Syntax

Syntactic actants

Actant 1: whose back it is
(a) Genitive
(b) Possessive
Спина её сильно ныла. Her back ached badly.
Спина Пети была чёрная от загара. Petya's back was brown from the sun.
NB When the word спина functions as a subject and does not have a descriptive modifier, the construction спина у + Genitive is often preferred to (a) and (b), above:
Спина у неё сильно ныла. Her back ached badly.