In terms of control of movement, the left hemisphere controls most of the movement (and sensation) of the right half of the body whereas the right hemisphere controls most of the left side. This is because about 80% of the neurons travelling from the brain to the body cross over at the base of the brain to travel the rest of the way down in the opposite side of the spinal cord. This is called the corticospinal tract; meaning that the nerves travel from the cortex (brain) to the spinal cord in order to connect to muscles to stimulate muscle contraction. However about 20% of nerve fibres do not cross in this way and instead stay on the same side to control movement of the same side of the body. This means that your right hemisphere controls 80% of the movement of the left side of the body and about 20% of the right side. Researchers have found that that most of the body’s complicated movements such as those for fine control of the arm and hand are controlled mainly by the crossed fibres. The 20% of uncrossed fibres seem to control muscles in the core of the body; the trunk, shoulder and hip muscles. Another way to look at this is that the right side of your body is mainly controlled by the left hemisphere of your brain except around your chest, back and waist which is controlled by both hemispheres. You can imagine that the fact that the core of your body (upper back, waist and pelvis) has input from both sides of your brain may provide us with some ‘back up’ in the case of a brain injury affecting only one hemisphere.
In the case of brain injury (a stroke, an MS attack, a tumour or a trauma) part of the corticospinal tract in one hemisphere (one half) of the brain is often disrupted. This results in some degree of paralysis of the opposite side of the body. However because the trunk, waist and pelvis are controlled by both sides of the brain (as opposed to the hand which is entirely controlled by the opposite brain hemisphere), we usually see recovery in the core of the body before the limbs. After a brain injury, the person may be able to roll or sit up with help even though the limbs are still paralysed.