
The shape of your spine has important consequences for the position of your shoulder and therefore your hands. The natural spinal curve allows the scapula to float around the posterior rib cage, contributing to the arm's significant range of movement. As soon as the upper body bends forward however the scapula can no longer float as freely and the shoulder needs to compensate.
Muscles in the back, the abdomen, the legs and the pelvis are responsible for movement and for the maintenance of the spinal curve. Strength in the hip flexors and your abdominal muscles maintain a stable lumbar curve while muscles in the pelvis and the legs ensure a stable base. Muscles in the spine vary from small structures spanning individual vertebrae to large, superficial muscles that control the movement of the spine as a whole.
The spine, apart from helping us to stand and walk upright, also contains the spinal cord. This important role will take precedence over anything else that you ask it do. Any strain on the spine that risks the integrity of the spinal curve will quickly be dealt with by muscles contracting to protect vulnerable areas, restrictions in movement, and subsequent adaptations in other parts of the body.