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Through the process of muscle contraction, tension is developed within muscle tissue, which may or may not lead to movement of a part of the body. The term contraction often means to shorten; however, during a muscle contraction the tension may cause muscles to stay the same, get longer or become shorter. The physical process of the contraction happens quickly and in only a few steps.
The two proteins that are responsible for muscle contractions are actin and myosin. When myosin and actin interact, it results in a change in the shape of the muscle cells. Contractions happen when actin filaments actually slide over myosin filaments. This slide causes a change in the shape of the muscle cells producing movement, which is considered a muscle contraction.
Depending on the muscle and the intended outcome of the contraction, muscles can either lengthen or shorten. A concentric contraction occurs when the muscle decreases in length, such as when performing a bicep curl. An eccentric contraction occurs when the muscle increases in length, such as when doing a push-up.
Muscle contractions can result in either movement or no movement. When attempts are made to push or pull an object that is physically unmovable, such as a building, the burden on the muscle exceeds the pressure produced by the contracting muscle. This results in no movement of the object or the muscle and is known as an isometric contraction. On the other hand, isotonic contractions result in movements taking place. If the muscle is stronger than the load it is trying to push or pull, a movement results.
There are two types of contractions: voluntary and involuntary. The difference between the two boils down to conscious thought vs. unconscious control. Skeletal muscles, also known as striated muscles, are voluntary muscles, while smooth muscle and cardiac muscle are involuntary muscles. Skeletal muscles are so named as they are usually attached to the skeleton to provide support and cause movement. Cardiac muscle is only found in the heart, and smooth muscle is found within internal organs, such as the stomach, bladder, and intestines.
Voluntary muscles are manipulated by conscious brain commands. These muscles must be stimulated by nerve impulses. This signal is known as an action potential and it motivates the muscle fibers to produce contractions. For example, to walk, one must consciously stimulate the body to perform the action. For this to happen, the brain sends a nerve impulse to the muscles in the legs so they contract and relax until the person sends another message to stop the action.
Smooth and cardiac muscles are involuntary muscles, so they do not need to be consciously stimulated for contractions to take place. Instead, a muscle contraction of smooth or cardiac muscle is initiated within the organ itself. Within the digestive system, the introduction of food is what causes the contractions to move the food through the system. Similarly, the heart controls its own contractions through electric impulses that originate within the heart.