Background:The precise in vivo motion of vertebrae and the way spinal segments contribute to overall trunk kinematics remain largely uncertain. This lack of direct measurements has led to a variety of simplifications and assumptions within musculoskeletal modelling, where the spine is often represented as a single rigid segment or with a fixed partition of motion across levels. Such modelling choices can significantly influence the predicted joint reaction forces and muscle activations, especially during daily activities involving complex trunk and pelvic motion, such as walking and sit-to-stand transitions. Investigating how different assumptions about spinal kinematics affect the mechanical outcome of the model is therefore essential to improve the physiological realism of computational simulations.