The upper part of the trapezius muscle is the primary target of the shoulder shrug. The trapezius is a triangular muscle that makes up the upper back, running from the back of the neck and down along the spine, lying over the shoulder blades.
In general terms, sex refers to the biological differences between males and females, such as the genitalia and genetic differences. Gender is more difficult to define but can refer to the role of a male or female in society (gender role), or an individual's concept of themselves (gender identity).
However, gender is much more than that. Gender is a social category. It contains the roles, dress, behavior, and expression expected of a person based on an arbitrary category given to them at birth. For example, the idea that women are nurturing and compassionate, whereas men are stoic and unemotional.