Effective arm movement and control is only possible when core (trunk) strength and stability are present.  A stable core provides a solid base of support from which the arms and legs are free to move with precision and control.  Core muscles are those muscles that stabilize 
the shoulder girdle, spine and pelvis.

Postural control refers to the ability to maintain an upright trunk position. Without good postural control, a child may have difficulty maintaining an upright sitting posture and may fatigue easily when sitting at a desk in school. For the child’s hands to work well, strength, stability and mobility are necessary in the shoulders and forearms. In addition to the shoulders being strong, wrist stability is an important component for having precise finger control.