In this study, we propose the design of a handle mechanism as an user interface for a power-assisted cart. The proposed power-assist cart has a motor, a differential gear, and brakes. This driving structure allows the cart to assist during going straight and turning motions. In other words, the cart can turn using brakes. The user applies brakes using the handle. The proposed mechanism is referred to as the "handle-brake mechanism". Based on the handle-brake mechanism concept, a novel handle mechanism was designed. There are different parameters that influence the maneuverability of the cart during the turning motion, such as the user's operating force and the movable range of the handle. In this research we investigated the relationship between these two parameters and their influence in the cart's maneuverability. We estimated the maneuverability of the proposed handle mechanism through a subjective evaluation. The experiments measured the subjects' mental workload while operating the cart. The results provide insight about the effect of the handle's range of motion in the maneuverability.