The part of your back that you can't reach is called the "acnestis."

💪A healthy young person with no signs of osteochondrosis can reach any point on their back. In fact, the severity of osteochondrosis is determined by the patient’s ability to bring their arms behind their back and interlock their fingers, either horizontally or across the shoulder with one hand. A healthy person can perform these interlocking gestures, but in the early stages of osteochondrosis, this becomes difficult. It is believed that the word “acnestis,” when translated from Ancient Greek, referred to a cheese grater, but this is not certain...