Parent handling of typical infants varies segmentally across development of postural control

Although trunk muscles extend across multiple vertebral joints, recent motor control studies have shown that a top-down progression of trunk control in typical infants occurs incrementally, one segment at a time, until independent sitting. The current study merges this surprising developmental pattern with parent behavior by exploring the relationship between how parents hold their infant and the segmental level for which the infant exhibits postural control. We measured trunk control of 60 infants (1-8 months) via the segmental assessment of trunk control. Spontaneous parental hold and variability was recorded during repeated sitting and standing conditions. Parent hold correlated with infant level of control in both sitting and standing, providing evidence for a positive interaction between parent behavior and segmental trunk development.

Stance width influences frontal plane balance responses to centripetal accelerations

Whenever the body is moving in a curvilinear path, inertial torques resulting from centripetal accelerations act on the body and must be counteracted to maintain stability. We tested the hypothesis that healthy subjects orient their center of mass in the position where gravitational torques offset the inertial torques due to centripetal accelerations. Ten healthy subjects stood on a platform that rotated in a circle at either a slow or fast speed, eyes open or closed, and in narrow or wide stance. Upper body, lower body, and center of mass (CoM) tilt with respect to vertical were measured and averaged across a 40 second time period of constant velocity. Body tilt was compared to the gravito-inertial acceleration (GIA) angle with respect to vertical. In all moving conditions, the upper body, lower body, and CoM tilted inward. However, this inward tilt did not reach the predicted GIA angle (CoM tilt was ~78% and 39% toward the predicted GIA angle in narrow and wide stance, respectively). Ratios of body tilt to GIA angle were minimally influenced by visual availability and magnitude of centripetal acceleration; but were largely influenced by stance width whereby narrow stance inward tilt was greater than wide stance. These results further highlight the important influence of the base of support on balance control strategies and enhance our understanding of how the balance control system compensates for inertial torques generated from centripetal accelerations.

Segmental trunk and head dynamics during frontal plane tilt stimuli in healthy sitting adults

A more detailed understanding of trunk behavior during upright sitting is needed to create a foundation to address functional posture impairments. Therefore, we characterized the dynamics of the trunk and head during perturbed sitting. A three-link inverted pendulum model of head and trunk segments was used to analyze kinematics of eight healthy sitting adults. Magnetic sensors were placed at the head and two locations of the trunk (C7 and T7). Six surface tilt stimuli (two spontaneous sway tests [no surface stimulus; eyes open, EO/eyes closed, EC] and four tests with continuous pseudorandom surface tilts [2 peak-to peak amplitudes of 2° or 8°; EO/EC]) were applied in the frontal plane. We used frequency-response functions (FRFs) to analyze sway across ~0.045-3Hz and found systematic differences in sway dynamics across segments. Superior segments exhibited larger fluctuations in gain and phase values across frequencies. FRF gains in superior segments were attenuated compared to other segments only at low frequencies but were larger at the higher frequencies. We also tested the influence of stimulus amplitude and visual availability on FRFs. Across all segments, increasing stimulus amplitude and visual availability (EO) resulted in lower gains, however, these effects were most prominent in superior segments. These changes in gain were likely influenced by changes in sensory reliance across test conditions. In conclusion, these results provide a benchmark for future comparisons to segmental responses from individuals with impaired trunk control. We suggest that a frequency-based approach provides detail needed to characterize multi-segment dynamics related to sensorimotor control.

Postural mechanisms in moderate-to-severe cerebral palsy

People with moderate-to-severe cerebral palsy (CP) have the greatest need for postural control research yet are usually excluded from research due to deficits in sitting ability. We use a support system that allows us to quantify and model postural mechanisms in nonambulatory children with CP. A continuous external bench tilt stimulus was used to evoke trunk postural responses in seven sitting children with CP (ages 2.5 to 13 yr) in several test sessions. Eight healthy adults were also included. Postural sway was analyzed with root mean square (RMS) sway and RMS sway velocity, along with frequency response functions (FRF, gain and phase) and coherence functions across two different stimulus amplitudes. A feedback model (including sensorimotor noise, passive, reflexive, and sensory integration mechanisms) was developed to hypothesize how postural control mechanisms are organized and function. Experimental results showed large RMS sway, FRF gains, and variability compared with adults. Modeling suggested that many subjects with CP adopted "simple" control with major contributions from a passive and reflexive mechanism and only a small contribution from active sensory integration. In contrast, mature trunk postural control includes major contributions from sensory integration and sensory reweighting. Relative to their body size, subjects with CP showed significantly lower damping, three to five times larger corrective torque, and much higher sensorimotor noise compared with the healthy mature system. Results are the first characterization of trunk postural responses and the first attempt at system identification in moderate-to-severe CP, an important step toward developing and evaluating more targeted interventions.NEW & NOTEWORTHY Cerebral palsy (CP) is the most common cause of motor disability in children. People with moderate-to-severe CP are typically nonambulatory and have major impairments in trunk postural control. We present the first systems identification study to investigate postural responses to external stimulus in this population and hypothesize at how the atypical postural control system functions with use of a feedback model. People with moderate-to-severe CP may use a simple control system with significant sensorimotor noise.

Influence of visual biofeedback and inherent stability on trunk postural control

BACKGROUND

For individuals who never achieve independent standing, rehabilitation is focused on trunk posture and balance control. Visual biofeedback has the potential to augment sitting balance training, however previous work in this area has been limited to standing.

RESEARCH QUESTION

To what extent do different types of visual biofeedback influence trunk sway in sitting?

METHODS

Twelve healthy young adults sat on an articulating bench. During 'sway referencing' trials, the bench tilted up and down in proportion to trunk sway in the frontal plane. This paradigm increased difficulty of the balance task and required participants to rely on visual and vestibular cues. Participants were provided different visual biofeedback through a rotating needle-gage display. Trials lasted 165 s, were ordered randomly, and included either direct feedback (needle rotated in proportion to body sway), inverted feedback (needle rotated in the opposite direction of sway), time delayed feedback (0.5 s), random feedback, eyes closed, or control (eyes open with screen off). To explore the impact of inherent stability, trials were repeated with and without external trunk support.

RESULTS

Body sway depended on feedback type. Specifically, direct and inverted feedback reduced root-mean-squared (RMS) sway the most, time delayed feedback had a smaller effect, and random visual feedback increased participants' RMS sway compared to control. Frequency domain analyses demonstrated direct and inverted visual feedback reduced sway amplitude at the lower frequencies while having minimal effect on (or increasing) sway amplitude at higher frequencies.

SIGNIFICANCE

This study extends previous work by showing that visual biofeedback can have powerful effects on sitting balance, even with external support. Results from the different types of feedback conditions further our understanding of how the brain interprets visual biofeedback. Frequency-based results were similar to previous studies using different modalities and suggest participants interpret biofeedback through sensory addition as opposed to sensory substitution.

Variation between individuals in sensorimotor feedback control of standing balance

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.