"1." Your back muscles are very clear and look strong. "
2. "Your back muscles are smooth and look harmonious."
"Your back muscles are very beautiful and look very healthy."
"Your back muscles are strong and you look confident."
5. "Your back is straight and looks elegant."
In short, when praising a person's back, we should emphasize his muscle lines, strength and upright posture, which are the key characteristics of fitness people and the result of their efforts.