Spontaneous body movements , as the word denote is that which is effected without recourse to the central nervous system as coordinated by the Brain. Since it does not involve the Brain; the Brain deads are not constrained from body, especially limb movement !!
This spontaneous Movement is not consciously effected as it is initiated independent of the Brain by electric impulses generated by chemical inequilibrium in the affected muscles!!
As these movements can't be moderated , they are of little or no use to the invalid and others!!
But what causes the chemical inbalances?? Little is known about this as at now.