This interactive fitness visualization presents a human muscle diagram derived from an SVG found on the Army Combat Fitness Test website, with the goal of tracking recently worked muscle groups. Users can select muscles to "hit" them, and those selections fade over time to reveal which areas are best targeted next. The example renders its static SVG paths directly using the `d3.select` API, while the fading effect would rely on `transition.attrTween` to manipulate opacity. The original SVG paths are embedded in the HTML, providing the anatomical base for the application.