from a common underlying disorder. Depression is a common feature of both syndromes, and treating this depression frequently improves the symptoms. This fact does not necessarily imply that the syndromes are fundamentally psychiatric, however, since depression can lead to fatigue and increased pain sensitivity.