The muscular system is made up of hundreds of muscles of various sizes, shapes and functions. Some muscles, such as your gluteus maximus, or butt muscle, are very big and powerful; others, such as your flexor digitorum profundus, or finger muscles, are much smaller and more dexterous. The skeletal system has a number of functions, both voluntary and involuntary.
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Skeletal muscles are arranged in pairs on opposite sides of joints. As one muscle contracts, the opposing muscle must relax to cause movement. This movement is commonly referred to as locomotion, although that term also can be applied to the mechanics of walking and running. The muscle that is contracting to generate movement is called the agonist and the muscle that is relaxing is called the antagonist. Other muscles, called synergists, assist the agonist, and joints are held in place by muscles called fixators.
Posture can be defined as maintaining optimum joint alignment against the forces of gravity, and includes activities such as keeping your head up, pulling your shoulders back and supporting your spine. Muscles involved in posture generate low amounts of force for long periods and are often referred to as tonic muscles. Tonic muscles do not generate much movement but instead hold your limbs in position by generating isometric contractions. An isometric contraction occurs when there is tension within a muscle but no corresponding change in muscle length.
The muscular system responds to a reduction in core body temperature by shivering. Shivering is an involuntary response in which muscles contract rapidly to generate heat. Shivering is correctly termed involuntary thermogenesis, which means making heat. The colder you are, the more you shiver. Shivering muscles demand an increase in oxygenated blood, which is drawn from nonessential organs to increase your body temperature.
Other Functions of the Muscular System
The muscular system has a number of other functions in addition to movement, posture and thermogenesis. Smooth muscles contract to propel food through the hollow tubes that make up your digestive system. Your stomach is a large and muscular bag that mixes your digested food before it passes into your small intestine for absorption. Your heart works every minute of every hour of every day to pump oxygen-rich blood around your body and, with the help of your blood vessels, ensures that your brain receives a constant supply of essential oxygen and glucose. Your muscular system also dictates your body shape and makes up a large percentage of your total body mass.