Muscle weakness (or "lack of strength") is a direct term for the inability to exert force with one's muscles to the degree that would be expected given the individual's general physical fitness. A test of strength is often used during a diagnosis of a muscular disorder before the etiology can be identified. Such etiology depends on the type of muscle weakness, which can be true or perceived as well as variable topically. True weakness is substantial, while perceived rather is a sensation of having to put more effort to do the same task. On the other hand, various topic locations for muscle weakness are central, neural and peripheral. Central muscle weakness is an overall exhaustion of the whole body, while peripheral weakness is an exhaustion of individual muscles. Neural weakness are somewhere between.

Muscle weakness can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction.

In a broader sense, muscle weakness is the physical part of fatigue (medical).