The muscle converts the free energy of ATP into work and heat. According to the second law of thermodynamics, in a system like muscle, which is at uniform temperature, it is impossible to convert heat into work. (This is possible in steam engine, which is not at uniform temperature, but there is a rather large temperature gradient within the engine). It follows that in muscle the heat produced is a lost free energy and the efficiency of the muscle is the ratio of the work produced to the free energy of ATP:
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