Figure 5.11a-c. The stable equilibrium of the humerus on top of the scapula. The arm, positioned vertically above the head, appears to defy the laws of gravity much like a ball standing on the nose of a seal (a). When a small lateral force is applied to a rod standing on one of its ends on a rough horizontal plane, the rod will begin to rotate and ultimately to lie flat on the plane (b). If, on the other hand, a translational acceleration is imposed on the supporting plane in the direction of the perturbation force, the resultant moment acting on the rod would be equal to zero. The rod would have a small displacement but not a rotation in the direction of the applied perturbation force (c).
a f of one member relative to the other. If the socket was not deep enough, as in the shoulder joint (or the articulating surface had varying curvature, as in the knee joint), an eccentric load could result in the disruption of the joint. The schematic diagrams shown in Fig. 5.12b indicate how muscle-tendon systems could prevent large displacements at the joint by adjusting the tendon tension accordingly. The glenohumeral joint articulating the humerus of the upper arm with the scapula is rather shallow, and therefore an eccentric loading could lead to instability. The joint is made stable by its capsular tissue, ligaments, and musculature, which provide the scapula the necessary movements to balance forces of perturbation.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.