This study explores the social and dynamic aspects of the concept "exercise identity". Previous research, mainly in psychology, has documented a link between exercise identity and exercise behaviour. However, the process of identity formation is not straightforward but rather something that can change with time, context and interaction with others. Subsequently, the present work is informed by a social constructivist approach that views exercise identity as a social product and the formation of it as a social process. Our case study of "Adrianna" examined through a biographical narrative analysis how such an identity may be constructed through interaction and over the life course. Three themes were identified; Adrianna's relationship to (1) significant others, (2) her body and (3) sociocultural norms and expectations. Reflecting this fluidity of exercise identities, we suggest the alternative concept "vulnerable exercise identity" to better understand the subtler dynamics of exercise identity formation and development. Adrianna's case is presented as a "recognizable story", representative of the struggle many people face when trying to become more physically active in contemporary western societies.