In our body, we often consciously use our skeletal muscles. Our nervous system sends an electrical signal to our muscles which affects proteins which cause our muscle to contract. Electrical energy is transmitted which begins a process of chemical energy being converted to mechanical energy.
Some smooth muscles behave like skeletal muscles, while others have contractions which are regularly and methodically induced by specific cells. The actions of these cells keep systems such as the digestive system working continuously without conscious thought.
The heart, the cardiac muscle, is controlled the same way as the smooth muscle. The cardiac muscle pumps blood through the body, using mechanical energy to bring oxygen to cells where it can facilitate more chemical reactions.