Final answer:
The hamstring muscle allows flexion of the hip joint, which is relevant when sitting in low chairs or toilet seats.
Step-by-step explanation:
The muscle being referred to in the question is the hamstring muscle. The hamstrings are a group of muscles located at the back of the thigh. They are responsible for flexing the hip joint, which means they allow you to bring your thigh closer to your chest. In this context, flexing the hip greater than 90 degrees is relevant because when sitting in low chairs or toilet seats, it requires the hamstrings to be stretched to a greater extent.