The human body is made up of nearly 700 muscles. The word muscle, which is derived from the Latin musculus, refers to specific tissues that contract when used. Muscles are like the engine that helps your body move. They perform a variety of functions in the human body such as helping us speak, eat, walk, pick up things, pump blood to our hearts and so much more. Without muscles, we wouldn't be able to do anything.
There are three types of muscles. Skeletal muscles are those that we can see and feel. They move your bones around. Think of a body builder's physique. It's the skeletal muscles that get exercised and become bigger with use. Skeletal muscles, which often work in teams because every set of muscles has an opposite set so that movements can be reversed, contract voluntarily, meaning you tell your nervous system what to do, and it does it.
Smooth muscles are found inside most of your organs (liver, kidneys, pancreas, etc.) and different systems of the body. The digestive system, for example, is comprised of smooth muscles in the stomach and intestines. Smooth muscles, which are able to stretch and hold tension for long periods of time, contract involuntarily. The system that uses them controls them. You don't. For the most part, you are not even aware of all the work these muscles are doing.
The third type of muscle is cardiac muscle. It is only found inside your heart. The cardiac muscles are built not for speed, but for stamina. They are hard-wired to work all the time. Cardiac muscles pump blood through the circulatory system using muscle contractions, which, of course, are voluntary.
All muscles are made of the same type of material. It's a stretchy fiber that is not unlike an elastic. Each fiber of muscle stretches to about 40 mm long and contains tons of smaller strands called fibrils. Fibrils are commanded by the nervous system so they can do the job they were meant to.
Obviously, your muscles work very hard each and every day. They can get overworked, or strained. Strains affect both muscles and their tendons. (Tendons connect muscles to bones.) Strains can be caused by pulling too far on a muscle or moving it one direction while it contracts in the other. Muscles have a range of motion. Going beyond that range of motion will not keep your muscles healthy.
Sometimes, the ligaments that connect the bones that the muscle works with can get injured. These are called sprains. A sprain causes a great deal of pain not just to the ligament, but the muscle(s) working with it. The most common sprain is a "twisted ankle." Howewer, knees and ankles are also connected by ligaments and can easily become sprained if you don't take proper precautions such as stretching before you exercise, wearing proper shoes for any activity you undertake, warming up before you exert yourself and/or traveling by foot on icy or uneven surfaces.
Muscle strains and sprains also result in muscle pain, stiffness and spasms. Muscle relaxants do not heal these injuries but help ease the discomfort and stop muscle spasms. Muscle relaxants are also sometimes used to treat Fibromyalgia, a condition that involves aches, pains, stiffness and tiredness.