What joint angle is regarded as 0 degree position when measuring with a goniometer?

When using a goniometer to measure joint angles, the 0-degree position is generally defined as the anatomical position of the body, which is standing upright with feet together, arms at the sides, and palms facing forward. In this context, hip adduction is the correct answer because at 0 degrees, the hip is aligned in the midline of the body, where the legs are straight and in a neutral position without any movement toward or away from the midline.

Hip adduction is specifically defined when the leg moves towards the body's midline from this neutral position. By contrast, the other options represent positions or actions that are away from the neutral position; for instance, elbow flexion refers to bending the arm at the elbow, knee extension straightens the knee, and shoulder flexion raises the arm in front of the body. These movements occur from the 0-degree starting point but do not represent the neutral anatomical position themselves. Hence, the 0-degree position is best represented by hip adduction, as it's the movement that returns to the neutral anatomical position.