We are accustomed to the strength of our body parts and are able to assess how much of strain our legs can endure. For most of us, our legs are strong enough for us to stand upright, walk, run, cycle and so on for a period of time. Depending on the level of physical conditioning, some people have stronger legs than others. However, when our leg begins to lose strength it is important to have it assessed for the underlying medical cause.
The strength of the legs are determined by several different structures and organs. Firstly the muscles and its tendons are responsible for moving the legs. As a result, the stronger these muscles the stronger the legs. Secondly muscles are controlled by nerves which can stimulate it to contract or relax. Stronger nerve impulses causes the muscles to contract more forcefully.
However, the muscles also need a strong support structure and this is determined by the bones. Since there are several bones that move (articulate), the joints are also an important factor in length strength. Ligaments further contribute to stabilizing and supporting joints. Blood flow to the area also assists by providing oxygen and nutrients to these different parts of the legs. Collectively, the muscles and tendons, nerves, bones, joints and ligaments contribute to leg strength in some way or the other.
Read more on leg weakness.
Causes of Weak Legs
There are a host of different causes of weakness in the legs. These causes can be categorized according to the structure or organ that is affected. Since weakness is a symptom, further diagnostc investigations are usually needed to identify the exact cause. Similarly other symptoms such as leg pain or numbness of the legs that may accompany leg weakness may help point towards a possible cause.
Read more on right and left leg pain.
Brain and Nerves
Signals from the brain are transmitted down the spinal cord to the nerves that control the leg muscles. These signals help to coordinate movement of the leg and control the muscle contractions. Therefore any disease or damage of the specific areas of the brain, the spinal cord or nerves supply the legs can led to weak legs.
- Stroke is where a portion of the brain tissue dies due to an interruption in the blood supply to the brain. It tends to cause paralysis on one side and may also affect the face and arm in a similar manner.
- Root compression is where the nerve root is compressed as it exits the spinal cord. This is usually due to conditions like a prolapsed disc or bone spurs. Sciatica is one such example.
- Pinched nerve is a broad term for nerve compression. It may include nerve root compression as discussed above or compression of the nerve along its path as is seen in carpal tunnel syndrome.
- Spinal cord injury due to traumatic events like a car accident or contact sport may damage the spinal cord below a certain level thereby leading to numbness and paralysis of the legs.
- Diabetic neuropathy is damage to the nerves caused by elevated blood glucose levels. It arises with long term and poorly managed diabetes mellitus. The nerves of the legs are more commonly affected.
- Amyotrophic lateral sclerosis (Lou Gehrig’s disease) is a condition that causes degeneration of nerves which causes weakness of muscles.
- Myasthenia gravis is where the communication between nerves and muscles is inhibited thereby affecting voluntary control of muscles.
- Multiple sclerosis (MS) is a condition where the immune system attacks the insulating covering around nerves known as the myelin sheath. It affects the transmission of nerve impulses.
- Guillain-Barre syndrome is an autoimmune condition where the immune system attacks the nerves and affects its normal function of conducting nerve impulses.
NOTE: Sudden muscle weakness on one side of the body must be immediately investigated by a medical professional. It could be a stroke which is one of the most common life-threatening conditions. Early diagnosis and proper medical treatment can save lives.
Muscles and Tendons
Nerve impulses from the brain are carried down the spinal cord through the peripheral nerves to various muscles throughout the body. The muscles, which attach to bones through tendons, contract and relax in response to these nerve impulses. Muscle problems may impair the contraction of the muscles in various ways.
- Strained muscles usually arises with overexertion. The muscles then become weak temporarily until it recovers with sufficient rest.
- Tendonitis is an inflammation of the tendons usually due to overexertion. It often accompanies muscle strain although in some conditions it can occur on its own.
- Torn muscle is where there is a tear in the muscle fibers due to sharp force trauma or extreme overuse. The extent of the muscle weakness correlates with the severity of the tear.
- Rhabdomyolysis is a condition where injury to muscle results in death and breakdown of muscle tissue. The components of muscle are released into the bloodstream and strains the kidneys.
- Muscular dystrophy is an inherited condition where there is a problem with the production of muscle protein. This leads to loss of muscle mass and ongoing muscle weakness over time.
- Polymyositis is an inflammatory condition where the muscles become weak. It is believed to be due to autoimmune factors although the exact cause is not known.
- Fibromyalgia is a chronic condition where there is widespread pain and fatigue. There may also be some degree of muscle weakness.
There are a host of other causes of muscle weakness. Some of these conditions may lead to pain where movement further aggravates the pain. Even though there may be no muscle weakness, the pain may limit normal muscle contraction. Some of these other causes includes:
- Arthritis, such as osteoarthritis and rheumatoid arthritis.
- Bone conditions, such as osteomyelitis and fractures.
- Poisons and toxins, such as lead poisoning and organophosphate poisoning.
- Medication, such as corticosteroids.
- Bursitis – inflammation of the bursae.
- Cachexia (wasting), seen in terminalcancer or AIDS.
- Psychological states like fear.
- Intoxication with alcohol, illicit substances or drugs like sedatives.