We may all get weak at the knees as we commonly say to describe leg weakness but this can be a serious symptom as well and needs to be medically assessed. There are number of different causes of leg weakness but it is primarily an issue with the muscles that control movement of the leg and/or the nerves that control these muscles. Sometimes it can be a disease limited to these structures or may be a systemic condition where multiple organs are affected, with the leg strength being of of them.
Meaning of Leg Weakness
The strength of any appendage like the lower limb depends on the strength of individual structures such as the:
The muscles contract and relax in response to signals carried to it by nerves. The stronger the signals, the stronger the muscle contraction. These contractions draw the tendons which pull against the bone. In order for two bones to articulate, a flexible joint has to be present. These joints are supported and strengthened by ligaments.
Leg muscle contractions do not move the limbs as is required when walking, running or kicking. It is also necessary to stabilize the upper body when standing. Leg weakness therefore refers to any reduction in the force generated by the legs when walking or when standing. It can lead to the inability to stand or walk, instability when standing or walking and even result in falling.
Symptoms of Leg Weakness
Leg weakness is a symptom on its own but may not always be as obvious. Episodes of poor balance and falling are typical when a person feels muscle weakness. Pain and cramping can also accompany leg weakness on one hand and other times there can be abnormal sensations (paresthesias) such as tingling, prickliness and even numbness. Changes in the color of the skin on the legs, such as purple or brown, may also be evident. Swelling can also occur.
Causes of Leg Weakness
Identifying the exact cause of leg weakness depends on the collection of symptoms present, the medical history of a person as well as the results of various diagnostic investigations. Below are some of the possible causes of leg weakness. While some causes may be obvious like leg weakness after a fall leading to fracture of the leg bones, at other times it may be less obvious as may occur with certain chronic autoimmune and neurodegenerative conditions.
Trauma is the most likely cause of acute leg weakness particularly when it is associated with event like a fall or sharp or blunt force injury. This includes trauma to the lower back, pelvis, thighs, lower legs and feet. It may result in the following injuries:
- Fractures – break in a bone.
- Sprain – injury or even tears of the ligaments.
- Strain – muscle and tendon injury or tears.
Nerve and Muscle
Any condition affecting the nerves that control the muscles will ultimately lead to leg weakness, as does any condition affecting the legs muscles alone. There are several muscular (muscle) and neuromuscular (nerve and muscle) conditions that can lead to leg weakness. This includes:
- Amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease is condition where the muscle shrinks and weakens due to death of the nerve cells that control these muscles.
- Cerebral palsy is caused by an abnormality in brain development or injury to the brain while the uterus, during childbirth or early in life which leads to movement and coordination problems.
- Guillain-Barre syndrome is an autoimmune condition where the immune system attacks the nerves and this leads to abnormal sensations and muscle weakness.
- Multiple sclerosis (MS) is another autoimmune condition where the immune system targets the myelin sheath (insulating layer) around the nerves.
- Muscular dystrophy is a genetic condition where there is abnormalities in protein production leading to a loss of muscle mass and weakness of the muscles.
- Myasthenia gravis is also an autoimmune condition where the immune systems disrupts the communication between nerves and muscles thereby impairing normal muscle strength and control.
- Polymyositis is a condition of unknown origin that is believed to be due to autoimmune factors leading to muscle inflammation and weakness.
- Rhabdomyolysis is a condition where the muscle breaks down that may occur with severe muscle strain, injury, infections, certain substances including some medication and high body temperature.
Blood and Circulation
- Stroke where a portion of the brain tissue dies (infarct) due to a disturbance in the blood supply to the brain. It can affect sensory and motor nerve function at distant sites from the brain and is usually one-sided.
- Peripheral arterial disease is where the arteries to the legs become narrowed an this reduces blood supply to the muscles of the legs which can lead to symptoms like pain and weakness especially upon exertion.
- Anemia is where the oxygen-carrying capacity of the blood is reduced due to abnormalities of the red blood cells and this can lead to insufficient oxygen to the muscles during physical activity.
Bones and Joints
Apart from a break in the bones, other bone conditions can also cause leg weakness. It is important to understand that muscles attach to bones by the way of tendons and if the bone is weak or deformed then the muscle may not be ale to generate the necessary force when contracting to function properly.
Weakness and inflammation of the joints may not only restrict range of motion but it can also cause weakness of the legs if it cannot withstand the force of standing and walking. Osteoarthritis, rheumatoid arthritis, septic arthritis and post-traumatic arthritis are some of the conditions that may present with leg weakness.
- Toxins and poisons such as botulinum and organophosphates.
- Drugs like corticosteroids and statins.
- Alcohol excess and certain illicit substances like heroine and cocaine.
The term “weak at the knees” is a common way of describing a sensation of feeling faint or about to fall usually associated with strong emotion. Sometimes a person does fall. This can be caused by momentary muscle weakness or loss of balance due to psychological factors and is a temporary problem that resolves thereafter.