48.23Hz actually, because of component availability. See page 6 and 7 in the paper for details.
The right side is for demonstration purpose only. Voltage sources simulate signal and mains AC noise separately, and the switches are for selecting either, or both. Signal to noise ratio in the input is set extremely low, to demonstrate filtering efficacy, and signal frequency was chosen for visual effect.
source: https://www.researchgate.net/publication/282404009_Body_Sensor_Network_for_Mobile_Health_Monitoring_a_Diagnosis_and_Anticipating_System (Fig. 8 on page 7)
