Syntax

Syntactic actants

Actant 1: whose fist it is
(a) Genitive
(b) Possessive
Она с ужасом смотрела на огромные кулаки Сергея. Horrified, she stared at Sergey's enormous fists.
Его кулаки были крепко сжаты. His fists were tightly clenched.
NB When the word кулак functions as a subject and does not have a descriptive modifier, the construction кулак у + Genitive is often preferred to (a) and (b), above:
Кулаки у него были крепко сжаты. His fists were tightly clenched.

Restrictive modifiers

(a) adjectival modifier
левый/правый кулак the left/right fist
У него был сжат правый кулак. His right fist was clenched.