The ability to perform sit-to-stand (STS) behaviours is fundamental for humans and other animals, yet the underlying high-level control goal driving the selection of a specific movement pattern for the STS transition remains elusive. Exploring the STS transition in non-human animals, particularly birds, offers an opportunity to deepen this understanding and allows for an independent assessment of how body sizes influence performance criteria. Computational biomechanical modelling and simulation, combined with optimal control methods, provide a rigorous and mechanistic approach to address these questions, offering insights that experimental limitations may hinder. Drawing inspiration from human studies, we systematically evaluate STS performance criteria (e.g., minimisation of effort and the muscle force rate) and examine how these criteria vary with body sizes, using a simplified two-link model to simulate vertical movements. We use this simplified simulation to inform fully predictive, three-dimensional, muscle-driven simulations of the STS transition in the emu (Dromaius novaehollandiae), a large bipedal bird. This study, focusing on emus, provides insights into avian species’ control strategies during the STS transition, with future investigations extending to simulate this transition in other bird species, contributing not only to our understanding of organismal morphofunctional specialisation but also to applications in robotics, prosthetics, and animal welfare, as well as aiding in the reconstruction of locomotion in extinct species.