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.

Muscle weakness from herniated discs in the spine is an extreme symptom that results from compression of the nerve roots in the central nervous system. These nerve roots send signals from the brain and spinal cord to the rest of the body – and back again – which explains why pain or discomfort in the extremities can often stem from back and neck issues. Should the soft cushions between the vertebrae – known as discs –dislodge or rupture, there is a risk of the adjoining nerve roots becoming compressed. If the nerve roots are under too much pressure, they may be unable to carry signals from the brain to various muscles in the body, making muscles feel weak and unresponsive.

While symptoms of a herniated back disc are not always present, a nerve root that has become compressed or inflamed by a herniated disc can potentially inhibit muscle reflexes or even basic function. The cause and extent of the weakness are entirely dependent on specific spinal nerves being compressed by the herniated disc. In the lumbar spine (lower back), a compressed nerve root can be the cause of lower back pain, but also numbness, tingling, and muscle weakness in the legs and feet. Meanwhile, a herniated cervical disc (in the neck) can cause neck pain and weakness in the arms and hands. The most common symptoms of a herniated disc are as follows:

  • Chronic pain and stiffness
  • Numbness and tingling in the extremities
  • The sensation of “pins and needles”
  • Muscle weakness

In the event that muscle weakness from a herniated disc is present, it is important to visit a physician to both identify the origin of the problem and develop a course of action. Treatment options to address the weakness can vary. Often the weakness can be treated with a conservative approach – including physical therapy, exercise, over-the-counter or prescription medication, and hot and cold packs, among others. Occasionally, a conservative approach isn’t entirely effective against muscle weakness, in which case a surgical alternative may be suggested.

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.