Recent improvements in the quality of life have led to a consumer need for emotional quality. This need is regarded as extremely important, particularly for products that require a close interaction between products and users and thus that directly lead to product purchase. As a result, research on how to design user-friendly products has become an important task for corporations. Discomfort evaluation in product use has been extensively researched for this purpose. Most of the research concludes that the joint angles of the human body are the main cause of discomfort and propose a discomfort evaluation method based on joint angles. In general, when a person uses great force, they feel discomfort, and the level of discomfort varies depending on the size of the force. Accordingly, it can be inferred that the force acting on the muscle is one of the important causes of discomfort, and research on the correlation between discomfort and muscle forces is needed. In this study, the authors developed a method to evaluate discomfort during ingress into a vehicle to design a side panel for comfortable ingress into a vehicle. The correlation between the muscle forces and discomfort was investigated, and a discomfort evaluation method based on muscle forces was developed. To calculate the muscle forces during the ingress motion, an experimental mock-up of a vehicle was made, and a motion capture experiment during the ingress motion was conducted with various side panel design parameters. The biomechanical simulation tool was used to perform motion simulation based on the motion data obtained. The mathematical correlation between the calculated muscle forces and discomfort was obtained by means of fuzzy logic, and the discomfort evaluation method developed in this study was used to propose a method for designing a comfortable side panel for a vehicle.