Muscle weakness is a lack of muscle strength. Its 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. It can be temporary or long-lasting (from seconds or minutes to months or years). The term myasthenia is from my- from Greek μυο meaning "muscle" + -asthenia  ἀσθένεια meaning "weakness". Types
Neuromuscular fatigue can be classified as either "central" or "peripheral" depending on its cause. Central muscle fatigue manifests as an overall sense of energy deprivation, while peripheral muscle fatigue manifests as a local, muscle-specific inability to do work. Neuromuscular fatigue
Nerves control the contraction of muscles by determining the number, sequence, and force of muscular contraction. When a nerve experiences synaptic fatigue it becomes unable to stimulate the muscle that it innervates. Most movements require a force far below what a muscle could potentially generate, and barring pathology, neuromuscular fatigue is seldom an issue.For extremely powerful contractions that are close to the upper limit of a muscles ability to generate force, neuromuscular fatigue can become a limiting factor in untrained individuals. In novice strength trainers, the muscles ability to generate force is most strongly limited by nerve’s ability to sustain a high-frequency signal.
After an extended period of maximum contraction, the nerve’s signal reduces in frequency and the force generated by the contraction diminishes. There is no sensation of pain or discomfort, the muscle appears to simply ‘stop listening’ and gradually cease to move, often lengthening. As there is insufficient stress on the muscles and tendons, there will often be no delayed onset muscle soreness following the workout. Part of the process of strength training is increasing the nerves ability to generate sustained, high frequency signals which allow a muscle to contract with their greatest force. It is this "neural training" that causes several weeks worth of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit. Past this point, training effects increase muscular strength through myofibrillar or sarcoplasmic hypertrophy and metabolic fatigue becomes the factor limiting contractile force. Central fatigue
Central fatigue is a reduction in the neural drive or nerve-based motor command to working muscles that results in a decline in the force output. It has been suggested that the reduced neural drive during exercise may be a protective mechanism to prevent organ failure if the work was continued at the same intensity. There has been a great deal of interest in the role of serotonergic pathways for several years because its concentration in the brain increases with motor activity. During motor activity, serotonin released in synapses that contact motoneurons promotes muscle contraction.