Physical fatigue
Physical fatigue, or muscle fatigue, is the temporary physical inability of a muscle to perform optimally. The onset of muscle fatigue during physical activity is gradual, and depends upon an individual's level of physical fitness, and also upon other factors, such as sleep deprivation and overall health. It can be reversed by rest. Physical fatigue can be caused by a lack of energy in the muscle, by a decrease of the efficiency of the neuromuscular junction or by a reduction of the drive originating from the central nervous system. The central component of fatigue is triggered by an increase of the level of serotonin in the central nervous system. During motor activity, serotonin released in synapses that contact motoneurons promotes muscle contraction. During high level of motor activity, the amount of serotonin released increases and a spillover occurs. Serotonin binds to extra synaptic receptors located on the axon initial segment
of motoneurons with the result that nerve impulse initiation and thereby muscle
contraction are inhibited.
Muscle strength testing can be used to determine the presence of a neuromuscular disease, but cannot determine its etiology. Additional testing, such as electromyography, can provide diagnostic information, but information gained from muscle strength testing alone is not enough to diagnose most neuromuscular disorders.
Musculoskeletal structures may have co-evolved with their corresponding brain structures in a way that allows them to adapt to environmental conditions.
People with multiple sclerosis experience a form of overwhelming lassitude or tiredness that can occur at any time of the day, for any duration, and that does not necessarily recur in a recognizable pattern for any given patient, referred to as "neurological fatigue".