In Franklin v. Workers' Comp. Appeals Bd. (1978) 79 Cal.App.3d 224, the court discussed apportionment under Labor Code section 4663, saying: "A preexisting disability cannot be established by a 'retroactive prophylactic work restriction' on the preexisting condition placed on the injured after the subsequent industrial injury in absence of evidence to show that the worker was actually restricted in his work activity prior to the industrial injury. Where the injured was actually under a prophylactic restriction for a pre-existing condition at the time of the industrial injury, apportionment to a preexisting disability is proper. It is only the retroactive application of a prophylactic restriction to an otherwise nonexistent previous disability that is prohibited. (Ibid.) The prohibition against 'retroactive prophylactic work restrictions' to establish a preexisting disability is not inconsistent with the fact that prophylactic restrictions are ratable factors of permanent disability stemming from the industrial injury. Applying a prophylactic work restriction retroactively creates 'a sort of factual or legal fiction of an otherwise nonexistent previous disability or physical impairment.' (Ibid.)" (79 Cal.App.3d at p. 238.)