definite limitation used to describe the human body in the passage means that

Read the passages below and answer the questions that follow:

 Everyone is tired or fatigued at some time. The major cycle in your life is work, fatigue and rest in that order. Fatigue is characteristic of your body. It does not occur in a man-made machine which operates as long as its parts are intact and it has fuel. But your body, a living machine, has a definite limitation – its work continues, it gradually loses its responsiveness, becomes less irritable, turns out less work and finally may not respond at all. The feeling of fatigue usually expresses itself in three ways: first, there is a feeling of tiredness and a marked desire for rest. Second, efficiency is greatly reduced. Third, there may be definite physiological changes in your body, low blood pressure, loss of muscle tone, tremors, and poor muscular coordination, and in other ways. Fatigue, however, may express itself in many ways for there are many different forms of it. The fatigue of a student. for example, who has worked all evening on a difficult lesson, is different from that of a labourer who has worked all day at a back-breaking task, or that of a business executive who worries with the stress and strain of organisation.