Previous research on human balance recovery suggests that, prior to an externally triggered postural perturbation, healthy subjects can pre-select their postural response based on the environmental context, but it is unclear whether this pre-selection includes the selection of a stepping leg when performing compensatory steps. We sought to determine how pre-selecting a stepping limb affects the compensatory steps and stability of young, healthy subjects when responding to postural perturbations. Nine healthy subjects (24-37 years of age) stepped in response to backward translations of a platform under their feet when, prior to the perturbations, the subjects either knew whether they were to step with their left or right leg to a visual target (the Predictable condition) or did not know whether to step with their left or right leg until one of two targets appeared at perturbation onset (the Unpredictable condition). The Unpredictable condition also included randomly inserted trials of toes-up rotations and backward translations without targets (catch trials). The results showed that, in the Predictable condition, the subjects consistently exhibited one anticipatory postural adjustment (APA; a lateral weight shift toward the stance limb) before stepping accurately to the target with the correct leg. In the Unpredictable condition, the subjects either (1) exhibited multiple APAs, late step onsets, and forward center-of-mass (CoM) displacements that were farther beyond their base of support, or (2) exhibited an early step with only one APA and kept their CoM closer to the base of support, but also stepped more often with the incorrect leg. Thus, when the subjects had to select a stepping leg at perturbation onset, they either became more unstable and used multiple APAs to delay stepping in order to provide enough time to select the correct stepping leg, or they stepped earlier to remain stable but often stepped with the incorrect leg. In addition, responses to catch trials in the Unpredictable condition included distorted step placements that resembled steps to anticipated targets, despite allowing the subjects to step with a leg of their choice and to a location of their choice. Lastly, the subjects' voluntary stepping latencies to visual targets presented without perturbations were twice as long as their stepping latencies to the backward platform translations. Therefore, healthy subjects appear to pre-select their stepping limb, even when the perturbation characteristics are unpredictable, because relying on visual input provided at perturbation onset requires a delayed response that leads to greater instability.
"Besides the aforementioned haptic feedback , the moving floor may also serve as a tool for generating platform perturbations. The platform perturbations elicit postural responses, actions that activate postural mechanisms  to prevent a fall and helps the subject to maintain upright posture without lifting or moving the feet. The postural mechanisms comprise ankle and hip strategies or combination of both, depending on the perturbation direction and strength. "
[Show abstract][Hide abstract] ABSTRACT: Balance and postural response strategies change when subjects are exposed to horizontal translations of the floor or virtual reality or both. This may impact the balance training strategy and balance capabilities assessment in the future telerehabilitation. In the study 15 neurologically intact volunteers participated. Balance standing frame with virtual reality tasks and our novel haptic floor able to generate horizontal translations were used. The postural responses were measured with center of gravity and muscle electromyography of plantardorsiflexors, quadriceps, hamstrings, hip and spine muscles in three scenarios. The results demonstrated that center of gravity and electromyographic activity were comparable; with low latency at translation only, longer latency at combination with virtual reality and long latency when only virtual reality was applied. Soleus and semimembranousis demonstrated lower latency at back-right horizontal translations when virtual reality was present. The outcomes suggests that the postural strategy changes from ankle to ankle-hip strategy with availability of additional sensory systems which may be an important issue for objective balance evaluation in the clinical environment and remote telerehabilitation.
IEEE transactions on neural systems and rehabilitation engineering: a publication of the IEEE Engineering in Medicine and Biology Society 04/2014; 22(4). DOI:10.1109/TNSRE.2014.2320320 · 3.19 Impact Factor
"The moments of relative quiescence on the PPN would therefore, allow increased signaling from the cholinergic cells within the PPN, which would still remain active via its connections with sensory signals via the dorsal root of the spinal cord (Pahapill and Lozano, 2000) (see Figure 2). This mechanism also provides a potential explanation for the relationship between the freezing phenomenon and impaired postural responses during tests of balance function (Jacobs and Horak, 2007). "
[Show abstract][Hide abstract] ABSTRACT: Freezing of gait (FOG) is a disabling symptom of advanced Parkinson's disease (PD) that leads to an increased risk of falls and nursing home placement. Interestingly, multiple lines of evidence suggest that the manifestation of FOG is related to specific deficits in cognition, such as set shifting and the ability to process conflict-related signals. These findings are consistent with the specific patterns of abnormal cortical processing seen during functional neuroimaging experiments of FOG, implicating increased neural activation within cortical structures underlying cognition, such as the Cognitive Control Network. In addition, these studies show that freezing episodes are associated with abnormalities in the BOLD response within key structures of the basal ganglia, such as the striatum and the subthalamic nucleus. In this article, we discuss the implications of these findings on current models of freezing behavior and propose an updated model of basal ganglia impairment during FOG episodes that integrates the neural substrates of freezing from the cortex and the basal ganglia to the cognitive dysfunctions inherent in the condition.
Frontiers in Systems Neuroscience 10/2013; 7:61. DOI:10.3389/fnsys.2013.00061
"In fact, we have hypothesized that freezing of gait is because of impaired coupling between postural and locomotor components of locomotion , specifically due to an inability to inhibit postural preparation and initiate stepping (Nutt et al, 2011; Cohen et al., 2013). Normal stepping, whether a voluntary step or a compensatory postural correction, is associated with a single anticipatory postural adjustment that shifts weight off the stepping leg (Jacobs and Horak, 2007b). In contrast, patients with Parkinson's disease who have freezing of gait show delayed step initiation associated with repetitive anticipatory postural adjustments as if they cannot inhibit their postural preparation and release the stepping programme (Jacobs and Horak, 2007b; Jacobs et al., 2009). "
[Show abstract][Hide abstract] ABSTRACT: Freezing of gait is one of the most debilitating symptoms in Parkinson's disease as it causes falls and reduces mobility and quality of life. The pedunculopontine nucleus is one of the major nuclei of the mesencephalic locomotor region and has neurons related to anticipatory postural adjustments preceding step initiation as well as to the step itself, thus it may be critical for coupling posture and gait to avoid freezing. Because freezing of gait and postural impairments have been related to frontal lesions and frontal dysfunction such as executive function, we hypothesized that freezing is associated with disrupted connectivity between midbrain locomotor regions and medial frontal cortex. We used diffusion tensor imaging to quantify structural connectivity of the pedunculopontine nucleus in patients with Parkinson's disease with freezing of gait, without freezing, and healthy age-matched controls. We also included behavioural tasks to gauge severity of freezing of gait, quantify gait metrics, and assess executive cognitive functions to determine whether between-group differences in executive dysfunction were related to pedunculopontine nucleus structural network connectivity. Using seed regions from the pedunculopontine nucleus, we were able to delineate white matter connections between the spinal cord, cerebellum, pedunculopontine nucleus, subcortical and frontal/prefrontal cortical regions. The current study is the first to demonstrate differences in structural connectivity of the identified locomotor pathway in patients with freezing of gait. We report reduced connectivity of the pedunculopontine nucleus with the cerebellum, thalamus and multiple regions of the frontal cortex. Moreover, these structural differences were observed solely in the right hemisphere of patients with freezing of gait. Finally, we show that the more left hemisphere-lateralized the pedunculopontine nucleus tract volume, the poorer the performance on cognitive tasks requiring the initiation of appropriate actions and/or the inhibition of inappropriate actions, specifically within patients with freezing. These results support the notion that freezing of gait is strongly related to structural deficits in the right hemisphere's locomotor network involving prefrontal cortical areas involved in executive inhibition function.
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