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.
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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 .