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.
Many people use the term weakness as a synonym for tiredness, although this is not technically correct. People suffering from certain medical conditions (for example, cancer 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.