The power and ability to move one's legs stems from the interaction of two processes. These are the nerve signal that is sent to the muscles from the brain and passes via the spinal cord, and the muscle's intrinsic power to contract. Any interruption in this pathway in the lower limb can result in leg weakness. As such, leg weakness can be caused by a muscle disease or a nerve disease.
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Sudden onset leg weakness may be a very serious symptom; as it could be a sign that the patient is experiencing a stroke. Leg weakness is common in stroke because many stroke patients have brain damage in the brain areas controlling the nerve signal sent to the legs. This damage results in weakness or paresis, which means complete paralysis. Sometimes this leg weakness is perceived as "heaviness" by the patient at the onset of the stroke. The patient suffers from right-sided leg weakness if the stroke is on the left side, and the opposite is true. Stroke-caused leg weakness is also associated with weakness of the same-side arm.
Leg weakness is one of the many complications diabetics can suffer from. Sometimes diabetics experience a form of nerve damage known as proximal neuropathy. This nerve damage affects the lower limbs and causes leg weakness. The National Diabetes Information Clearinghouse estimates that around 60 percent of diabetics have some form of nerve damage.
Familial Periodic Paralysis
A clinical entity known as familial periodic paralysis can cause leg weakness. Merck Manual describes familial periodic paralysis as a rare inherited condition where patients experience episodes of severe limb weakness that resolve spontaneously. Familial periodic paralysis is classified based on its relation to the amount of potassium in the patient's blood during the symptoms. And a treatment plan will be set out for the patient according to whether he has normal, low or high potassium levels in his blood during the episode.
Nerve Cell Degeneration
Motor neuron disease is a group of disorders where the nerve cells controlling voluntary movement degenerate and die off. In some types, like primary lateral sclerosis, it is the nerve cells responsible for movement in the brain that die.
Primary lateral sclerosis (PLS) is a progressive disease of unknown cause where there is specific damage in the nerve cells that control movement of the leg, trunk and upper extremities muscles. The weakness usually develops in that order.
Muscle wasting, medically known as muscle atrophy, can also be a cause of leg weakness. An example of this is the weakness a fracture patient experiences after prolonged disuse of her leg placed in a cast. This type of weakness can be easily overcome with strength training and rehabilitation.