Muscular tissue is of three types. Skeletal (striated) muscle is attached to bones and works under our will, so it is responsible for voluntary movements such as moving the arm. Smooth (unstriated) muscle is found in the walls of blood vessels and internal organs and contracts involuntarily, often rhythmically. Cardiac muscle is found only in the heart and contracts rhythmically throughout life.

The patient cannot move the arm voluntarily, so the skeletal muscle (or its control) is impaired. The rhythmic contractions that continue during sleep are involuntary, and involuntary rhythmic contraction is the characteristic action of smooth muscle, which in the arm lines the walls of its blood vessels. So the involuntary smooth muscle is still working normally; only voluntary control through skeletal muscle has been lost.

A statement that skeletal muscle is working normally contradicts the loss of voluntary movement, and a statement that all muscle types are normal cannot explain it either. Calling cardiac muscle impaired is also unsupported, because cardiac muscle is not in the arm and the heart keeps working in a living patient.

Hence, Skeletal muscle is impaired, but smooth muscle is functioning normally.