There are basically two types of fatigue: physical (muscular) and general (mental). Physical fatigue is characterized by acute pain localized in the muscles, particularly those used for a long period in the exercise of a particular task. General fatigue is characterized by decreasing motivation and willingness to work, unusual sensitivity to situations and to other people, irrational stubbornness and disintegration of attention. Early researchers emphasized the importance of the accumulation of chemical products, such as lactic acid, in the production of fatigue. A more recent view is that fatigue is an outcome of frustration and conflict within the individual.