Massage & Bodywork

MARCH | APRIL 2018

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OTHER EXAMPLES There are many other examples in the human body of muscles that change their joint action when the position of the body changes. Image 5 shows the clavicular head of the pectoralis major. From anatomic position, it is an adductor of the arm at the GH joint because its line of pull is below the (anteroposterior) axis of motion for frontal plane motion (see right side of the client's body). But if the arm were first abducted approximately 100 degrees or more, the line of pull moves to be above the axis, so the clavicular head of the pectoralis major becomes an abductor of the arm (see left side of the client's body). Images 6A–6B depict the adductor longus. From anatomic position, the adductor longus is a flexor of the thigh at the hip joint (in addition to being an adductor). And if the thigh is in extension, it is also a flexor (Image 6B). But if the thigh is in full flexion, then the line of pull of the adductor longus moves posterior to the (mediolateral) axis of sagittal plane motion and the adductor longus becomes an extensor (Image 6A). The advantage to this ability to change its joint action is that when walking or running, the adductor longus contributes to both flexion and extension of the thigh depending on the position during the gait/running cycle. Perhaps one of the most well- known examples of a muscle that can change its joint action is the piriformis (Image 7). From anatomic position (Image 8A), the piriformis is a lateral rotator of the thigh at the hip joint because its line of pull passes posteriorly to the axis of motion in the transverse plane. But if the thigh is first flexed approximately 60 degrees or more (Image 8B), the piriformis becomes a medial rotator because its line of pull is now anterior to the axis of motion. For a video demonstration, visit www.learnmuscles. com/mb to watch "Transverse Plane Motions of the Piriformis." 88 m a s s a g e & b o d y w o r k m a r c h / a p r i l 2 0 1 8 Anterior View Adductor longus Adductor longus A B *OXWHXV PHGLXV 3LULIRUPLV 6XSHULRU JHPHOOXV $ Clavicular head of the pectoralis major. From anatomic position (as seen on the person's right side), the clavicular head adducts the arm at the glenohumeral joint. When the arm is first abducted to approximately 100 degrees or more (as seen on the person's left side), it becomes an abductor. Permission Joseph E. Muscolino, Kinesiology: The Skeletal System and Muscle Function, 3rd ed. ( Elsevier, 2017). Adductor longus. 6A: When the thigh is in full flexion, the adductor longus is an extensor. 6B: When the thigh is extended, it becomes a flexor. Permission Joseph E. Muscolino, Kinesiology: The Skeletal System and Muscle Function, 3rd ed. ( Elsevier, 2017). Posterior view of the piriformis bilaterally. The gluteus medius and superior gemellus have been ghosted in on the left side. Permission Joseph E. Muscolino, The Muscular System Manual: The Skeletal Muscles of the Human Body, 4th ed. ( Elsevier, 2017). 5 6 7

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