I am assuming that the person who is autistic is physically unable to lift his or her arms, and not because he or she is unwilling to lift his or her arms.
I am also assuming that the person who is autistic was physically able to lift his or her arms before.
Also, is he or she an autistic child, teen, or adult? The cause may be different for each of them.
Has there been a physical episode perhaps recently that may have caused any head injuries?
Now, here is a question that the caretaker would not be able to know because the autistic person is unable to convey this. The question is: Does it hurt for the autistic person to lift his or her arms?
There is one way to find out, maybe. If the autistic person allows the caretaker to touch and hold him or her, then hold one of the arms and slowly lift it upwards and observe the facial expression. The facial expression would convey two emotions: If the facial expression is that of a frown and there is a violent jerking away movement, then the autistic person is resisting. Therefore it might be difficult to tell if his or her arm is hurting. If the facial expression is that of a frown without any violent jerking away movement, and the autistic person allows the caretaker to continue to lift the arm, and then suddenly the facial expression changes to an expression of agony, suffering or “pain”, and at this moment, look straight into the eyes of the autistic person, then we can assume that lifting the arm is causing pain.
In this case, has there been a physical episode perhaps recently that may have twisted the arm muscles, especially at the shoulder joints? Distortion of the arms, impact trauma of the arms and the arm joints may have caused the pain. Acupressure of the shoulder and upper arm muscles should help to ease the inability to lift the arms. The reference here is the case of the lady who could not lift her arms because of rough sex. Acupressure restored her ability to lift her arms. This is a case of a physical inability to lift the arms.