Effect of spinal cord injury on neural encoding of spontaneous postural perturbations in the hindlimb sensorimotor cortex

Supraspinal signals play a significant role in compensatory responses to postural perturbations. While the cortex is not necessary for basic postural tasks in intact animals, its role in responding to unexpected postural perturbations after spinal cord injury (SCI) has not been studied. To better understand how SCI impacts cortical encoding of postural perturbations, the activity of single neurons in the hindlimb sensorimotor cortex (HLSMC) was recorded in the rat during unexpected tilts before and after a complete midthoracic spinal transection. In a subset of animals, limb ground reaction forces were also collected. HLSMC activity was strongly modulated in response to different tilt profiles. As the velocity of the tilt increased, more information was conveyed by the HLSMC neurons about the perturbation due to increases in both the number of recruited neurons and the magnitude of their responses. SCI led to attenuated and delayed hindlimb ground reaction forces. However, HLSMC neurons remained responsive to tilts after injury but with increased latencies and decreased tuning to slower tilts. Information conveyed by cortical neurons about the tilts was therefore reduced after SCI, requiring more cells to convey the same amount of information as before the transection. Given that reorganization of the hindlimb sensorimotor cortex in response to therapy after complete mid-thoracic SCI is necessary for behavioral recovery, this sustained encoding of information after SCI could be a substrate for the reorganization that uses sensory information from above the lesion to control trunk muscles that permit weight-supported stepping and postural control.

Integration of vestibular and hindlimb inputs by vestibular nucleus neurons: multisensory influences on postural control

Vestibular nucleus neurons receive convergent information from hindlimb somatosensory inputs and vestibular inputs. In this study, extracellular single-unit recordings of vestibular nucleus neurons during conditions of passively applied limb movement, passive whole body rotations, and combined stimulation were well fit by an additive model. The integration of hindlimb somatosensory inputs with vestibular inputs at the first stage of vestibular processing suggests that vestibular nucleus neurons account for limb position in determining vestibulospinal responses to postural perturbations.

Promoting Generalized Learning in Balance Recovery Interventions

Recent studies have shown balance recovery can be enhanced via task-specific training, referred to as perturbation-based balance training (PBT). These interventions rely on principles of motor learning where repeated exposure to task-relevant postural perturbations results in more effective compensatory balance responses. Evidence indicates that compensatory responses trained using PBT can be retained for many months and can lead to a reduction in falls in community-dwelling older adults. A notable shortcoming with PBT is that it does not transfer well to similar but contextually different scenarios (e.g., falling sideways versus a forward trip). Given that it is not feasible to train all conditions in which someone could fall, this limited transfer presents a conundrum; namely, how do we best use PBT to appropriately equip people to deal with the enormous variety of fall-inducing scenarios encountered in daily life? In this perspective article, we draw from fields of research that explore how general learning can be promoted. From this, we propose a series of methods, gleaned from parallel streams of research, to inform and hopefully optimize this emerging field where people receive training to specifically improve their balance reactions.

Differential Effects of Perturbation Magnitude on Reactive Balance Control in Young Sedentary Adults

This study investigates postural responses to unexpected perturbations induced by a load release of different weights. Groups of 26 men (age 22.6 ± 2.4 years, height 178.0 ± 9.1 cm, and body mass 86.9 ± 11.5 kg) and 21 women (age 21.9 ± 2.7 years, height 168.8 ± 6.8 cm, and body mass 65.3 ± 8.7 kg) underwent load-triggered postural perturbations by 1 and 2 kg while standing on a force plate with either eyes open or eyes closed. Postural perturbations induced by a heavier load, representing about 2% and 3% of body weight in men and women, respectively, led to significantly higher peak anterior and peak posterior center of pressure displacements when compared with a lighter load (29.6% and 45.4%, respectively) both with eyes open (36.9%) and closed (42.1%). Their values were significantly lower in men than women only when a higher load was used (∼25%). However, there were no significant differences in time to peak anterior and posterior center of pressure displacements. These findings indicate that heavier load-induced postural perturbations are greater in women than men regardless of visual conditions. This underlines the importance of loading dose in the magnitude of postural responses to externally induced perturbations.

Is it me or the room moving? Recreating the classical “moving room” experiment with virtual reality for postural control adaptation

Postural control is a complex process requiring both sensory and motor responses. Perturbation-based balance training has emerged as an effective fall prevention intervention, which provides physical postural perturbations for postural control training and adaptation. With the advent of technology, virtual reality (VR) has also been used for fall prevention training by providing visual postural perturbations. This article addresses such VR studies, including a recent experiment which involved recreating the classical “moving room” paradigm into a “virtual moving room-wall paradigm” to assess the impact of VR-induced visual postural perturbations on postural stability and control. Evidence of both compensatory and anticipatory postural responses during unexpected and expected visual postural perturbations is presented. The future scope, required virtual environment set-up variations, limitations, and significance of a “virtual moving wall” paradigm in the learning and adaptation of postural control behavior are also discussed.

Impaired Scaling of Step Length in Parkinsonian Postural Instability

Background: Postural stepping is an important strategy for recovery of balance in response to postural perturbations. It is disrupted by Parkinson's disease (PD) and other conditions. The nature of this disruption remains poorly understood. Understanding the motor control nature of this impairment can guide the development of novel interventions. Objectives: To identify the motor control abnormalities responsible for parkinsonian impairment of postural stepping. Methods: We studied four groups of participants: control, aged, PD, and normal-pressure hydrocephalus (NPH). We performed kinematic analysis of postural stepping by recording participants' body motion during a modified version of the clinical pull test, which was performed multiple times with different amounts of pulling forcefulness. Results: Successful postural stepping in the control group was accompanied by linear scaling of their first step's length and latency to the body's initial motion: more forceful pulls caused larger initial body acceleration, which resulted in longer steps that began earlier. PD patients exhibited reduced scaling of step length: they maintained normal reaction time but took steps that were inadequately short. Reduced step length scaling was present, but less severe, in aged individuals, and was more severe in NPH patients. Aged individuals and PD patients exhibited partial compensation for reduced step length scaling: their step length included a component that was independent of initial body acceleration, which was absent in control and NPH groups. Conclusions: the impairment of postural stepping caused by PD and related conditions is due to inadequate scaling of movement amplitude and is thus a form of hypokinesia.

Impact of pain on reactive balance and falls in community-dwelling older adults: a prospective cohort study

Background pain is associated with increased postural sway and falls in older adults. However, the impact of pain on reactive balance induced by postural perturbations and how this might predispose older adults to falls is not known. Objective to investigate whether any pain, back/neck pain and lower limb pain are associated with poor reactive balance and prospective fall outcomes in older adults. Design 12-month prospective cohort study. Setting community. Subjects 242 community-dwelling older adults aged 70+ years. Methods participants completed a questionnaire on the presence of pain and underwent force-controlled waist-pull postural perturbations while standing. Force thresholds for stepping, step initiation time, step velocity and step length were quantified. Falls were monitored with monthly falls calendars for 12-months. Results participants with lower limb pain had significantly lower force thresholds for stepping. Those with any pain or pain in the back/neck had longer step initiation time, slower step velocity and shorter step length. The three pain measures (any pain, back/neck pain, lower limb pain) were significantly associated with multiple falls when adjusted for age, sex, body mass index, use of polypharmacy, strength and walking speed. In mediation analyses, there was a significant indirect effect of reactive balance for the relationship between back/neck pain and falls with fractures. Conclusions older people with pain have impaired reactive balance and an increased risk of falls. Reactive balance partially mediated the association between pain and fall-related fractures. Further research is required to confirm the findings of this study.