Final answer:
The b. sartorius muscle is the one involved in crossing one leg over the other while sitting, as it enables leg flexion at the knee and lateral rotation at the hip.
Step-by-step explanation:
The muscle involved in crossing one leg over the other while in a sitting position is the b. sartorius.
This versatile muscle originates at the anterior superior iliac spine on the pelvic girdle, and it inserts on the medial surface of the proximal tibia.
The sartorius muscle flexes the leg at the knee and also flexes, abducts, and laterally rotates the leg at the hip, which facilitates the action of sitting cross-legged.
Other muscles in the vicinity such as the hamstrings (including the biceps femoris, semitendinosus, and semimembranosus) and the quadriceps femoris (comprising the rectus femoris, vastus lateralis, vastus medius, and vastus intermedius) have different primary functions.
The hamstrings are primarily involved in flexing the knee, while the quadriceps extend the knee. Despite being large muscle groups in the legs, they are not the main muscles used for the action of crossing one's legs.