Noticed that too. Maybe lack of circulation and cramping from being bound? Or possibly they have been trained to sit very still and watch with eyes wide open during therapy so clenching their hands is one of the few signs of stress they show?
I also wonder if their eyebrows fell off because of the hormones they give to delay puberty for the child sex slaves.
Or did they fall off as a result of the adrenochrome released during the torture?