Arm and leg weakness can be caused by a variety of different conditions, some of them temporary and others chronic. Excessive exercise is one cause of temporary fatigue in the major muscle groups, although this is not considered true muscle weakness. Conditions which may cause clinical weakness include neurological disorders, muscular problems or injuries, toxic overload, and certain metabolic illnesses.

There are many common reasons for arm and leg weakness that are not cause for alarm and generally get better with treatment. Muscle injuries due to playing sports are one example, as well temporary toxic overload in the body. Dehydration can also cause weakness in severe cases and can be remedied by replenishing fluids. Toxins can be removed from the body by doing a detoxifying cleanse, such as a juice fasting diet, or by discontinuing any activities or medications which may be causing the overload in the first place.

Certain neurological disorders can also cause arm and leg weakness. Multiple sclerosis is one condition that is hallmarked by increased weakness, trouble with balance and coordination, and sometimes vision problems. More severe but much less likely conditions are Parkinson’s disease and Lou Gehrig's disease. These often strike the elderly more so than young adults and teens, and are generally considered rare. This is particularly true for Lou Gehrig's disease.

Sometimes illnesses such as cancer can also cause arm and leg weakness, although other symptoms are generally noticed first. Another possible illness is diabetes, which can lead to all-over body fatigue and malaise when untreated. Other possible causes include an electrolyte imbalance, certain viruses like influenza, torn muscles or ligaments, and stroke.

If arm and leg weakness is severe and lasts longer than a couple of days with no known trauma being done to the area of weakness, a doctor should be consulted for an exam. Although weakness is very rarely a serious problem, there are conditions which should be ruled out. Weakness accompanied by other symptoms should always be examined promptly, especially if neurological symptoms are present. These can include muscle twitching, loss of balance, decreased coordination, personality changes, vision changes, dizziness, fatigue, memory loss, or vertigo.

Injury-related weakness is most often accompanied by pain and tenderness in the same area. Even after pain has subsided, weakness may continue until the muscle has been exercised sufficiently enough to rebuild tissue. This should be done under the supervision of a doctor or physical therapist.