Muscles aren't just for weight lifting. They serve important purposes in your body. The muscular system allows you to move freely and stand up straight. The heart, one of the most important muscles of all, pumps blood throughout your entire body. Some muscles you can control and others, like your heart, are completely autonomous. Reflexes and small muscle contractions allow you to coordinate and balance with little mental effort. For example, during a road collision, your muscles may direct you safely through reflexes faster than your brain can think of a solution.
A muscle is made from strong, but elastic tissue. Depending on how you exercise, certain muscles can be stiffer or more flexible than others. Muscles are susceptible to tearing, pulling, and soreness. When exercising it is important to not push your muscles too hard. While soreness can feel like a sign of a successful workout, excessive soreness is never a good thing.