Title 1 Trunk Stabilization during Sagittal Pelvic Tilt: from Trunk-on-pelvis to Trunk-in-space Due to Vestibular and Visual 2 Feedback 3 Corresponding Author *

19 The goal of this study was to investigate the human ability to stabilize the trunk in space during pelvic 20 tilt. Upper body sway was evoked in kneeling-seated healthy subjects by angular platform perturbations with a 21 rotation around a virtual low-back pivot point between the L4 and L5 vertebrae. To investigate motor control 22 modulation, variations in task instruction (balance naturally or minimize trunk sway), vision (eyes open or 23 closed) and perturbation bandwidth (from 0.2 up to 1, 3 or 10Hz) were applied. Co-contraction and 24 proprioceptive muscle spindle feedback were associated with minimizing low-back flexion/extension (trunk-on25 pelvis stabilization), while vestibular and visual feedback were supposed to contribute to trunk-in-space 26 stabilization. 27 Trunk-in-space stabilization was only observed with the minimize trunk sway task instruction, while the 28 task instruction to balance naturally led to trunk-on-pelvis stabilization with trunk rotations even exceeding the 29 perturbations. This indicates that vestibular feedback is used when minimizing trunk sway, but has only a minor 30 contribution during natural trunk stabilization in the sagittal plane. The eyes open condition resulted in 31 reduced global trunk rotations and increased global trunk reflexive responses, demonstrating effective visual 32 contributions to trunk-in-space stabilization. On the other hand, increasing perturbation bandwidth caused a 33 decreased feedback contribution leading to deteriorated trunk-in-space stabilization. 34