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Common sites for intramuscular injection are the gluteus medius, vastus lateralis, and deltoid muscles. However for infants, the recommended site for injection is the middle of the vastus lateralis in the thigh, which can be located in between the greater trochanter and the lateral femoral condyle.
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Vastus lateralis
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Origin: Linea aspera of femur and greater trochanter
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Insertion: Patella via quadriceps tendon and then tibial tuberosity via patellar ligament.
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Action: Extend leg at knee joint
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For adults, the preferred site for intramuscular injection is at the deltoid because it is the easiest to expose. To find the optimal injection site, make 2 fingerbreadths below the acromion process and make a triangle with two more fingers, then inject in the center of the triangle.
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Deltoid
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Origin: Spine of scapula, acromion of scapula, and acromial extremity of clavicle
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Insertion: Deltoid tuberosity of humerus
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Action: Abduct, rotate, and extend arm at shoulder joint
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