Answer
Most body heat is generated by chemical heat-producing (exergonic) reactions-- including oxidation of fuel nutrients, and the utilization of ATP. Smaller amounts of heat derive from joint friction, blood flow, and movements of other body structures and organs.
When the body is at rest, most of the heat generation is done by brain, heart, endocrine organs-- the skeletal muscle produces about of the total heat generated in the resting person. This situation is reversed in the exercising body. During physical activity, the skeletal muscles may contribute as much as forty fold X40) the amount of heat produced by the rest of the structures/organs of the body.
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Brown fat is also a source of heat in infants.
