Officer was conducting a clothing exchange in the strip frisk room on inmate during his SHU admission. After mechanical restraints were removed, inmate refused to follow several direct orders. The inmate then turned to his right and struck officer in the face with a closed fist. Inmate then stepped toward officer and the officer grabbed the inmate and lost his balance and fell to the floor with inmate on top of him. Officers responded and were able to force the inmate off of the officer and gain control of the situation.