Automated postural responses are modified in a fun

Authors: Vivian Weerdesteyn Andrew C Laing Stephen N Robinovitch

Publish Date: 2008/01/09

Volume: 186, Issue: 4, Pages: 571-580

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Abstract

The restoration of upright balance after a perturbation relies on highly automated and to a large extent stereotyped postural responses Although these responses occur before voluntary control comes into play previous research has shown that they can be functionally modulated on the basis of cognitive set experience advanced warning instruction etc It is still unknown however how the central nervous system deals with situations in which the postural response is not necessarily helpful in the execution of a task In the present study the effects of instruction on automated postural responses in neck trunk shoulder and leg muscles were investigated when people were either instructed to recover balance after being released from an inclined standing posture balance recovery BR trials or not to recover at all and fall onto a safety mattress in the most comfortable way fall F trials in both backward and leftward directions Participants were highly successful in following the instructions consistently exhibiting stepping responses for balance recovery in BR trials and suppressing stepping in the F trials Yet EMG recordings revealed similar postural responses with onset latencies between 70 and 130 ms in both BR and F trials with slightly delayed responses in F trials In contrast very pronounced and early differences were observed between BR and F trials in response amplitudes which were generally much higher in BR than in F trials but with clear differentiation between muscles and perturbation directions These results indicate that a balance perturbation always elicits a postural response irrespective of the task demands However when a specific balance recovery response is not desired after a perturbation postural responses can be selectively downregulated and integrated into the motor output in a functional and goaloriented wayThe restoration of upright balance after a perturbation relies on highly automated and to a large extent stereotyped postural responses involving a complex pattern of activation of upper and lower leg trunk shoulder and neck muscles Allum et al 2002 Bloem et al 2000 Carpenter et al 2004 McIlroy and Maki 1995 Moore et al 1988 Thelen et al 2000 Woollacott et al 1988 These responses typically occur at onset latencies of ∼100 ms In response to a perturbation both feetinplace and stepping strategies can be used to recover balance with the incidence of stepping responses becoming larger as the perturbation magnitude increases Hsiao and Robinovitch 1998 McIlroy and Maki 1993 These responses are triggered and modulated on the basis of sensory information with lower extremity and trunk proprioception as well as vestibular inputs as possible sources Allum and Honegger 1998 Bloem et al 2000 Do et al 1988Although the onsets of automated postural responses occur before voluntary cognitive control comes into play and the characteristics of the responses are distinctly different from those of voluntary movements Nashner and Cordo 1981 previous studies have shown that higher brain levels presumably involving the cerebral cortex can modulate postural responses by changing the activity of the pathways that are involved in their generation for review see Jacobs and Horak 2007 In 1976 Nashner was the first to report that experience causes functional adaptations in response amplitudes following an unexpected change of support surface motion Subsequent studies have shown that response amplitudes depend on the predictability of perturbation magnitudes Beckley et al 1991 Horak et al 1989 restrictions on the balance recovery strategy feetinplace vs stepping Burleigh and Horak 1996 Burleigh et al 1994 McIlroy and Maki 1993 and the task of holding an object Bateni et al 2004 Marsden et al 1981 In general these factors do not cause corresponding changes in onset latencies and activation sequences On the other hand both response amplitudes and onset latencies may be affected by advanced warning of the perturbation Mawston et al 2007 McChesney et al 1996 and increased postural anxiety Carpenter et al 2004 Hence the literature indicates that changes in initial contexts generally affect response amplitudes preserve activation sequences and have limited influence on onset latenciesOverall this pattern of results suggests that the sensory information as generated by the induced perturbation to upright balance inevitably launches a postural response directed toward recovering or maintaining an upright stance that can be scaled by cognitive set by changing the gain but not fully suppressed However in all of these previous studies the final goal was the same in every condition namely balance recovery It has been postulated Nashner and McCollum 1985 that in order to reduce the degrees of freedom the CNS composes complex postural responses from a combination of stereotyped synergies When a synergy is used to recover balance in response to a perturbation selective cancellation of single muscle activation within the synergy might not be possible This raises the question of whether postural responses are also immutable when they are not necessarily helpful in the execution of a task Would it be possible to turn off these highly automated postural responses when functionally undesired or can they be integrated into the motor output in a meaningful way The answer could provide important knowledge of the functional organization of the central nervous system in such conflicts In order to obtain insight into this issue an experiment would be needed to study postural responses to identical mechanical perturbations but with distinct task demandsInstructionrelated modulation of longlatency stretch reflexes in response to identical singlejoint mechanical perturbations has been extensively studied by instructing participants either to resist or not to resist the perturbation eg Hammond 1956 Rothwell et al 1980 Gottlieb and Agarwal 1980 Capaday et al 1994 These studies demonstrated that response amplitudes of the stretched muscles except for the flexor pollicis longus were heavily modulated as a result of the instruction characterized by divergence between instructions almost immediately after response onset Whether such instructionrelated modulation also applies to automated postural responses is not knownIn the present study the effects of instruction on automated postural responses in neck trunk shoulder and leg muscles were investigated when people were either instructed to recover balance after being released from an inclined standing posture or not to recover at all and fall onto a safety mattress in the most comfortable way It was hypothesized that instruction would have a profound influence on the wholebody postural response Two possible scenarios were anticipated In the first scenario the instruction of not recovering balance would result in completely different muscle onset latencies activation sequences and amplitudes indicative of a separate motor program ie a set of muscle commands that are structured before a movement sequence begins to prepare for a safe landing and consequently a cancellation of the automated postural response In the second scenario the instruction would differentially affect response amplitudes with no or only limited changes in onset latencies or activation patterns This pattern of results would imply that the automated postural response could not be suppressed but could be tailored by differential feedback and/or feedforward gain settings in order to meet specific task constraints or demandsA total of 10 healthy young adults 3 women 7 men mean age 283 ± 43 years range 22–37 height 174 ± 013 m weight 68 ± 13 kg participated in this study They all provided written informed consent to participate and the study was approved by the Simon Fraser University Office of Research Ethicsa Schematic diagram of the experimental setup backward perturbation position illustrated Subjects stood supported at an angle of 15° to the vertical The tether was released unexpectedly inducing a balance perturbation b Raw data from a typical backward balance recovery trial dark gray area and black dashed lines and a fall trial light grey area and gray solid lines showing left sternocleidomastoid EMG SCL anterior deltoid EMG DAL rectus femoris EMG RFL right tibialis anterior TAR lateral movement of the left elbow marker and upward movement of the right foot marker Tether release is at time = 0 ms

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