When a muscle contracts, it shortens, in many cases pulling bone like a lever across its hinge. Muscles move and by their motions we move. We are capable of performing a wide variety of actions, but despite this, muscle itself moves only by becoming shorter. They shorten and then they rest - in other words, a muscle can pull but it cannot push. We can see whole muscles contracting in this way but, in reality, they consist of millions of tiny, finely tuned protein filaments working in total concert.
Muscles produce large amounts of heat.