Muscles produce energy and heat: mammals
Muscles are contractile tissues that produce force and cause motion through a process involving electrical impulses and metabolization of glucose, producing ATP and lactic acid.
"The muscle consumes oxygen and fuel that can be transported via a circulation system; the muscle itself supports the chemical reaction that leads to mechanical work; electrochemical circuits can act as nerves, controlling actuation; some energy is stored locally in the muscle itself; and, like natural muscle, the materials studied…contract linearly." (Madden 2006:1559)
Learn more about this functional adaptation.