The shoulder is the most mobile joint in the body. It enjoys an amazing range of motion; it can rotate 360 degrees and can extend upward, sideways, across the body, outward, inward, every which way. That makes it the most useful joint we have, and, not surprisingly, we use it the most. Lifting, pushing, pulling, throwing, catching, hugging: the shoulder experiences more motion than any other joint. So it is perhaps not surprising that, sooner or later, it becomes overtaxed fatigued. After all, as with anything, if you keep applying the same pressure over and over, the strength and stability...