Unless there is absolute lights out, like a decapitation, the brain will fight to survive and cause muscle movement as reflex. According to some reports of guillotine executions, there can be seemingly intentional movements for up to around 2 minutes after separation. As long as there is the ability for nerves to send signals, muscles will tense or twitch. You can remove a muscle group and it will still move until it is dead-dead. Usually the muscles will tense and then release once there is no ability for the reptile brain to try and fight.