Issue link: https://www.massageandbodyworkdigital.com/i/943527
ON-AXIS AND OFF-AXIS ATTACHMENTS OF A MUSCLE In order to understand the concept of whether a muscle can or cannot create rotation (long axis rotation), we need to see where the muscle attaches relative to the axis of motion for rotation. Images A and B show a generic muscle that crosses a hypothetical joint. Image B shows this muscle attaches from one bone (designated as fixed) to the other bone (designated as mobile), and attaches ("on-axis") directly over the axis of motion for rotation. Therefore, when this muscle contracts and shortens, it would pull the mobile bone toward the fixed bone, but it would not create any rotation. But in Image C, the muscle is instead shown attaching "off-axis," to one side. Now, the muscle would pull the mobile bone toward the fixed bone but would also rotate the mobile bone, as represented by the arrow seen in the figure. And in Image D, the muscle is now seen attaching "off-axis" on the other side, and the arrow in the figure shows that the muscle now has the opposite rotation motion. The ability to create rotation, or any motion for that matter, is based on whether the muscle in question attaches over the axis of motion—in other words, on-axis—or whether the muscle attaches off-axis. Permission Joseph E. Muscolino, Kinesiology: The Skeletal System and Muscle Function, 3rd ed. (Elsevier, 2017). A B M P m e m b e r s e a r n F R E E C E a t w w w. a b m p . c o m / c e b y r e a d i n g M a s s a g e & B o d y w o r k m a g a z i n e 85 A B C D So, is the coracobrachialis a medial rotator, a lateral rotator, or not a rotator at all? The answer is yes, yes, and yes, depending on the position the humerus is in when the motion starts.