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Neural control of standing posture
Swedish School of Sport and Health Sciences, GIH, Department of Sport and Health Sciences, Laboratory for Biomechanics and Motor Control. (Karolinska Institutet)
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When humans are asked to stand normally, they are not completely motionless. Rather, small amounts of body movement, termed postural sway, can be observed. Although the postural sway of standing has been well described, the manner in which this sway is neurally controlled and its influence in tasks involving postural re-stabilization are not known. Therefore, the aim of this thesis was to investigate the neural control of human standing posture, with a special emphasis on 1) whether the neuromuscular responses to an unexpected perturbation are influenced by the postural sway, 2) whether spinallymediated changes occur as a function of postural sway position and/or direction, and 3) whether the excitability of the cortical and corticospinal pathways are altered with respect to postural sway. In each study, subjects stood quietly on a force platform. For Studies I-III, the anteroposterior center of pressure (COP) signal from the force platform was monitored online such that when the position and/or velocity of the COP was of the desired magnitude and direction, a perturbation was administered to the subject. The perturbation consisted of either a sudden support surface translation (Study I) or a percutaneous electrical stimulation to the posterior tibial nerve (Studies II-IV). In Study IV, a perturbation, in the form of either a transcranial magnetic (TMS) or electric (TES) stimulation to the left motor cortex, was triggered at a random time, regardless of the COP signal. The neuromuscular responses to the mechanical, electrical or magnetic perturbations were assessed by measuring the body kinematics from a motion capture system or electromyographic (EMG) recordings from surface electrodes placed over various lower limb muscles. Specific dependent measures included the number of stepping responses, the latencies and amplitudes of the EMG recordings, the peak-to-peak amplitudes of the Hoffmann reflex (Hreflex) and M-wave from tibial nerve stimulation, as well as the peak-to-peak amplitudes of the motor evoked potentials (MEPs) elicited by TMS and TES. Study I indicated that when subjects were standing normally, the position of postural sway influenced the postural responses to an unexpected surface translation. EMG activity of various lower limb and trunk muscles were generally delayed in time and larger in amplitude when subjects were swaying in the direction opposite to the upcoming perturbation. The altered postural responses may be related to the ongoing modulation of the synaptic efficacy, as reflected by the size of the H-reflex, to the triceps surae Ia pathways. In Studies II-IV, it was found that when subjects were swaying in the forward as compared to the backward direction or position, depolarization of the soleus and medial gastrocnemius motoneurone pools, via synaptic transmission of the Ia afferents, was easier to achieve. However, this sway direction- and sway position-dependent modulation of neural excitability was limited to the spinal and corticospinal levels. Study IV revealed that TMS- and TES-evoked MEPs were similarly modulated during the naturally occurring sway of normal standing, suggesting that the excitability of the motor cortex was not dependent on postural sway. A facilitation in cortical excitability, as shown by the differential MEP response between TMS and TES, was however found during normal as compared supported (i.e. no postural sway) standing. This thesis demonstrates that human standing posture is controlled via an overall enhancement of cortical excitability, concurrently with an ongoing sway-dependent modulation of spinal and corticospinal processes. The constantly changing neural inputs to the motoneurone pool may give insight into the influence of postural sway to the neuromuscular responses to an unexpected perturbation. 

Place, publisher, year, edition, pages
Stockholm: Karolinska institutet , 2007.
National Category
Medical and Health Sciences
Research subject
Medicine/Technology
Identifiers
URN: urn:nbn:se:gih:diva-1955ISBN: 978-91-7357-396-2 (print)OAI: oai:DiVA.org:gih-1955DiVA: diva2:451798
Public defence
2007-12-07, Aulan, GIH, Lidingövägen 1, Stockholm, 09:00 (English)
Opponent
Supervisors
Note
Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007Available from: 2011-10-27 Created: 2011-10-21 Last updated: 2011-10-27Bibliographically approved
List of papers
1. The influence of natural body sway on neuromuscular responses to an unpredictable surface translation.
Open this publication in new window or tab >>The influence of natural body sway on neuromuscular responses to an unpredictable surface translation.
2006 (English)In: Experimental Brain Research, ISSN 0014-4819, E-ISSN 1432-1106, Vol. 174, no 1, 19-28 p.Article in journal (Refereed) Published
Abstract [en]

Previous research has shown that the postural configuration adopted by a subject, such as active leaning, influences the postural response to an unpredictable support surface translation. While those studies have examined large differences in postural conditions, it is of additional interest to examine the effects of naturally occurring changes in standing posture. Thus, it was hypothesized that the normal postural sway observed during quiet standing would affect the responses to an unpredictable support surface translation. Seventeen young adults stood quietly on a moveable platform and were perturbed in either the forward or backward direction when the location of the center of pressure (COP) was either 1.5 standard deviations anterior or posterior to the mean baseline COP signal. Postural responses, in the form of electromyographic (EMG) latencies and amplitudes, were recorded from lower limb and trunk muscles. When the location of the COP at the time of the translation was in the opposite, as compared to the same, direction as the upcoming translation, there was a significantly earlier onset of the antagonists (10-23%, i.e. 15-45 ms) and a greater EMG amplitude (14-39%) in four of the six recorded muscles. Stepping responses were most frequently observed during trials where the position of the COP was opposite to the direction of the translation. The results support the hypothesis that postural responses to unpredictable support surface translations are influenced by the normal movements of postural sway. The results may help to explain the large variability of postural responses found between past studies.

Identifiers
urn:nbn:se:gih:diva-564 (URN)10.1007/s00221-006-0414-x (DOI)16544134 (PubMedID)
Available from: 2009-02-16 Created: 2009-02-16 Last updated: 2011-10-27Bibliographically approved
2. Control of the triceps surae during the postural sway of quiet standing.
Open this publication in new window or tab >>Control of the triceps surae during the postural sway of quiet standing.
Show others...
2007 (English)In: Acta Physiologica, ISSN 1748-1708, E-ISSN 1748-1716, Vol. 191, no 3, 229-36 p.Article in journal (Refereed) Published
Abstract [en]

AIM: The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing. METHODS: Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway. RESULTS: A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity. CONCLUSIONS: It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.

Identifiers
urn:nbn:se:gih:diva-572 (URN)10.1111/j.1748-1716.2007.01727.x (DOI)17635414 (PubMedID)
Available from: 2009-02-16 Created: 2009-02-16 Last updated: 2011-10-27Bibliographically approved
3. Sway-dependent modulation of the triceps surae H-reflex during standing.
Open this publication in new window or tab >>Sway-dependent modulation of the triceps surae H-reflex during standing.
Show others...
2008 (English)In: Journal of applied physiology, ISSN 8750-7587, E-ISSN 1522-1601, Vol. 104, no 5, 1359-65 p.Article in journal (Refereed) Published
Abstract [en]

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P = 0.075), with an 8.9 +/- 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.

Identifiers
urn:nbn:se:gih:diva-575 (URN)10.1152/japplphysiol.00857.2007 (DOI)18369094 (PubMedID)
Note
At the time of Craig Tokuno's dissertation the article was submitted to the journal.Available from: 2009-02-16 Created: 2009-02-16 Last updated: 2011-12-13Bibliographically approved
4. Changes in cortical and corticospinal excitability during standing
Open this publication in new window or tab >>Changes in cortical and corticospinal excitability during standing
2007 (English)In: Journal of Physiology, ISSN 0022-3751, E-ISSN 1469-7793Article in journal (Refereed) Submitted
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:gih:diva-1966 (URN)
Note

At the time of Craig Tokuno's dissertation the article was submitted.

Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2017-12-08Bibliographically approved

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