Muscle weakness or
myasthenia (my- from Greek μυο meaning "muscle" + -asthenia ἀσθένεια meaning "
weakness") is a lack of muscle strength. The causes are many and can be divided into conditions that have either true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including
muscular dystrophy and
inflammatory myopathy. It occurs in
neuromuscular junction disorders, such as
myasthenia gravis. Muscle weakness can also be caused by low levels of
potassium and other
electrolytes within muscle cells.