The sudden flailing arms and legs are medically known as myoclonic jerks and are characteristic of the muscle contractions which occur as you drift into a superficial level of sleep, somewhere between consciousness and deep sleep.
As said above, it is likely that the problem becomes worse at times when someone is anxious or stressed and the problem is not abnormal.
If you suffer from this (or your bed partner does!), the best suggestion is that you aid muscular relaxation by having a hot bath before bed, by winding down at the end of the day and by taking regular exercise to use up reserves of energy within the muscles themselves. A night cap may also be beneficial as alcohol in small quantities can be a muscle relaxant, and if someone is still suffering after all that, then muscle-relaxing sedatives may be used.