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I t p a y s t o b e A B M P C e r t i f i e d : w w w. a b m p . c o m / g o / c e r t i f i e d c e n t r a l 91 17 A chin-up switches the action of the brachialis; now the proximal end (P) moves toward its distal end (D). STORY 4 REVERSE ACTIONS A muscle (via its tendons) can pull on any bone it attaches to. This means that your deltoid can technically tug the scapula and clavicle just as much as it can swing the humerus. In truth, that doesn't happen, as each muscle demonstrates a biomechanical preference to pull one bone more than another. Thus, a muscle's origin is the attachment to the more stationary bone, while its insertion is the connection to the more mobile bone. For example, the majority of your brachialis contractions pull the distal forearm (the insertion) toward the proximal humerus (its origin). This is logical as the humerus attaches to your stable torso, while your forearm and hand are freer to move (Image 16). This disposition can switch, however, with the origin and insertion swapping roles to produce a reverse muscle action. In the case of your brachialis, this is easily seen when you grab a bar to do a chin-up (Image 17). Now, the insertion is stationary and the humerus (the origin) must move toward the hand. Please note that the same action—fl exion of the elbow—occurs, but now with the (typically stable) origin moving toward the (usually mobile) insertion. BLOOD, MUSCLES, AND WASTE Myofascial units (a muscle belly and its fascial elements) are outfi tted with an ample blood supply. Yet, all of those tiny blood vessels are useless if the belly is pinched off. Here's the story. A muscle serving as a neutralizer, fi xator, or longstanding postural unit spends a great deal of time isometrically contracted. Even when you are relaxed, its tone can be excessive. This chronic holding pattern not only prevents arteries from delivering fresh blood, but also restricts veins and lymph vessels (which depend on the pumping action of muscle contractions) from fulfi lling their tasks. Soon, a vicious cycle ensues: built- up waste material begins to irritate the nerves. This further tightens the muscles, exacerbating the ischemia (loss of blood supply). Finally, your client lies down on the table where you discover that certain tissues feel like angry harp strings. D P origin moving toward the (usually mobile) insertion. D P 1 2 P D 3 D P 16 Raising an entire pineapple to your mouth (as you do!) requires the brachialis to pull its distal end (D) toward its proximal end (P).