I don't think if the arm gets tired means we are arming the ball. The arm swings the racquet as well in a FH else why is the arm finishing windshield wiper flow when the torso has finished turning? The weight behind the short should not be supplied by the arm but the whole torso swinging with the arm. Hence arm can get tired as well.
The probability of a payer arming the ball is higher with a lighter racquet than a heavier one.
Donay Gold 99 with Tourna Bighitter blue 16g.