Hard labour is an activity that intensely employs skeletal muscles. These can convert chemical energy into work by moving body segments against internal and external resistances. The skeletal muscles make the body work by moving body segments against internal and external resistances. The measurement of the maximal amount of oxygen an individual's body can use during a specified period of intense exercise reflects the aerobic physical fitness of the person. The mitochondria inside muscles can convert chemical energy into physical energy. In the cylinder of the engine, an explosive combustion of a fuel–air mixture transforms chemically stored energy into physical kinetic energy and heat. Health and fitness tests are important means to make sure that only persons who are fit to do heavy physical work are employed to do so. Providing breaks is essential in hard physical labour and desirable even at lighter work for physiological and psychological reasons.