I was dismayed that my NDT certification course did not teach me hand function comes from my gut. I learned this when a PT had me do abdominal exercises after my back spasms returned.
I was surprised to feel abdominal muscles on my paralyzed side burning from effort. My ability to reach improved after my abdominals got stronger.
When you reach for an object (shoulder flexion) the shoulder blade (scapula) helps the arm by sliding forward. Scapular muscles are attached to the ribs so if weak abdominal muscles do not keep the ribs still, the scapular muscles have a wobbly home base. Having weak stomach muscles meant my shoulder and elbow muscles were overworked when scapular muscles could not do their job. I repeatedly got tendonitis in the long head of the biceps because this elbow muscle assisted weak shoulder muscles. My biceps tendon used to swell so badly that it made a popping sound when it slipped out of its groove. Exercise sessions had to end with ice to reduce the swelling.
I wonder how much faster I would have progressed if my OTs had worked on abdominal strength. I do not blame my OTs. As an NDT certified OT I do not know of a motor theory that links hand function to core muscles. However, athletes like weight lifters and javelin throwers who need superior arm strength know how important abdominal strength is to upper extremity performance. Stroke survivors exert this same kind of extreme effort when they lift their arm with paralyzed muscles. Don't believe me? Therapists use two hands to lift a paralyzed arm. Even they are challenged when lifting all that dead weight.