The rectus abdominis is a long, flat muscle that runs vertically along the front of the abdomen. It is responsible for compressing the abdomen, which helps in activities like coughing, sneezing, and defecation. Additionally, the rectus abdominis is involved in flexing the trunk, which means it helps in bending the torso forward. However, it does not have any role in compressing the heart or flexing the bicep.