Final answer:
The nurse should instruct the UAP to help Mr. Mathis move his lower legs up and back toward the buttocks and assist in moving his thigh diagonally upward and outward, in line with knee and thigh exercises, while ensuring his comfort and safety.
Step-by-step explanation:
The instruction that the nurse should give to the unlicensed assistive personnel (UAP) for positioning Mr. Mathis' legs is closely related to demonstrating the anatomical position, which is essential in the medical field for standardizing how the body is described no matter the orientation of the individual. The anatomical position is a reference point and is described as a person standing erect, facing forward, with feet together and parallel, and arms at the sides with palms facing forward. However, Mr. Mathis needs a different position, specifically to move the back of his lower legs up and back toward the buttocks as when kneeling, which helps in moving the thigh diagonally upward and outward. Thus, the UAP should be instructed to help Mr. Mathis into a position that facilitates this movement, while ensuring comfort and safety.