Populations with moderate-to-severe motor control impairments often exhibit degraded trunk control and/or lack the ability to sit unassisted. These populations need more research, yet their underdeveloped trunk control complicates identification of neural mechanisms behind their movements. The purpose of this study was to overcome this barrier by developing the first multi-articulated trunk support system to identify visual, vestibular, and proprioception contributions to posture in populations lacking independent sitting. The system provided external stability at a user-specific level on the trunk, so that body segments above the level of support required active posture control. The system included a tilting surface (controlled via servomotor) as a stimulus to investigate sensory contributions to postural responses. Frequency response and coherence functions between the surface tilt and trunk support were used to characterize system dynamics and indicated that surface tilts were accurately transmitted up to 5Hz. Feasibility of collecting kinematic data in participants lacking independent sitting was demonstrated in two populations: two typically developing infants, ~2-8 months, in a longitudinal study (8 sessions each) and four children with moderate-to-severe cerebral palsy (GMFCS III-V). Adaptability in the system was assessed by testing 16 adults (ages 18-63). Kinematic responses to continuous pseudorandom surface tilts were evaluated across 0.046–2Hz and qualitative feedback indicated that the trunk support and stimulus were comfortable for all subjects. Concepts underlying the system enable both research for, and rehabilitation in, populations lacking independent sitting.
We examined inter-subject variation in human balance, focusing on sensorimotor feedback. Our central hypothesis was that inter-subject variation in balance characteristics arises from differences in central sensorimotor processing. Our second hypothesis was that similar sensorimotor feedback mechanisms are used for sagittal and frontal balance. Twenty-one adults stood on a continuously rotating platform with eyes-closed in the sagittal or frontal plane. Plant dynamics (mass, height, inertia) and feedback control were included in a model of sensory weight, neural time delays, and sensory-to-motor scaling (stiffness, damping, and integral gains). Sway metrics (root-mean-square (RMS) sway and velocity) were moderately correlated between planes of motion (RMS: R=0.66-0.69 and RMS velocity: R=0.53-0.58). Sensory weight and integral gain exhibited the highest correlations between plane of motion (R=0.59 for sensory weight and R=0.75 for integral gain during large stimuli). Compared to other subjects, people who adopted a high vestibular weight or large integral gain in one condition did so across all tests. Inter-subject variation in sensory weight, stiffness, and integral gain were significantly associated with inter-subject variation in RMS sway while sensory weight and time delay were the strongest significant predictors of RMS velocity. A multiple linear regression showed that inter-subject variation in sway metrics were predicted better by inter-subject variation in central feedback mechanisms vs. plant dynamics. Together, results supported the first hypothesis and partially supported the second hypothesis because only a subset of feedback processes were moderately or strongly correlated (mostly during large surface tilts) between planes of motion.
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