We have developed a novel cable robotic device, Trunk Support Trainer (TruST), which allows training of seated posture at and beyond an individual’s point of stability failure during volitional trunk displacement. This is accomplished by creating a force tunnel to support posture beyond the sitting stability region. The system provides an assist-as-needed force strategy to support the trunk. Our study supports the hypothesis that a single training session with TruST at and beyond the point of stability failure, increases lower trunk COM displacement and increases the lower trunk and pelvic rotation. The findings suggest that this approach may be useful for training patients with neurological and musculoskeletal disorders where children or adults have compromised postural stability, as seen in Cerebral Palsy.