An example rotational joint assembly for a robotic medical system, the rotational joint assembly comprising at least one arm segment and a rotational joint provided at one end of the arm segment. The rotational joint is to allow the arm segment to rotate about a rotational axis. The rotational joint comprising a brake to lock rotation of the arm segment at the rotational joint and an actuator to selectively engage or disengage the brake. The actuator comprising a cam having two stable regions separated by two transition regions, the two stable regions comprising a first stable region corresponding to engagement of the brake and a second stable region corresponding to disengagement of the brake.