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 - Critical Thinking and Clinical Application Questions - Page 384: 5b


The deltoid is a large triangular shaped ( deltoid) muscle at the top of the shoulder. It can flex , medially rotate and abduct the arm. The action of abduction of the arm by the deltoid is an example of the third class ( order) of lever. In this class of action the effort (E) is exerted in the middle , between the resistance/load (R) (weight/load) and the fulcrum F) This is usually represented as REF.

Work Step by Step

The deltoid has three origins-the end of the lateral clavicle, and the acromion and the spine of the scapula. It crosses the shoulder joint and inserts on the deltoid tuberosity of the humerus. The action of abduction of the arm by the deltoid is a demonstration of the working of the third class of levers .
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