a. In the case of income received from separate property during the marriage, such income shall be marital property only to the extent it is attributable to the personal efforts of either party. In the case of the increase in value of separate property during the marriage, such increase in value shall be marital property only to the extent that marital property or the personal efforts of either party have contributed to such increases, provided that any such personal efforts must be significant and result in substantial appreciation of the separate property.
For purposes of this subdivision, the nonowning spouse shall bear the burden of proving that (i) contributions of marital property or personal effort were made and (ii) the separate property increased in value. Once this burden of proof is met, the owning spouse shall bear the burden of proving that the increase in value or some portion thereof was not caused by contributions of marital property or personal effort.
"Personal effort" of a party shall be deemed to be labor, effort, inventiveness, physical or intellectual skill, creativity, or managerial, promotional or marketing activity applied directly to the separate property of either party.