Intracortical inhibition in the soleus muscle is reduced during the control of upright standing in both young and old adults

Purpose: In a previous study, we reported that a short-interval intracortical inhibition (SICI) decreases in old but not in young adults when standing on foam vs. a rigid surface. Here, we examined if such an age by task difficulty interaction in motor cortical excitability also occurs in easier standing tasks.Methods: Fourteen young (23 ± 2.7 years) and fourteen old (65 ± 4.1 years) adults received transcranial magnetic brain stimulation and peripheral nerve stimulation, while they stood with or without support on a force platform.Results: In the soleus, we found that SICI was lower in unsupported (35 % inhibition) vs. supported (50 %) standing (p = 0.007) but similar in young vs. old adults (p = 0.591). In the tibialis anterior, SICI was similar between conditions (p = 0.597) but lower in old (52 %) vs. young (72 %) adults (p = 0.030). Age and standing with or without support did not affect the Hoffmann reflex in the soleus.Conclusions: The current data suggest that the motor cortex is involved in standing control, and that its role becomes more prominent with an increase in task difficulty

Postural challenge affects motor cortical activity in young and old adults

When humans voluntarily activate a muscle, intracortical inhibition decreases. Such a decrease also occurs in the presence of a postural challenge and more so with increasing age. Here, we examined age-related changes in motor cortical activity during postural and non-postural contractions with varying levels of postural challenge. Fourteen young (age 22) and twelve old adults (age 70) performed three conditions: (1) voluntary contraction of the soleus muscle in sitting and (2) leaning forward while standing with and (3) without being supported. Subthreshold transcranial magnetic stimulation was applied to the soleus motor area suppressing ongoing EMG, as an index of motor cortical activity. The area of EMG suppression was ~ 60% smaller (p  0.05). Even though in absolute terms young compared with old adults leaned farther (p = 0.018), there was no age effect or an age by condition interaction in EMG suppression. Leaning closer to the maximum without support correlated with less EMG suppression (rho = − 0.44, p = 0.034). We conclude that the critical factor in modulating motor cortical activity was postural challenge and not contraction aim or posture. Age did not affect the motor control strategy as quantified by the modulation of motor cortical activity, but the modulation appeared at a lower task difficulty with increasing age

Age-related reversal of spinal excitability during anticipatory postural control

IntroductionAn internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.MethodsWe compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n=11, age 19-24 years), middle-aged (n=10, age 37-56 years), and older healthy adults (n=11, age 63-78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120ms, 100ms, 80ms, 60ms, 40ms, 20ms, and 0ms before as well as 20ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.ResultsAll subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.ConclusionBecause APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs

Age-related reversal of spinal excitability during anticipatory postural control

IntroductionAn internal perturbation of standing balance activates muscles critical for maintaining balance and is preceded by anticipatory postural adjustments (APAs). In healthy younger adults, a measure of spinal excitability in the form of the Hoffmann (H) reflex becomes depressed during APAs but how aging affects the reflex control of APAs is unknown.MethodsWe compared H reflex excitability profiles in the right soleus muscle, indirectly indicating APA, between younger (n=11, age 19-24 years), middle-aged (n=10, age 37-56 years), and older healthy adults (n=11, age 63-78 years). Subjects rapidly raised the right-dominant arm in response to an auditory cue. The H reflex was evoked 120ms, 100ms, 80ms, 60ms, 40ms, 20ms, and 0ms before as well as 20ms after the onset of the right anterior deltoid muscle activation. For data processing, each trial was controlled for the corresponding background EMG activity before normalizing the standing data to the data in sitting in the 8 time bins.ResultsAll subjects showed a silent period in the soleus background electromyographic activity, suggesting the presence of APA. We found that the stereotypical H reflex depression associated with APAs in younger adults was reduced in middle-aged adults and reversed to facilitation in older adults. The depression occurred in 10 out of 11 younger adults, whereas all 11 older adults exhibited facilitation.ConclusionBecause APAs are organized at the supraspinal level, we speculate a supraspinal origin of the age-related reflex facilitation during APAs

Age-related changes in neural control of posture

As we get older many physiological functions decline, including muscle strength, flexibility, and memory. Also in the aging brain there are changes, such as shrinkage of its volume. Since we need our brain to keep our balance while standing, it seems likely that these changes also affect our balance control. In this thesis we therefore aimed to investigate whether and how the role of the brain in balance control changes with aging and how this affects stance stability. For our study we used a TMS (transcranial magnetic stimulation) device with which we stimulated the brain of young and old adults during various balance tasks. The effect of this stimulation is a brief muscle contraction that allows us to measure the excitability of the brain. The results show that both young and old brains are more active during difficult compared with easier balance tasks. The good news is that the ability to adjust the brain activity to the balance task is not affected by age. However, the threshold is lower in old compared with young adults, meaning that old adults activate the brain in easier tasks. We also investigated whether the increased brain activation is the reason that old adults have more difficulty with dual-tasking than young adults. Although this hypothesis is often mentioned, our study suggests that this is incorrect. Overall, the findings of this thesis help us better understand why elderly are more unstable than young adults and therefore forms a basis for better fall prevention programs

Age-related changes in the neural control of standing balance

Controlling body sway while standing is an active process involving lower as well as higher neural centers. This chapter examines how the central nervous system controls undisturbed standing balance and summarizes the current knowledge concerning the effects of task difficulty and old age on postural control of standing. There is an age-related reorganization of neural control of standing, with decreased efficacy of Ia afferents to activate spinal motor neurons, increased cortical activation and corticospinal excitability, and reduced intracortical inhibition. Age does not affect the motor control strategy of reducing intracortical inhibition with increasing postural challenge. However, the threshold for down-modulation decreases with aging. It thus seems that motor cortical involvement in the control of standing balance becomes more prominent with age and postural task difficulty. Future studies will determine if it is beneficial and necessary through interventions to reduce the cortical involvement in the control of standing balance in healthy old adults especially if this involvement should increase in old adults with a history of falls.</p

Age-related changes in the neural control of standing balance

Controlling body sway while standing is an active process involving lower as well as higher neural centers. This chapter examines how the central nervous system controls undisturbed standing balance and summarizes the current knowledge concerning the effects of task difficulty and old age on postural control of standing. There is an age-related reorganization of neural control of standing, with decreased efficacy of Ia afferents to activate spinal motor neurons, increased cortical activation and corticospinal excitability, and reduced intracortical inhibition. Age does not affect the motor control strategy of reducing intracortical inhibition with increasing postural challenge. However, the threshold for down-modulation decreases with aging. It thus seems that motor cortical involvement in the control of standing balance becomes more prominent with age and postural task difficulty. Future studies will determine if it is beneficial and necessary through interventions to reduce the cortical involvement in the control of standing balance in healthy old adults especially if this involvement should increase in old adults with a history of falls

Neural correlates of motor-cognitive dual-tasking in young and old adults

When two tasks are performed simultaneously, performance often declines in one or both tasks. These so-called dual-task costs are more pronounced in old than in young adults. One proposed neurological mechanism of the dual-task costs is that old compared with young adults tend to execute single-tasks with higher brain activation. In the brain regions that are needed for both tasks, the reduced residual capacity may interfere with performance of the dual-task. This competition for shared brain regions has been called structural interference. The purpose of the study was to determine whether structural interference indeed plays a role in the age-related decrease in dual-task performance. Functional magnetic resonance imaging (fMRI) was used to investigate 23 young adults (20-29 years) and 32 old adults (66-89 years) performing a calculation (serial subtraction by seven) and balance-simulation (plantar flexion force control) task separately or simultaneously. Behavioral performance decreased during the dual-task compared with the single-tasks in both age groups, with greater dual-task costs in old compared with young adults. Brain activation was significantly higher in old than young adults during all conditions. Region of interest analyses were performed on brain regions that were active in both tasks. Structural interference was apparent in the right insula, as quantified by an age-related reduction in upregulation of brain activity from single-to dual-task. However, the magnitude of upregulation did not correlate with dual-task costs. Therefore, we conclude that the greater dual-task costs in old adults were probably not due to increased structural interference.</p

Age-related decrease in postural control is related to different modulation in motor cortical inhibition between postural tasks

info:eu-repo/semantics/publishe