Human Anatomy & Physiology (9th Edition)

Published by Pearson
ISBN 10: 0321743261
ISBN 13: 978-0-32174-326-8

Chapter 10 - The Muscular System - Review Questions - Page 383: 18

Answer

Most of the actions of skeletal muscles in the body can be analyzed using the principle of levers . A lever is a rigid bar used in a work operation to provide a mechanical advantage or to enhance speed or distance with which or to which a load can be moved. There are three elements involved in the application of the principle of levers in work operations. These are effort (or power, load) ,weight or resistance), and fulcrum -- a stationary point around which the lever rotates. There are thee classes (or orders ) of lever based on the relationship between the fulcrum (F) , the effort (E) and the load or resistance(R). 1. First class of levers In this order the fulcrum is positioned between the effort and the resistance or load -- E -F- R. 2.. Second class of levers In this class the resistance or load R) is in the middle between the effort (E) and the fulcrum (F): -- F-R-E 3. Third class of levers; In this class the effort is exerted between the resistance (load) and the fulcrum (F). --F-E-R Most of the skeletal muscles of the body operate on the principle of the third class of levers. Some arrangements of effort, load, and fulcrum confer a mechanical advantage, while others operate at a mechanical disadvantage If the effort is exerted closer to the fulcrum than the load, this is called a speed lever arrangement . In this case there is no gain in the lessening of effort needed for a task; however, there is gain in speed and range of motion or distance of application. On the other hand, if the effort is applied farther from the fulcrum than the load , this is called a power lever application. In this case the application is slow, but there is great gain in strength: there is a multiplication of the force applied.

Work Step by Step

Examples of the three classes of levers: The first class of levers. A common example of the operation of the first class of levers is the operation of a regular pair of scissors in cutting some thing --cloth or paper. The load is where thecloth is offering resistance to cutting; the fulcrum is where the pair of blades of the scissors are joined; the power or effort is applied by our fingers as we squeeze the handles of the blades. An example in the body of the operation of the first is the raising of the head off the chest by someone who is sitting upright with chin resting on chest. The fulcrum is where the occipital condyles rotate on the atlas; the head and face constitute the load or resistance and the effort or force is applied by the muscles at the back of the head and neck A well-known example of he second class of levers is the operation of a wheelbarrow; In this example the fulcrum is where the distal end of the levers fit into the wheel( or the wheel positioned on the ground); the effort is applied at the proximal end of the handles where the operator lifts; the load or resistance is positioned in between fulcrum and effort in the tray(or bin) of the wheel barrow. An example of the second class of levers in the human body is standing on tiptoe;:The load or resistance is the weight of the body; the fulcrum is the joints of the ball of the foot; the effort is applied by the posterior calf muscle (gastrocnemius, plantaris, popliteus tibialis posterior...) pulling up on the heel. The third order of levers: A common example of the application of the third order of levers is the use a pair of tweezers to pick up a small object. In this case the fulcrum is at the proximal end of the instrument where the two parts (levers) are joined. The load or resistance is the object being held by the distal ends of the two levers of the tweezer. The force or effort is the pressure the user applies between fulcrum and resistance, to enable the two levers of the tweezer to grasp and hold the object(load or resistance). An example of the working of the third class of levers in the body is the raising of the forearm( with or without extra weight) by the contraction of the biceps brachaii. In this case, the fulcrum is the elbow joint; the load or resistance is the distal forearm and hand( including any weight being held; the power or effort is applied at the insertion of the biceps on the proximal end of the proximal radius. Most skeletal muscles operate on the arrangement of the third class of levers
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