A4 waist refers to the waist width narrower than vertical A4 paper. A4 paper is defined by ISO 2 16, and its specification is 21* 29.7 cm (210 mm× 297 mm). So as long as the waist width is less than 2 1cm(2 10mm), it can be called A4 waist.
social context
The trend of "showing off your figure" in social networks has virtually become the driving force for many people to keep fit. On the one hand, people who have no conditions to show off will inevitably be impatient when they see the good figure displayed by others in the circle of friends, and may "abuse" themselves in the gym; On the other hand, people who already have a good figure will inevitably continue to sweat in order to continue to show off their figure in the circle of friends and maintain the "leading" advantage.