Kinect, which is useful as a multimodal input device, offers multiple features such as voice recognition, facial tracking, object recognition, and gesture recognition. Games with such human-centred, interactive interfaces create an environment that bridges between artificial and natural cognitive capabilities, enabling them to work together more efficiently. This study specifically examines a usability evaluation method that uses Kansei engineering knowledge to identify and remedy difficulties related to user interface development. As a practical application of the suggested evaluation method, we used a particular serious game, “The Glider,” a prototype developed by Serious Games Finland. It exploits Kinect as its game controller. Serious games are usu- ally aimed at promoting specific effects that can be achieved through playing the game. However, such effects can also present tradeoffs in terms of usability. Controlling motions in “The Glider” has been developed in close cooperation with medical doctors, physiotherapists, and patients in terms of well-being and light exercise. However, modulating the difficulty balance and usability of the game is also a very important aspect of game development. As described in this paper, our goal is to evaluate the control motion of the game from a usability perspective. Evaluation consists of state and appearance observations conducted with 12 Japanese subjects. Observed data were tagged into several categories to quantize their behaviours and speeches, and were analysed using both cognitive bias evaluation and statistical evalu- ation. Results show that difficulties for subjects in control motions of the game were curved out, which is a key to balancing usability and difficulty while maintaining the genuine purpose of the serious game.