40 ILCS 5/7-149

The Board shall conduct periodic checks to determine if any participating employee is disabled. Such checks may consist of periodic examinations by a physician or physicians appointed by the Board, requiring the employee to submit evidence of continuing disability and such other investigations as the Board may deem appropriate. The following shall constitute prima-facie evidence of termination of temporary disability: (a) A written report by a physician appointed by the Board stating that the temporary disability has ceased; (b) The earning of compensation by the employee from any source for personal services, in excess of 25% of the monthly rate of earnings upon which his disability benefits are based.