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The Muscular System Muscle tissue has four main properties: Excitability (ability to respond to stimuli), Contractibility (ability to contract),Extensibility (ability of a muscle to be stretched without tearing) and Elasticity (ability to return to its normal shape). Through contraction, the muscular system performs three important functions: Motion - walking, running etc. Heat production - maintain normal body temperature Maintenance of posture - standing, sitting etc. Motion To understand how the muscles combine with the skeleton in providing motion we must look at the basic mechanics of movement. The main framework of the body is covered by muscle, whose function is to permit movement. We know that to move or lift a load against another force, it is easier to use levers, and it is this principle which the musculoskeletal system adopts and which we must examine. The component parts that are used in a lever are as follows: Lever - nearly always the bone Fulcrum - pivot point of the lever, which is usually the joint Muscle Force - force that draws the opposite ends of the muscles together Resistive Force - force generated by a factor external to the body (e.g. gravity, friction etc.) that acts against muscle force Torque - the degree to which a force tends to rotate an object about a specified fulcrum There are different types of levers dependent upon the position of fulcrum, effort and resistive force. First Class lever: Muscle force and resistive force is on different sides of the fulcrum e.g. the head resting on the vertebral column. As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back. Second Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the level longer than that through which the resistive force acts - e.g. raising the body up onto the toes. The body is the resistance, the ball of the foot is the fulcrum, and the effort is the contraction of the calf muscle. Third Class lever: Muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through the lever shorter than that through which the resistive force acts - e.g. adduction of the thigh. The weight of the thigh is the resistance, the hip joint is the fulcrum, and the contraction of the adductor muscle is the effort. Most of the limbs of the human body are articulated by third class levers. Agonist, Antagonist, Fixator & Synergist Muscles Muscles can only exert a pulling force so work in pairs. When we move a limb one muscle, the agonist muscle also known as the prime mover, causes the movement and an antagonist muscle works in opposition to the agonist muscle. Example: Biceps curl – the biceps is the agonist muscle causing the movement and the triceps are the antagonist muscle working in opposition to the biceps. The function of a fixator muscle is to stabilize the origin of the agonist muscle so that it can move efficiently. Other muscles, known as synergist muscles, stabilize muscle movements to keep them even, and control the movement so that it falls within a range of motion which is safe and desired.