Weakness is the inability to exert force with ones muscles to the degree that would be expected given the individual's general physical condition. A top-down view of skeletal muscle Muscle is a contractile form of tissue. ...

The term subsumes two other more specific terms, true weakness and perceived weakness.

True weakness (or "objective weakness") describes a condition where the instantaneous force exerted by the muscles is less than would be expected. For instance, if a patient suffers from amyotrophic lateral sclerosis (ALS), motor neurons are damaged and can no longer stimulate the muscles to exert normal force. Motor Neurone Disease (MND) is a term used to cover a number of illnesses of the motor neurone. ... In vertebrates, motoneurons (also called motor neurons) are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. ...

Perceived weakness (or "subjective weakness") describes a condition where it seems to the patient that more effort than normal is required to exert a given amount of force. For instance, a person with chronic fatigue syndrome may struggle to climb a set of stairs when feeling especially fatigued, but if their muscle strength is objectively measured (eg, the maximum weight they can press with their legs) it is essentially normal. Chronic fatigue syndrome (CFS), also known as myalgic encephalomyelitis (ME), post-viral fatigue syndrome (PVFS) and various other names, is a disorder of the central nervous system (CNS) characterized by a number of features, the most common and notable one being severe fatigue, usually made worse by even modest exertion. ...

In some conditions such as myasthenia gravis muscle strength is normal when resting, but true weakness occurs after the muscle has been subjected to exercise. Myasthenia gravis (MG, Latin: grave muscle weakness) is a neuromuscular disease leading to fluctuating weakness and fatiguability. ...