Exp Brain Res (2014) 232:3515–3524 DOI 10.1007/s00221-014-4040-8
RESEARCH ARTICLE
Pain reported during prolonged standing is associated with reduced anticipatory postural adjustments of the deep abdominals Paul W. M. Marshall · Rick Romero · Cristy Brooks
Received: 12 June 2013 / Accepted: 9 July 2014 / Published online: 25 July 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Within the context of low back pain, the measurement of deep abdominal anticipatory postural adjustments (APAs) during rapid limb movement has received much interest. There is dispute about the association between APAs and back pain. Moreover, there is limited evidence examining compensatory postural adjustments (CPAs) in back pain. This study examined the relationship between APAs and CPAs with pain reported in the low back during 2 h of prolonged standing. Twenty-six participants with no history of severe back pain performed 2-h prolonged standing. APAs and CPAs of the deep abdominal muscles (transverse abdominis/internal obliques) were measured by surface electromyography during rapid shoulder flexion and extension. APAs and CPAs measured prestanding revealed symmetrical anticipatory activity, but an asymmetry between the different sides of the abdominal wall for CPAs. APAs and CPAs measured pre-standing were not associated with pain reported during standing. For the whole group, APA amplitudes were reduced post-standing during shoulder flexion (p = 0.005). Pain reported during standing was associated with the changes in APA amplitudes post-standing (rs = 0.43, p = 0.002). These findings support previous research using hypertonic saline injections to induce back pain that showed reduced APA amplitudes, and extends findings to suggest pain does not effect compensatory postural adjustments. Keywords Anticipatory motor activation · Compensatory postural adjustment · Transversus abdominis · Prolonged standing · Back pain P. W. M. Marshall (*) · R. Romero · C. Brooks School of Science and Health, University of Western Sydney, Locked Bag 1797, Sydney, NSW 2751, Australia e-mail: [email protected]
Introduction Anticipatory postural adjustments (APAs) are involuntary and automatic adjustments in muscle activation that occur prior to a predictable perturbation (Belen’kii et al. 1967; Aruin and Latash 1995). APAs of the transversus abdominis (TA) muscle have received particular interest for the research of postural control in low back pain (LBP). Nearly 20-years ago, delayed onsets of TA during rapid shoulder movements in patients with chronic LBP were first reported (Hodges and Richardson 1996). This delay was sufficiently late to not be classed as anticipatory or feedforward activation [muscle onset 85 dB @ 50 Hz, input impedance 200 MΩ) with 16-bit analog to digital conversion, sampled at 2,000 Hz (ADI instruments, Australia). Raw signals were initially filtered with a fourth-order Bessel filter between 20 and 500 Hz. Subsequent data processing was performed using MATLAB (MATLAB v7.1, The Mathworks Inc, Natwick, MA, USA). EMG signals were rectified and digitally filtered using a fourth-order, 100 Hz Savitsky–Golay low pass prior to onset determination. The difference between the onset of the right and left TA/IO with the right deltoid prime movers formed the basis of the APA analysis. Muscle onset determination Initially, signals were visually inspected by enlarging the collection screen to a resolution of 25 ms epochs to determine that onsets were not obscured by signal artifact introduced from cable movement or ECG contamination. A physiologically relevant analysis window from 150 ms pre-deltoid onset to 400 ms post-deltoid onset (window based on visual inspection of deltoid onset) was selected
3517
for calculation of muscle onsets and signal amplitudes. In addition, a 50 ms baseline epoch preceding the analysis window was selected to perform a computer-based verification of signal-to-noise ratios and for normalization of signal amplitudes. An initial algorithm was used to verify that an onset was present in the analysis window. The mean and SD for the 50 ms baseline epoch were calculated, and a threshold of 5 SD above this mean was established as a minimum signalto-noise ratio for computer-based verification that a muscle onset was present within the analysis window. Of the 1,040 abdominal muscle trials recorded, 201 (19.3 %) were discarded from prescreening of the signal for a verifiable onset based on a combination of the initial visual inspection and/or a signal-to-noise ratio
RESEARCH ARTICLE
Pain reported during prolonged standing is associated with reduced anticipatory postural adjustments of the deep abdominals Paul W. M. Marshall · Rick Romero · Cristy Brooks
Received: 12 June 2013 / Accepted: 9 July 2014 / Published online: 25 July 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Within the context of low back pain, the measurement of deep abdominal anticipatory postural adjustments (APAs) during rapid limb movement has received much interest. There is dispute about the association between APAs and back pain. Moreover, there is limited evidence examining compensatory postural adjustments (CPAs) in back pain. This study examined the relationship between APAs and CPAs with pain reported in the low back during 2 h of prolonged standing. Twenty-six participants with no history of severe back pain performed 2-h prolonged standing. APAs and CPAs of the deep abdominal muscles (transverse abdominis/internal obliques) were measured by surface electromyography during rapid shoulder flexion and extension. APAs and CPAs measured prestanding revealed symmetrical anticipatory activity, but an asymmetry between the different sides of the abdominal wall for CPAs. APAs and CPAs measured pre-standing were not associated with pain reported during standing. For the whole group, APA amplitudes were reduced post-standing during shoulder flexion (p = 0.005). Pain reported during standing was associated with the changes in APA amplitudes post-standing (rs = 0.43, p = 0.002). These findings support previous research using hypertonic saline injections to induce back pain that showed reduced APA amplitudes, and extends findings to suggest pain does not effect compensatory postural adjustments. Keywords Anticipatory motor activation · Compensatory postural adjustment · Transversus abdominis · Prolonged standing · Back pain P. W. M. Marshall (*) · R. Romero · C. Brooks School of Science and Health, University of Western Sydney, Locked Bag 1797, Sydney, NSW 2751, Australia e-mail: [email protected]
Introduction Anticipatory postural adjustments (APAs) are involuntary and automatic adjustments in muscle activation that occur prior to a predictable perturbation (Belen’kii et al. 1967; Aruin and Latash 1995). APAs of the transversus abdominis (TA) muscle have received particular interest for the research of postural control in low back pain (LBP). Nearly 20-years ago, delayed onsets of TA during rapid shoulder movements in patients with chronic LBP were first reported (Hodges and Richardson 1996). This delay was sufficiently late to not be classed as anticipatory or feedforward activation [muscle onset 85 dB @ 50 Hz, input impedance 200 MΩ) with 16-bit analog to digital conversion, sampled at 2,000 Hz (ADI instruments, Australia). Raw signals were initially filtered with a fourth-order Bessel filter between 20 and 500 Hz. Subsequent data processing was performed using MATLAB (MATLAB v7.1, The Mathworks Inc, Natwick, MA, USA). EMG signals were rectified and digitally filtered using a fourth-order, 100 Hz Savitsky–Golay low pass prior to onset determination. The difference between the onset of the right and left TA/IO with the right deltoid prime movers formed the basis of the APA analysis. Muscle onset determination Initially, signals were visually inspected by enlarging the collection screen to a resolution of 25 ms epochs to determine that onsets were not obscured by signal artifact introduced from cable movement or ECG contamination. A physiologically relevant analysis window from 150 ms pre-deltoid onset to 400 ms post-deltoid onset (window based on visual inspection of deltoid onset) was selected
3517
for calculation of muscle onsets and signal amplitudes. In addition, a 50 ms baseline epoch preceding the analysis window was selected to perform a computer-based verification of signal-to-noise ratios and for normalization of signal amplitudes. An initial algorithm was used to verify that an onset was present in the analysis window. The mean and SD for the 50 ms baseline epoch were calculated, and a threshold of 5 SD above this mean was established as a minimum signalto-noise ratio for computer-based verification that a muscle onset was present within the analysis window. Of the 1,040 abdominal muscle trials recorded, 201 (19.3 %) were discarded from prescreening of the signal for a verifiable onset based on a combination of the initial visual inspection and/or a signal-to-noise ratio
