Weakness is a decrease in the strength in one or more muscles. In the strictest sense, the medical definition of weakness refers to loss of muscle strength, and this article is focused upon conditions that can result in a measurable loss of muscle function. Weakness may be generalized or may affect one muscle or muscle group exclusively. Diseases of the neuromuscular system, injuries, metabolic diseases, and toxins can all cause of measurable muscle weakness. Signs and symptoms of weakness can include trouble doing daily tasks, such as grooming or writing or problems with gait and loss of balance.

Many people use the term weakness as a synonym for tiredness, weariness, lack of energy or fatigue, although this is not technically correct. People suffering from certain medical conditions (for example, chronic fatigue, insomnia, cancer, heart disease, adrenal disease, or influenza) may describe feelings of total body weakness, referring to feelings of malaise or tiredness, even though no detectable loss of muscle strength is present.

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