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).
1 Gender refers to society's expectations about how we should think and act as girls and boys, and women and men. It is our biological, social, and legal status as women and men.  Gender identity is how we feel about and express our gender and gender roles â clothing, behavior, and personal appearance.