(1) In cases of permanent disability less than total, if the degree or duration of disability resulting from an industrial injury or occupational disease is increased or prolonged because of a preexisting physical impairment, the employer shall be liable only for the additional disability from the industrial injury or occupational disease.
(2) Any income benefits previously paid an injured workman for permanent disability to any member or part of his body shall be deducted from the amount of income benefits provided for the permanent disability to the same member or part of his body caused by a change in his physical condition or by a subsequent injury or occupational disease.