Designing ergonomic products that accommodate diverse user populations is a significant challenge, particularly when limited to a single size and material. To address this, we developed a digital numerical model of the human arm, simulating the biomechanical response of soft tissues to mechanical interactions. This model was employed to design an armrest optimized to minimize peak contact pressure and achieve uniform pressure distribution, ensuring comfort and usability for a wide variety of users with different arm sizes, shapes, and resting forces. The digital model provides detailed insights into the interaction between the human arm and the armrest, accounting for variations in user anatomy and usage angles. Using these insights, a deformable cellular metamaterial was developed to conform to diverse arm geometries and distribute forces evenly. The iterative design process, guided by simulations, enabled the optimization of material geometry to reduce peak pressures significantly while ensuring consistent pressure profiles across the population. Results from the simulation demonstrate that the armrest designed using the digital model achieves a notable reduction in peak contact pressure, improving comfort and providing a consistent user experience across various demographics. This uniformity in mechanical response is expected to result in similar subjective comfort levels, enhancing overall satisfaction and usability. The development and application of the digital numerical arm model introduce a novel framework for designing ergonomic products. This methodology can be extended to other interface materials, such as helmets, prosthetics, orthotics, and protective gear, offering a scalable approach for improving comfort and inclusivity in product design.