Muscle is the largest single tissue in human body, comprising more than 40% body mass in adults. The contraction of muscles is the most apparent and dramatic macroscopic signs of life and have therefore excited the imagination since the times of ancient. It is ironic that after using muscles day and night, for many thousands of years homo-sapiens still does not know at molecular level. Now it is known that muscles are transducers capable of converting chemical energy into mechanical energy. Mechanical contraction of muscle is initiated by the arrival of electrical nerve impulse and is set into action through calcium ions which in force sliding of actin and myosin filaments. Mechanical analogue of muscle comprising of contractile and elastic components is powerful asset in explaining isotonic and isometric contraction of muscles. A description of this model is provided in this chapter. Further, features of velocity of shortening, tension, latent period, force velocity curve and length tension relationship of contracting muscle have been analysed in the text. To compute the force exerted by muscle, various joints in the body are analysed in the term of a lever. Types of levers and there relative importance in body movements have also been explained. To integrate information of the muscle machine, a note on the work output, power output and mechanical efficiency of working muscle has been added in the chapter.