Neural Control Of Balance Research Articles

Brain Activation During Active Balancing and Its Behavioral Relevance in Younger and Older Adults: A Functional Near-Infrared Spectroscopy (fNIRS) Study.

Age-related deterioration of balance control is widely regarded as an important phenomenon influencing quality of life and longevity, such that a more comprehensive understanding of the neural mechanisms underlying this process is warranted. Specifically, previous studies have reported that older adults typically show higher neural activity during balancing as compared to younger counterparts, but the implications of this finding on balance performance remain largely unclear. Using functional near-infrared spectroscopy (fNIRS), differences in the cortical control of balance between healthy younger (n = 27) and older (n = 35) adults were explored. More specifically, the association between cortical functional activity and balance performance across and within age groups was investigated. To this end, we measured hemodynamic responses (i.e., changes in oxygenated and deoxygenated hemoglobin) while participants balanced on an unstable device. As criterion variables for brain-behavior-correlations, we also assessed postural sway while standing on a free-swinging platform and while balancing on wobble boards with different levels of difficulty. We found that older compared to younger participants had higher activity in prefrontal and lower activity in postcentral regions. Subsequent robust regression analyses revealed that lower prefrontal brain activity was related to improved balance performance across age groups, indicating that higher activity of the prefrontal cortex during balancing reflects neural inefficiency. We also present evidence supporting that age serves as a moderator in the relationship between brain activity and balance, i.e., cortical hemodynamics generally appears to be a more important predictor of balance performance in the older than in the younger. Strikingly, we found that age differences in balance performance are mediated by balancing-induced activation of the superior frontal gyrus, thus suggesting that differential activation of this region reflects a mechanism involved in the aging process of the neural control of balance. Our study suggests that differences in functional brain activity between age groups are not a mere by-product of aging, but instead of direct behavioral relevance for balance performance. Potential implications of these findings in terms of early detection of fall-prone individuals and intervention strategies targeting balance and healthy aging are discussed.

Frequency-dependent force direction elucidates neural control of balance

BackgroundMaintaining upright posture is an unstable task that requires sophisticated neuro-muscular control. Humans use foot–ground interaction forces, characterized by point of application, magnitude, and direction to manage body accelerations. When analyzing the directions of the ground reaction forces of standing humans in the frequency domain, previous work found a consistent pattern in different frequency bands. To test whether this frequency-dependent behavior provided a distinctive signature of neural control or was a necessary consequence of biomechanics, this study simulated quiet standing and compared the results with human subject data.MethodsAiming to develop the simplest competent and neuromechanically justifiable dynamic model that could account for the pattern observed across multiple subjects, we first explored the minimum number of degrees of freedom required for the model. Then, we applied a well-established optimal control method that was parameterized to maximize physiologically-relevant insight to stabilize the balancing model.ResultsIf a standing human was modeled as a single inverted pendulum, no controller could reproduce the experimentally observed pattern. The simplest competent model that approximated a standing human was a double inverted pendulum with torque-actuated ankle and hip joints. A range of controller parameters could stabilize this model and reproduce the general trend observed in experimental data; this result seems to indicate a biomechanical constraint and not a consequence of control. However, details of the frequency-dependent pattern varied substantially across tested control parameter values. The set of parameters that best reproduced the human experimental results suggests that the control strategy employed by human subjects to maintain quiet standing was best described by minimal control effort with an emphasis on ankle torque.ConclusionsThe findings suggest that the frequency-dependent pattern of ground reaction forces observed in quiet standing conveys quantitative information about human control strategies. This study’s method might be extended to investigate human neural control strategies in different contexts of balance, such as with an assistive device or in neurologically impaired subjects.

Flexible Recruitment of Balance Mechanisms to Environmental Constraints During Walking

Background: Humans need to actively control their upright posture during walking to avoid loss of balance. We do not have a comprehensive theory for how humans regulate balance during walking, especially in complex environments. The nervous system must process many aspects of the environment to produce an appropriate motor output in order to maintain balance on two legs. We have previously identified three balance mechanisms that young healthy adults use to maintain balance while walking: 1) The ankle roll mechanism, a modulation of ankle inversion/eversion; 2) The foot placement mechanism, a shift of the swing foot placement; and 3) The push-off mechanism, a modulation of the ankle plantar flexion angle during double stance. We know that these mechanisms are inter dependent and can be influenced by internal factors such as the phase of the gait cycle and walking cadence. Here we seek to determine whether there are changes in neural control of balance when walking in the presence of environmental constraints. Methods: Subjects walked on a self-paced treadmill while immersed in a virtual environment that provides three different colored pathways. Subjects were instructed not to step in the No-Step Zone, which appeared either on the right or left side of the subject. While walking, subjects received balance perturbations in the form of galvanic vestibular stimulation,providing the sensation of falling sideways, either toward the No-Step zone or toward the Neutral zone on the other side. Results: The results indicate that the use of the balance mechanisms are altered depending on whether the perceived fall is toward the No-Step or the Neutral zone. Participants increased the use of the lateral ankle and foot placement mechanisms for No-Step stimuli resulting in a larger shift of the center of mass (CoM) compared to the Neutral stimuli. Conclusion:This experiment provides further evidence that the balance control system during walking is extremely flexible, recruiting multiple mechanisms at different times in the gait cycle to adapt to environmental constraints.

Walking Cadence Affects the Recruitment of the Medial-Lateral Balance Mechanisms.

We have previously identified three balance mechanisms that young healthy adults use to maintain balance while walking. The three mechanisms are: (1) The lateral ankle mechanism, an active modulation of ankle inversion/eversion in stance; (2) The foot placement mechanism, an active shift of the swing foot placement; and (3) The push-off mechanism, an active modulation of the ankle plantarflexion angle during double stance. Here we seek to determine whether there are changes in neural control of balance when walking at different cadences and speeds. Twenty-one healthy young adults walked on a self-paced treadmill while immersed in a 3D virtual reality cave, and periodically received balance perturbations (bipolar galvanic vestibular stimulation) eliciting a perceived fall to the side. Subjects were instructed to match two cadences specified by a metronome, 110 bpm (High) and 80 bpm (Low), which in this experiment, led to faster and slower gait speeds, respectively. The results indicate that subjects altered the use of the balance mechanisms at different cadences. The lateral ankle mechanism was used more in the Low condition, while the foot placement mechanism was used more in the High condition. There was no difference in the use of the push-off mechanism between cadence conditions. These results suggest that neural control of balance is altered when gait characteristics, such as cadence change, suggesting a flexible balance response that is sensitive to the constraints of the gait cycle. We speculate that the use of the balance mechanisms may be a factor resulting in well-known characteristics of gait in populations with compromised balance control, such as slower gait speed in older adults or higher cadence in people with Parkinson's disease.

Examining changes in corticospinal excitability and balance performance in response to social-comparative feedback

BackgroundSocial-comparative feedback informs an individual that their performance was better or worse than the group. Previous studies have found that compared to knowledge of results alone, social-comparative feedback produces a valence response that results in larger improvements in balance performance. However, the neural processes contributing to these motor improvements have not yet been examined. Research questionDoes social-comparative feedback alter corticospinal excitability and consequently, balance performance? MethodsThirty-six healthy young adults stood and maintained their balance on a stabiliometer for eight trials. After three of the trials, the neutral (i.e., only knowledge of results) group received their performance feedback (i.e., time on balance) while the other two groups also received positive (i.e., performed better than the group) or negative (i.e., performed worse than the group) social-comparative feedback. To measure corticospinal excitability, soleus motor-evoked potentials were elicited using transcranial magnetic stimulation at the beginning of the experiment, after the presentation of feedback, and at the end of the experiment. Pre- and post- ratings of confidence, perceived skill, motivation, and anxiety were also collected. ResultsThe negative feedback group reported decreases in perceived skill (43 ± 29%) and balance confidence (26 ± 28%), while the positive group reported a 13 ± 17% increase in perceived skill. Despite these group differences in feedback perception, all three groups improved their balance performance by ≈35% (p < 0.001) by the eighth trial. However, this improvement in balance performance was not matched by any changes in corticospinal excitability over time (19.2 ± 55.9% change; p = 0.340) or between groups (p = 0.734). SignificanceOur findings suggest that social-comparative feedback, as presented in this study, does not affect corticospinal excitability and balance performance differently than knowledge of results (neutral feedback) alone. More arousing and more frequent forms of social-comparative feedback may be necessary for observing larger changes in the functional or neural control of balance.

Neural Control of Balance During Walking.

Neural control of standing balance has been extensively studied. However, most falls occur during walking rather than standing, and findings from standing balance research do not necessarily carry over to walking. This is primarily due to the constraints of the gait cycle: Body configuration changes dramatically over the gait cycle, necessitating different responses as this configuration changes. Notably, certain responses can only be initiated at specific points in the gait cycle, leading to onset times ranging from 350 to 600 ms, much longer than what is observed during standing (50–200 ms). Here, we investigated the neural control of upright balance during walking. Specifically, how the brain transforms sensory information related to upright balance into corrective motor responses. We used visual disturbances of 20 healthy young subjects walking in a virtual reality cave to induce the perception of a fall to the side and analyzed the muscular responses, changes in ground reaction forces and body kinematics. Our results showed changes in swing leg foot placement and stance leg ankle roll that accelerate the body in the direction opposite of the visually induced fall stimulus, consistent with previous results. Surprisingly, ankle musculature activity changed rapidly in response to the stimulus, suggesting the presence of a direct reflexive pathway from the visual system to the spinal cord, similar to the vestibulospinal pathway. We also observed systematic modulation of the ankle push-off, indicating the discovery of a previously unobserved balance mechanism. Such modulation has implications not only for balance but plays a role in modulation of step width and length as well as cadence. These results indicated a temporally-coordinated series of balance responses over the gait cycle that insures flexible control of upright balance during walking.

Strategies for the Control of Balance During Locomotion

The neural control of balance during locomotion is currently not well understood, even in the light of considerable advances in research on balance during standing. In this paper, we lay out the control problem for this task and present a list of different strategies available to the central nervous system to solve this problem. We discuss the biomechanics of the walking body, using a simplified model that iteratively gains degrees of freedom and complexity. Each addition allows for different control strategies, which we introduce in turn: foot placement shift, ankle strategy, hip strategy, and push-off modulation. The dynamics of the biomechanical system are discussed using the phase space representation, which allows illustrating the mechanical effect of the different control mechanisms. This also enables us to demonstrate the effects of common general stability strategies, such as increasing step width and cadence.

Quantifying Balance Control in Concussed Varsity Athletes

Previous research has demonstrated that even at return to play (RTP), there are continued balance control deficits in previously concussed athletes. However, these deficits were only distinguishable using higher order measures, which might have been missed using traditional measures (Powers et al., 2013). Higher-order analyses provide better insights into the neural control of balance. Sparato and colleagues (2013) quantified the number of lateral posture adjustments (PA) during a stepping task and found young adults tend to elicit one PA whereas older adults with history of falls tended to use two. PURPOSE: To determine whether a PA measure can distinguish control differences between groups of young adults during gait initiation (GI) task. HYPOTHESIS: During GI, concussed individuals will make more PA than non-athletes and baseline athletes, respectively. METHOD: Participants were all females (17-24 years) and included: varsity soccer players (N=28, baseline), non-athletes (N=6), and concussed soccer players during the symptomatic phase (N=3). Participants were asked to stand quietly on a Bertec force plate (FP) with their feet together and hands by their sides with their eyes open or closed for 45s. After 45s, an auditory cue sounded and participants initiated gait from the FP by taking two steps. Postural adjustments made during the gait initiation phase were calculated using the vertical force and counting the number of inflection points from the start of GI to when the participants were completely off the FP. Participants were taken from three groups: Baseline Female Laurier Varsity Soccer Players, Non-Athletes, and Concussed Varsity Athletes at the asymptomatic and RTP stage of recovery. RESULTS: During eyes open, non-athletes made more PA than the baseline athletes (p=.034) but concussed soccer players were not different from the baseline athletes (p=.13). With eyes closed, all participants produced similar number of PA. CONCLUSSION: These preliminary findings suggest that a PA measurement during GI can distinguish athletes from non-athletes and hopefully with more concussed athlete participants, may be able to distinguish between concussed and non-concussed athletes. If this is true, this would be an effective measure to not only determine if an athlete is concussed, but also to determine if the concussed athlete is ready to return to activity.