Limits...
Automated postural responses are modified in a functional manner by instruction.

Weerdesteyn V, Laing AC, Robinovitch SN - Exp Brain Res (2008)

Bottom Line: The restoration of upright balance after a perturbation relies on highly automated and, to a large extent, stereotyped postural responses.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.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 goal-oriented way.

View Article: PubMed Central - PubMed

Affiliation: Department of Rehabilitation, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. v.weerdesteyn@reval.umcn.nl

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 goal-oriented way.

Show MeSH
EMG amplitudes of balance recovery (BR) trials minus the amplitudes in fall (F) trials in response to backward and leftward perturbations. Average differences (±SE) are shown as a function of time after tether release for bilateral sternocleidomastoid (SC), anterior deltoid (DA), posterior deltoid (DP), rectus abdominis (AB), rectus femoris (RF), and tibialis anterior (TA). Muscles on the left side of the body are shown as black dashed lines, muscles on the right are shown as gray solid lines. The arrows indicate at which instant EMG amplitudes started to deviate significantly between BR and F trials
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2279151&req=5

Fig3: EMG amplitudes of balance recovery (BR) trials minus the amplitudes in fall (F) trials in response to backward and leftward perturbations. Average differences (±SE) are shown as a function of time after tether release for bilateral sternocleidomastoid (SC), anterior deltoid (DA), posterior deltoid (DP), rectus abdominis (AB), rectus femoris (RF), and tibialis anterior (TA). Muscles on the left side of the body are shown as black dashed lines, muscles on the right are shown as gray solid lines. The arrows indicate at which instant EMG amplitudes started to deviate significantly between BR and F trials

Mentions: With respect to EMG amplitudes, there were no significant differences in baseline activity levels between BR and F trials [F(1,9) = 2.149, P = 0.177]. Hence, the analysis of response amplitudes was not compromised by instruction-related baseline differences. EMG amplitudes after tether release were generally higher in BR than in F trials and these instruction-related differences could often be detected shortly after onset (mostly within 40 ms). For backward trials, significant Instruction × Bin interactions were found for all muscles [values for F(7,63) ranging from 2.964 to 19.863, all P values <0.010], except ABL, ABR, and RFR [values for F(7,63) ranging from 0.433 to 1.815, P values > 0.100]. Post hoc contrasts revealed that EMG amplitudes started to increase more steeply in BR than in F trials at 60–80 ms after release for SCR, at 80–100 ms after release for SCL and TAL, followed by DAL, RFL, and TAR at 100–120 ms, and DAR at 120–140 ms [values for F(1,9) ranging from 5.946 to 18.575, P values between 0.002 and 0.037] (Fig. 3).Fig. 3


Automated postural responses are modified in a functional manner by instruction.

Weerdesteyn V, Laing AC, Robinovitch SN - Exp Brain Res (2008)

EMG amplitudes of balance recovery (BR) trials minus the amplitudes in fall (F) trials in response to backward and leftward perturbations. Average differences (±SE) are shown as a function of time after tether release for bilateral sternocleidomastoid (SC), anterior deltoid (DA), posterior deltoid (DP), rectus abdominis (AB), rectus femoris (RF), and tibialis anterior (TA). Muscles on the left side of the body are shown as black dashed lines, muscles on the right are shown as gray solid lines. The arrows indicate at which instant EMG amplitudes started to deviate significantly between BR and F trials
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2279151&req=5

Fig3: EMG amplitudes of balance recovery (BR) trials minus the amplitudes in fall (F) trials in response to backward and leftward perturbations. Average differences (±SE) are shown as a function of time after tether release for bilateral sternocleidomastoid (SC), anterior deltoid (DA), posterior deltoid (DP), rectus abdominis (AB), rectus femoris (RF), and tibialis anterior (TA). Muscles on the left side of the body are shown as black dashed lines, muscles on the right are shown as gray solid lines. The arrows indicate at which instant EMG amplitudes started to deviate significantly between BR and F trials
Mentions: With respect to EMG amplitudes, there were no significant differences in baseline activity levels between BR and F trials [F(1,9) = 2.149, P = 0.177]. Hence, the analysis of response amplitudes was not compromised by instruction-related baseline differences. EMG amplitudes after tether release were generally higher in BR than in F trials and these instruction-related differences could often be detected shortly after onset (mostly within 40 ms). For backward trials, significant Instruction × Bin interactions were found for all muscles [values for F(7,63) ranging from 2.964 to 19.863, all P values <0.010], except ABL, ABR, and RFR [values for F(7,63) ranging from 0.433 to 1.815, P values > 0.100]. Post hoc contrasts revealed that EMG amplitudes started to increase more steeply in BR than in F trials at 60–80 ms after release for SCR, at 80–100 ms after release for SCL and TAL, followed by DAL, RFL, and TAR at 100–120 ms, and DAR at 120–140 ms [values for F(1,9) ranging from 5.946 to 18.575, P values between 0.002 and 0.037] (Fig. 3).Fig. 3

Bottom Line: The restoration of upright balance after a perturbation relies on highly automated and, to a large extent, stereotyped postural responses.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.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 goal-oriented way.

View Article: PubMed Central - PubMed

Affiliation: Department of Rehabilitation, Radboud University Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. v.weerdesteyn@reval.umcn.nl

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 goal-oriented way.

Show MeSH