Raymond Chong

Control of reactive balance adjustments in perturbed human walking: roles of proximal and distal postural muscle activity

Abstract Studies on the proactive control of gait have shown that proximal (hip/trunk) muscles are the primary contributors to balance control, while studies on reactive balance control during perturbed gait, examining only activity in distal (leg/thigh) muscles, have shown that these muscles are important in compensating for a gait disturbance. This study tested the hypothesis that proximal muscles are also primary contributors to reactive balance control during perturbed gait. Thirty-three young adults participated in a study in which an anterior slip was simulated at heel strike by the forward displacement of a force plate on which they walked. Surface electromyographic data were recorded from bilateral leg, thigh, hip and trunk muscles. Kinematic data were collected on joint angle changes in response to the perturbation. The results did not support the hypothesis that the proximal muscles contribute significantly to balance control during perturbed gait. The proximal muscles did not demonstrate more consistent activation, earlier onset latency, longer burst duration or larger burst magnitude than distal muscles. Moreover, although proximal postural activity was often present in the first slip trial, it tended to adapt away in later trials. By contrast, the typical postural responses exhibited by young adults consisted of an early (90–140 ms), high-magnitude (4–9 times muscle activity during normal walking) and relatively long duration (70–200 ms) activation of bilateral anterior leg muscles as well as the anterior and posterior thigh muscles. Thus, postural activity from bilateral leg and thigh muscles and the coordination between the two lower extremities were the key to reactive balance control and were sufficient for regaining balance within one gait cycle. The adaptive attenuation of proximal postural activity over repeated trials suggests that the nervous system overcompensates for a novel balance threat in the first slip trial and fine-tunes its responses with experience.

Time-dependent influence of sensorimotor set on automatic responses in perturbed stance

Abstract These experiments tested the hypothesis that the ability to change sensorimotor set quickly for au-tomatic responses depends on the time interval between successive surface perturbations. Sensorimotor set refers to the influence of prior experience or context on the state of the sensorimotor system. Sensorimotor set for postural responses was influenced by first giving subjects a block of identical backward translations of the support surface, causing forward sway and automatic gastrocnemius responses. The ability to change set quickly was inferred by measuring the suppression of the stretched antagonist gastrocnemius responses to toes-up rotations causing backward sway, following the translations. Responses were examined under short (10–14 s) and long (19–24 s) inter-trial intervals in young healthy subjects. The results showed that subjects in the long-interval group changed set immediately by suppressing gastrocnemius to 51% of translation responses within the first rotation and continued to suppress them over succeeding rotations. In contrast, subjects in the short-interval group did not change set immediately, but required two or more rotations to suppress gastrocnemius responses. By the last rotation, the short-interval group suppressed gastrocnemius responses to 33%, similar to the long-interval group of 29%. Associated surface plantarflexor torque resulting from these responses showed similar results. When rotation and translation perturbations alternated, however, the short-interval group was not able to suppress gastrocnemius responses to rotations as much as the long-interval group, although they did suppress more than in the first rotation trial after a series of translations. Set for automatic responses appears to linger, from one trial to the next. Specifically, sensorimotor set is more difficult to change when surface perturbations are given in close succession, making it appear as if set has become progressively stronger. A strong set does not mean that responses become larger over consecutive trials. Rather, it is inferred by the extent of difficulty in changing a response when it is appropriate to do so. These results suggest that the ability to change sensorimotor set quickly is sensitive to whether the change is required after a long or a short series of a prior different response, which in turn depends on the time interval between successive trials. Different rate of gastrocnemius suppression to toes-up rotation of the support surface have been reported in previous studies. This may be partially explained by different inter-trial time intervals demonstrated in this study.

Specific interference between a cognitive task and sensory organization for stance balance control in healthy young adults: Visuospatial effects

We tested the hypothesis that a computational overload results when two activities, one motor and the other cognitive that draw on the same neural processing pathways, are performed concurrently. Healthy young adult subjects carried out two seemingly distinct tasks of maintaining standing balance control under conditions of low (eyes closed), normal (eyes open) or high (eyes open, sway-referenced surround) visuospatial processing load while concurrently performing a cognitive task of either subtracting backwards by seven or generating words of the same first letter. A decrease in the performance of the balance control task and a decrement in the speed and accuracy of responses were noted during the subtraction but not the word generation task. The interference in the subtraction task was isolated to the first trial of the high but not normal or low visuospatial conditions. Balance control improvements with repeated exposures were observed only in the low visuospatial conditions while performance in the other conditions remained compromised. These results suggest that sensory organization for balance control appear to draw on similar visuospatial computational resources needed for the subtraction but not the word generation task. In accordance with the theory of modularity in human performance, the contrast in results between the subtraction and word generation tasks suggests that the neural overload is related to competition for similar visuospatial processes rather than limited attentional resources.

The strategies to regulate and to modulate the propulsive forces during gait initiation in lower limb amputees

We used the framework of motor program adaptability to examine how unilateral above-knee (AK) or below-knee (BK) amputee subjects organize the global and local biomechanical processes of generation of the propulsive forces during gait initiation to overcome the segmental and neuro-muscular asymmetry. The organization of the global biomechanical process refers to the kinematics behavior of the couple center of foot pressure (CoP) and center of mass (CoM); the organization of the local biomechanical process refers to the propulsive forces generated by the prosthetic or intact limb during the anticipatory postural adjustment phase and the step execution phase. Specifically, we examined: i) the strategy to regulate the progression velocity, i.e., to maintain it comparably when the leading limb changed from the prosthetic limb to the intact limb; and ii) the strategy to modulate the progression velocity, i.e., to increase it when gait was initiated with the prosthetic limb vs. intact limb. The kinematics of the CoM and CoP in the amputees showed the same global biomechanical organization that is typically observed in able-bodied subjects, i.e., the production of the forward disequilibrium torque was obtained by a backward shift of the CoP, followed by a forward acceleration of the CoM. However, gait initiation was achieved by using a different local strategy depending on which limb was used to initiate the step. For the regulation of the CoM progression velocity, when the gait was initiated with the intact limb, the slope of the progression velocity during the anticipatory postural adjustment phase (APA) was steeper and lasted longer, the step execution duration was shorter, and the variation of the CoM speed was lower. In other words, to regulate the speed of progression, the amputee subjects controlled the spatial and temporal parameters of the propulsive forces. In the modulation of the CoM progression velocity, when the gait was initiated with the intact limb, the amputees controlled only the intensity of the propulsive forces during both the APA and step execution phases. In contrast, when the gait was initiated with the prosthetic limb, the modulation resulted mainly from the propulsive forces generated during the step execution phase. These different strategies are discussed in terms of the subject’s capacity to adapt the motor program for gait initiation to new constraints.

Age-related changes in the center of mass velocity control during walking.

During walking, the body center of mass oscillates along the vertical plane. Its displacement is highest at mid-swing and lowest at terminal swing during the transition to double support. Its vertical velocity (CoMv) has been observed to increase as the center of mass falls between mid- and late swing but is reduced just before double support. This suggests that braking of the center of mass is achieved with active neural control. We tested whether this active control deteriorates with aging (Experiment 1) and during a concurrent cognitive task (Experiment 2). At short steps of <0.4m, CoMv control was low and similar among all age groups. All groups braked the CoMv at longer steps of >0.4m but older subjects did so to a lesser extent. During the cognitive task, young subjects increased CoMv control (i.e. increase in CoMv braking) while maintaining step length and walking speed. Older subjects on the other hand, did not increase CoMv control but rather maintain it by reducing both step length and walking speed. These results suggest that active braking of the CoM during the transition to double support predominates in steps >0.4m. It could be a manifestation of the balance control system, since the braking occurs at late stance where body weight is being shifted to the contralateral side. The active braking mechanism also appears to require some attentional resource. In aging, reducing step length and speed are strategic to maintaining effective center of mass control during the transition to double support. However, the lesser degree of control in older adults indicates a true age-related deficit.

Rapid assessment of postural instability in Parkinson's disease (RAPID): a pilot study.

Background:  The Fahn’s pull (or retropulsion) test is an item in the motor section of the Unified Parkinson’s Disease Rating Scale, which is used almost exclusively to classify postural instability in Parkinson’s disease (PD). However, the test is hard to standardize and is often performed incorrectly, making it hard to interpret. Moreover, it may not be safe to administer in patients who experience pain in the shoulders, neck, trunk and/or lower extremities. Identifying and grading postural instability in PD without requiring a physical challenge would not only be useful for the clinician but would assist patients and caregivers in its recognition. We propose the use of the rapid assessment of postural instability in Parkinson’s disease (RAPID) questionnaire as a non‐physical assessment tool.

Factors influencing the quick onset of stepping following postural perturbation.

It has been shown that the stepping to recover balance following a forward fall occurs at a constant time (on average 293 ms) (Do et al. Journal of Biomechanics 15, 1982, 933-939). In this study, we tested the hypothesis according to which programming to make fast movement could trigger the movement earlier than when programming self-pace movement. The same experimental paradigm of forward fall was used (see Do et al., 1982) to induce stepping. Different extents of stepping were manipulated by instructions: Subjects were instructed to step to recover their balance naturally (control condition); to make shorter steps than in the control condition; longer steps; faster steps. Lastly, a fast step was also induced by the biomechanical constraint on the initial posture, i.e. by inclining the subject forward at his maximum capacity. Data were collected from 12 subjects. The variables analyzed were the onset latency of step execution and other classical parameters (time of heel-contact, duration of the swing phase, step length, center of mass progression velocity, and step velocity). The results showed that the onset of stepping was unchanged in the longer- and faster-step conditions, relative to the control condition (mean control value = 280 ms). In contrast, the onset of stepping was significantly earlier in the short-step condition, and when the initial inclination was greater (250 and 252 ms, respectively). The swing phase duration in these two conditions averaged 140 and 185 ms, was significantly shorter than in the other conditions, whereas step length was obviously expected to be shorter in the shorter-step condition and longer in the longer-step condition than in the other conditions. Step length was similar between the other conditions. We conclude that neither step length or step velocity programming could induce an earlier onset latency of stepping. Step programming in relation to these specific instructions seemed to concern the extent of step execution and not the time of triggering of the stepping. We suggest that the control of short swing phase duration resulted in an earlier onset latency of stepping to recover the balance. This control depends on the combination of biomechanical constraints and cognitive processes, including subjects interpretation of the instructions and evaluation of the risk of fall.

Upregulation of GPR109A in Parkinson's disease.

Background Anecdotal animal and human studies have implicated the symptomatic and neuroprotective roles of niacin in Parkinson’s disease (PD). Niacin has a high affinity for GPR109A, an anti-inflammatory receptor. Niacin is also thought to be involved in the regulation of circadian rhythm. Here we evaluated the relationships among the receptor, niacin levels and EEG night-sleep in individuals with PD. Methods and Findings GPR109A expression (blood and brain), niacin index (NAD-NADP ratio) and cytokine markers (blood) were analyzed. Measures of night-sleep function (EEG) and perceived sleep quality (questionnaire) were assessed. We observed significant up-regulation of GPR109A expression in the blood as well as in the substantia nigra (SN) in the PD group compared to age-matched controls. Confocal microscopy demonstrated co-localization of GPR109A staining with microglia in PD SN. Pro and anti-inflammatory cytokines did not show significant differences between the groups; however IL1-β, IL-4 and IL-7 showed an upward trend in PD. Time to sleep (sleep latency), EEG REM and sleep efficiency were different between PD and age-matched controls. Niacin levels were lower in PD and were associated with increased frequency of experiencing body pain and decreased duration of deep sleep. Conclusions The findings of associations among the GPR109A receptor, niacin levels and night-sleep function in individuals with PD are novel. Further studies are needed to understand the pathophysiological mechanisms of action of niacin, GPR109A expression and their associations with night-sleep function. It would be also crucial to study GPR109A expression in neurons, astrocytes, and microglia in PD. A clinical trial to determine the symptomatic and/or neuroprotective effect of niacin supplementation is warranted.

Influence of sensory inputs and motor demands on the control of the centre of mass velocity during gait initiation in humans

Human gait requires the simultaneous generation of goal-directed continuous movement (locomotion) and the maintenance of balance (postural control). In adults, the centre of mass (CoM) oscillates in the vertical plane while walking. During the single support phase of gait initiation, its vertical (vCoM) velocity increases as the CoM falls and is actively reversed prior to foot-contact. In this study we investigated whether this active control, which is thought to reflect balance control during gait initiation, is controlled by visual and somatosensory inputs (Experiment 1) and whether it is modified by a change in motor demands, two steps versus one step (Experiment 2). In all healthy adults, the vCoM velocity was braked, or controlled, by contraction of the soleus muscle of the stance leg. The elimination of visual input alone had no effect on braking, although its amplitude decreased when somatosensory inputs were disrupted (-47%), and further decreased when both visual and somatosensory inputs were disrupted (-83%). When subjects performed only one step, with no trailing of the stance foot, the vCoM velocity braking also decreased (-42%). These results suggest that active braking of the CoM fall during the transition to double support, an indicator of balance control, is influenced by both multisensory integration and the demands of the current motor program. The neural structures involved in this mechanism remain to be elucidated.

Heart rate, blood pressure, and running speed responses to mesencephalic locomotor region stimulation in anesthetized rats

Abstract The decerebrate rat locomotor preparation described in a previous study requires extensive brain surgery with the possibility of significant blood loss. The purpose of this study was to improve on the previous model by using lightly anesthetized instead of decerebrated rats. After initial surgery consisting of boring a small hole through the parietal bone, the animals were maintained on low levels of halothane anesthetic. The mesencephalic locomotor region was then located by physiological criteria using stereotaxic coordinates from the previous study. Locomotor speed, blood pressure and heart rate responses were then measured over a wide range of stimulation currents that elicited a maximal running speed. Stimulation currents ranged from 36 μA for walking to 82 μA for fast galloping. Locomotor speeds ranged from 20 m/min for walking to 64 m/min for fast galloping. Some animals easily achieved galloping speeds beyond 100 m/min. Blood pressure and heart rate increased with increasing stimulation currents. Blood pressure also increased during stimulation after muscular paralysis. This was not due to current spread, suggesting that the mesencephalic locomotor region might be involved in central command mechanisms. Heart rate did not increase after paralysis. This supports other multi-joint dynamic studies suggesting that movement per se may be necessary to induce heart rate changes, presumably via joint mechanoreceptors. The range of locomotor patterns and cardiovascular responses were obtained under self-supported conditions. By defining the mesencephalic locomotor region via physiological criteria, and by grouping blood pressure and heart rate measurements by gait rather than by stimulation currents, the potential use of the intact model for cardiovascular control studies was demonstrated. The animals were able to run and gallop at high speeds considering they were anesthetized. The simplified preparation will be useful for more complex cardiovascular experiments requiring intact and self-supported conditions.