慢性腰痛患者深层多裂肌EMG信号时频特征变化

目的:采用表面肌电(sEMG)分析系统结合丝电极观察慢性腰痛(cLBP)患者与健康对照者腰部深层多裂肌的肌电活动,比较二者的EMG信号时域和频域特征,探讨腰痛深层多裂肌功能状态变化。方法:选取22例cLBP患者(10男12女)和30例健康对照者(15男15女)采用一次性无菌针头导入不锈钢丝电极(直径0.16mm)至L4水平深层多裂肌后完成改良BST动作,采集多裂肌最大随意收缩时的EMG信号,比较两组平均肌电值(AEMG)、均方根值(RMS)、中位频率(MF)、平均功率频率(MPF)及其频率的特征差异。结果:cLBP腰部深层多裂肌EMG信号时域指标AEMG、RMS明显低于健康对照组(P0.001);频域指标MF、MPF明显高于对照组(P0.05),MFs、MPFs绝对值明显低于对照组(P=0.001/0.001)。结论:最大等长收缩运动中,cLBP患者腰部深层多裂肌电活动异常,可能是持续疼痛刺激引起多裂肌募集能力降低,耐力代偿性增加。     

帕金森病患者前臂肌群负重前后表面肌电特征

目的探讨负重前后帕金森病患者前臂表面肌电(sEMG)的变化特征。方法26 例帕金森病患者和28 例正常对照参加本研究。于坐位上肢屈肘无负重或负重1.5 kg 静态负荷下采用表面电极引导,记录sEMG信号,并进行线性时、频分析。结果无负重时,帕金森病患者屈腕屈肌的中位频率(MF)和平均功率频率(MPF)均大于正常对照(P<0.05),两组平均肌电值(AEMG)相比无显著性差异(P>0.05);负重1.5 kg 时,帕金森病患者AEMG显著小于正常对照(P<0.001),两组MF和MPF相比无显著性差异(P>0.05)。帕金森病患者无负重时MF和MPF均显著大于负重时(P<0.001),AEMG显著小于负重时(P<0.001);而正常对照MF和MPF在负重前后均无显著性差异(P>0.05),只有无负重时AEMG显著小于负重时(P<0.001)。结论帕金森病患者静止性震颤在sEMG上主要表现为频域参数的变化,而肢体乏力主要表现为时域参数的变化。     

脑卒中患者桥式运动下竖脊肌与多裂肌表面肌电图信号特征研究

目的:观察脑卒中患者桥式运动下竖脊肌与多裂肌表面肌电(s EMG)信号变化特征,为桥式运动的临床应用提供电生理理论基础。方法:选取20例健康同龄成年人(正常组)与20例脑卒中偏瘫患者(患者组),对两组受试者行桥式运动时的竖脊肌与多裂肌s EMG信号参数进行比较分析。结果:1正常组双侧竖脊肌、多裂肌配对比较,时域指标平均肌电值(AEMG)、频域指标中位频率值(MF)均无显著性差异(P0.05);患者组竖脊肌患侧AEMG值大于健侧、患侧MF值均小于健侧,差异有显著性意义(P0.05);患者组双侧多裂肌AEMG值、MF值组内配对比较,差异无显著性意义。2两组受试者同侧竖脊肌与多裂肌AEMG值的比值差异无显著性意义(P0.05)。结论:桥式运动可作为脑卒中患者躯干肌训练的一种方法,但须注意训练时双侧竖脊肌的均衡性。     

慢性腰痛患者深层多裂肌非线性肌电信号的特征分析

目的 采用非线性动力学方法提取慢性腰痛（cLBP）患者与健康对照者深度多裂肌不同状态下肌电图(EMG)信号中所蕴含的非线性动力学信息，比较二者的非线性指标Lempel-Ziv（LZ）复杂度的变化特点。 方法 选取符合标准的cLBP患者25例设为cLBP组，另选取性别、年龄、教育程度等相匹配的正常受试者28例设为正常组。采用sEMG结合丝电极记录2组受试者腰部L4深层多裂肌EMG信号，分析静息状态、最大肌力、耐力收缩、持续收缩后放松等4种不同状态下多裂肌电非线性特征LZ复杂度的差异，并比较cLBP组腰部痛侧与非痛侧多裂肌LZ复杂度的差异性。采用Pearson相关分析cLBP组的疼痛持续时间、疼痛强度和功能障碍指数与LZ复杂度的相关性。 结果 cLBP组患者的VAS评分平均为（4.00±1.04）分，持续疼痛时间平均为（5.96±4.69）年，ODI指数平均为（17.12±10.49）。cLBP组患者肌电信号采集各时间点（进针时、退针后、肌肉收缩时和动作终止时）的VAS评分均显著高于正常组，差异均有统计学意义（P＜0.01）。cLBP组在肌力测试、耐力测试和持续收缩后放松的深层多裂肌LZ复杂度与正常组比较，差异均有统计学意义（P＜0.01）。2组受试者肌力测试、耐力测试和持续收缩后放松的深层多裂肌LZ复杂度与组内静息状态比较，差异均有统计学意义（P＜0.01）；2组受试者持续收缩后放松的深层多裂肌LZ复杂度与组内肌力测试和耐力测试时比较，差异均有统计学意义（P＜0.01）。cLBP组疼痛持续时间与其深层多裂肌最大等长收缩（肌力测试时和耐力测试时）的LZ复杂度呈显著负相关（P＜0.01）。 结论 持续的疼痛刺激会影响大脑对深层多裂肌的协调控制，进而导致中枢对核心稳定肌肉的控制能力下降，提示中枢参与核心稳定肌调控障碍可能是cLBP病因机制的关键因素。     

应用表面肌电图评定乒乓球运动员的上肢肌疲劳

目的:对北京市优秀乒乓球运动员进行提拉弧圈球训练时握拍上肢主要做功肌肉表面肌电(surfaceelectromyography,sEMG)信号进行分析,利用时、频域指标的变化来评价上肢肌疲劳情况。方法:北京市乒乓球队7名男性运动员,进行乒乓球训练的主要内容为提拉弧圈球训练,采集准备活动后训练开始与训练结束前各20min的握拍上肢:肱三头肌、肱二头肌、肱桡肌、三角肌、腕屈肌和腕伸肌肌电信号,利用Megawin软件进行平均功率频率(meanpowerfrequency,MPF),中位频率(medianfrequency,MF),均方根振幅(root-mean-square,RMS)的分析,两个实验测试阶段的频域和时域值的差异性利用t检验进行统计处理。结果:第2阶段所测六块肌肉的MPF,MF均比第1阶段的明显降低(P<0.001),具有非常显著性差异;第2阶段与第1阶段的RMS值相比,所测肌肉的RMS值变化趋势不稳定。结论:频域指标MPF,MF可较敏感地反映乒乓球训练上肢肌肉功能状况,可作为评价肌肉动态负荷水平的生理学指标。     

脑卒中患者下肢肌肉动态运动负荷下表面肌电信号特征研究

目的:观察动态运动负荷对脑卒中患者下肢肌肉表面肌电信号变化的影响,探讨在动态运动负荷下脑卒中患者_F肢肌肉表面肌电信号特征.方法:11例脑卒巾患者和10例正常人(对照组)参加本实验研究,利用动态运动负荷诱发双侧下肢屈伸膝关节,采用表面电极引导和记录肌电信号并进行线性时、频分析.结果:两组各活动肢体间比较除AEMG斜率均值外MF、MPF、AEMG及MF斜率及MPF斜率均值差异无显著性意义;各活动肌肉间比较MF、MPF、AEMG均值差异有显著性意义,而MF斜率、MPF斜率及AEMG斜率均值无显著性差异;活动肢体因素四个水平间比较除偏瘫组患侧与健侧之间的MF和AEMG、AEMG斜率均值和偏瘫组患侧与正常组左侧的MPF斜率均值有显著性差异外,其余参数在偏瘫组与正常组组内、组间的差异无显著性差异;活动肌肉因素4组肌肉间对比:主动肌与拮抗肌以及协同肌AEMC、MPF差异有显著性,协同肌之间MF、MPF差异有非常显著性,拮抗肌与协同肌差异无显著性.结论:采用快速傅立叶变换方法进行线性功率谱分析和平均肌电值及疲劳试验分析并不能很好地反映动态运动负荷下脑卒中患者下肢肌肉表面肌电信号变化,特别是活动肢体因素,原因尚待深入研究.     

慢性非特异性下背痛患者悬吊运动疗法治疗后表面肌电信号的变化

摘要 目的：研究慢性非特异性下背痛患者（CNLBP）悬吊运动疗法治疗前后竖脊肌、多裂肌的表面肌电信号的变化特征。 方法：15例CNLBP患者,给予患者每周3次,持续4周的悬吊运动疗法（SET）治疗,分别在治疗前后采集患者在做半桥动作时痛侧竖脊肌和多裂肌的肌电信号,取时域指标：平均肌电值（AEMG）及频域指标：平均功率频率（MPF）、中位频率（MF）,进行统计学比较。 结果：治疗后竖脊肌、多裂肌痛侧频域指标MF、MPF较治疗前下降(治疗前MF 74.95±6.85、MPF 98.10±5.45,治疗后MF 62.90±4.02、MPF 83.89±3.84);痛侧时域指标AEMG较治疗前增高(治疗前AEMG 48.76±5.08,治疗后AEMG 86.27±9.43),差异有显著性意义（P<0.05）。 结论：SET治疗后,CNLBP患者痛侧竖脊肌、多裂肌的疲劳程度下降,收缩能力增强。     

恢复期脑卒中患者膝关节拮抗肌群在动态运动负荷下对协同收缩率的影响

目的观察动态运动负荷对脑卒中患者膝关节拮抗肌群协同收缩率变化的影响。方法11例脑卒中患者(偏瘫组)和14例正常人(正常组)利用动态运动负荷诱发双侧下肢屈伸膝关节,采用表面电极引导和记录肌电信号并进行线性时、频分析。结果偏瘫组与正常组组内中位频率(MF)、平均功率频率(MPF)、平均肌电值(AEMG)膝关节拮抗肌群协同收缩率均值变化无显著性差异(P>0.05);组间比较,MF膝关节拮抗肌群协同收缩率均值变化无显著性差异(P>0.05);偏瘫组MPF膝关节拮抗肌群协同收缩率明显增高,AEMG膝关节拮抗肌群协同收缩率明显降低(均P<0.01)。结论MPF、AEMG膝关节拮抗肌群协同收缩率能反映动态运动负荷下脑卒中患者下肢肌肉表面肌电信号变化,而MF膝关节拮抗肌群协同收缩率并不能很好地反映动态运动负荷下脑卒中患者下肢肌肉表面肌电信号变化。     

应用表面肌电图评定乒乓球运动员的上肢肌疲劳

目的：对北京市优秀乒乓球运动员进行提拉弧圈球训练时握拍上肢主要做功肌肉表面肌电(surface electromyography，sEMG)信号进行分析，利用时、频域指标的变化来评价上肢肌疲劳情况。方法：北京市乒乓球队7名男性运动员，进行乒乓球训练的主要内容为提拉弧圈球训练，采集准备活动后训练开始与训练结束前各20min的握拍上肢：肱三头肌、肱二头肌、肱桡肌、三角肌、腕屈肌和腕伸肌肌电信号，利用Megawin软件进行平均功率频率(mean power frequency，MPF)，中位频率(median frequency，MF)．均方根振幅(root—mean—square，RMS)的分析，两个实验测试阶段的频域和时域值的差异性利用t检验进行统计处理。结果：第2阶段所测六块肌肉的MPF，MF均比第1阶段的明显降低(P&;lt;0．001)，具有非常显著性差异；第2阶段与第1阶段的RMS值相比，所测肌肉的RMS值变化趋势不稳定。结论：频域指标MPF，MF可较敏感地反映乒乓球训练上肢肌肉功能状况，可作为评价肌肉动态负荷水平的生理学指标。     

sEMG信号变化的对侧负荷效应

目的：观察一侧肢体疲劳负荷条件下对侧相应肌肉的sEMG信号变化特征。方法：10名女性受试者完成两臂不同负荷等长疲劳实验，同时记录两侧肱二头肌sEMG信号并考察线性指标平均肌电值（AEMG），中位频率（MF）和非线性指标复杂度（C（n））、确定性线段百分比（％DET）的变化特征。结果：30％受试者出现对侧肌肉MF斜率的随变现象，且观察臂MF下降百分数为负荷臂下降百分数的50％以上；对所有受试者对照组和实验组各指标变化斜率的均值进行配对t检验，MF，C（n）及％DET差异无显著性意义（P〉0．05），而AEMG斜率存在显著性差异（t=-4．342．P〈0．01）。结论：一侧肱二头肌等长疲劳负荷对对侧未疲劳肱二头肌AEMG斜率变化有显著影响，存在对侧肌肉MF斜率的随变现象但具有个体差异性，这可能是中枢神经双侧交叉控制策略的结果。     

Surface EMG electrodes do not accurately record from lumbar multifidus muscles

OBJECTIVE: This study investigated whether electromyographic signals recorded from the skin surface overlying the multifidus muscles could be used to quantify their activity. DESIGN: Comparison of electromyography signals recorded from electrodes on the back surface and from wire electrodes within four different slips of multifidus muscles of three human subjects performing isometric tasks that loaded the trunk from three different directions. BACKGROUND: It has been suggested that suitably placed surface electrodes can be used to record activity in the deep multifidus muscles. METHODS: We tested whether there was a stronger correlation and more consistent regression relationship between signals from electrodes overlying multifidus and longissimus muscles respectively than between signals from within multifidus and from the skin surface electrodes over multifidus. RESULTS: The findings provided consistent evidence that the surface electrodes placed over multifidus muscles were more sensitive to the adjacent longissimus muscles than to the underlying multifidus muscles. The R(2) for surface versus intra-muscular comparisons was 0.64, while the average R(2) for surface-multifidus versus surface-longissimus comparisons was 0.80. Also, the magnitude of the regression coefficients was less variable between different tasks for the longissimus versus surface multifidus comparisons. CONCLUSIONS: Accurate measurement of multifidus muscle activity requires intra-muscular electrodes. RELEVANCE: Electromyography is the accepted technique to document the level of muscular activation, but its specificity to particular muscles depends on correct electrode placement. For multifidus, intra-muscular electrodes are required.     

Motor Training of the Lumbar Paraspinal Muscles Induces Immediate Changes in Motor Coordination in Patients With Recurrent Low Back Pain

Recurrent low back pain (LBP) is associated with altered motor coordination of the lumbar paraspinal muscles. Whether these changes can be modified with motor training remains unclear. Twenty volunteers with unilateral LBP were randomly assigned to cognitively activate the lumbar multifidus independently from other back muscles (skilled training) or to activate all paraspinal muscles with no attention to any specific muscles (extension training). Electromyographic (EMG) activity of deep (DM) and superficial multifidus (SM) muscles were recorded bilaterally using intramuscular fine-wire electrodes and that of superficial abdominal and back muscles using surface electrodes. Motor coordination was assessed before and immediately after training as onsets of trunk muscle EMG during rapid arm movements, and as EMG amplitude at the mid-point of slow trunk flexion-extension movements. Despite different intentions of the training tasks, the pattern of activity was similar for both. After both training tasks, activation of the DM and SM muscles was earlier during rapid arm movements. However, during slow trunk movements, DM and SM activity was increased, and EMG activity of the superficial trunk muscles was reduced only after skilled training. These findings show the potential to alter motor coordination with motor training of the lumbar paraspinal muscles in recurrent LBP.     

Muscle activity onset in the lumbar multifidus muscle recorded simultaneously by ultrasound imaging and intramuscular electromyography

BACKGROUND: Delayed anticipatory muscle activity response in deep abdominal and back muscles has been observed in patients with low back pain, indicative of a pathological condition. Muscle activity onset is traditionally recorded by intramuscular electromyography, but there is a need for a non-invasive and less cumbersome recording method in large clinical studies. An experimental study was carried out to explore whether high-frame rate m-mode ultrasound could measure anticipatory muscle responses ("onset") in the lumbar multifidus muscle reliably and comparably accurate to intramuscular electromyography. METHODS: Muscle activity onset was recorded by ultrasound m-mode and intramuscular electromyography. Ultrasound m-mode with a temporal resolution of 500 s(-1) (frames per second) was used to record rapid movements caused by muscle deformations in multifidus. In ultrasound m-mode, the frequency of each echo signal from 0.15 mm incremental depth levels is analysed. The frequency of these signals is proportional to the velocity of the interrogated tissue. The mean amplitude of the high-pass filtered echo signals within a pre-set depth range was plotted against time, and used to indicate onset. The results were compared to muscle activity onset in the multifidus recorded simultaneously by intramuscular electromyography. FINDINGS: High inter-rater agreement was found for visual determination of onset within both methods. The smallest detectable difference was 21 and 24 ms for electromyography and the ultrasound methods, respectively. The ultrasound m-mode method recorded muscle activity onset in the deep multifidus on average 16 ms (SD 21) later than intramuscular electromyography. For single trials, large variation and thus unacceptable method agreement was found. INTERPRETATION: Ultrasound m-mode imaging at high time resolution can detect onset of muscle activity comparably accurate to intramuscular electromyography, but with a small systematic delay that should be corrected for in onset determination by m-mode ultrasound. Regardless of recording method, onset estimates should be based on averaged values of repeated trials. Further studies are needed to explore the applicability of the ultrasound method in clinical settings.     

腰椎间盘突出症患者首次核心稳定性训练前后表面肌电信号分析

目的：分析腰椎间盘突出症患者首次核心稳定性训练前后即刻的表面肌电信号特征。方法收集诊断明确且符合纳入标准的腰椎间盘突出症患者30例,均在专业的治疗师指导下进行一次核心稳定性训练,应用Biering-Sorensen腰背肌等长收缩测试方法,于腰部两侧竖脊肌及多裂肌处记录核心稳定性训练前和训练后即刻的表面肌电信号,并提取平均肌电值（AEMG）、中位频率值（MF）进行统计学分析。结果训练前的痛侧竖脊肌MF值、多裂肌MF值、多裂肌AEMG值均小于非痛侧,差异均有统计学意义（t分别=3.44、6.06、4.02,P均＜0.05）;训练后的痛侧竖脊肌MF值、多裂肌MF值、多裂肌AEMG值仍小于非痛侧,差异均有统计学意义（t分别=2.24、6.27、4.12,P均＜0.05）;训练后痛侧竖脊肌与多裂肌AEMG值较训练前增大,但均差异无统计学意义（t分别=1.65、1.23,P均＞0.05）;训练后痛侧多裂肌MF值较训练前低,差异有统计学意义（t=3.91,P＜0.05）。结论核心稳定性训练可激活腰椎间盘突出症患者腰背部核心肌群,可针对性的应用于深层核心肌群多裂肌的强化训练。     

Multiplying linear dimension techniques may predict the cross-sectional area of multifidus muscle at all levels of lumbar spine

Objective: This study was designed to find out the relationship between the multiplied linear dimensions (MLD) measurement and the cross-sectional area (CSA) measurement of the lumbar multifidus muscle. Methods: Sixty healthy female subjects participated in this study. The CSA of the lumbar multifidus muscle from L2 to L5 was bilaterally measured by tracing around the muscle margins with an on- screen cursor. The linear dimensions including lateral dimension (LD), anteroposterior dimension (APD) and MLD of the muscles were also calculated at the same levels of lumbar vertebrae. Results: The linear regression between CSA and MLD for each vertebral level was significant (r=0.89 to 0.97, p<0.0001). In addition, there was significant correlation between CSA and APD (r=0.69 to 0.87, p<0.05).Conclusions: MLD method can be used to predict the CSA of the lumbar multifidus muscle. The method described for assessing the multifidus muscle is a potentially valuable, quick and easy way to evaluate muscle size at different levels of the lumbar vertebrae in clinical practice.     

Selective Activation of Lumbar Paraspinal Muscles during Various Exercises in the Prone Position as Measured by EMG

[Purpose] This study compared the effects of three exercises performed in a prone position on the selective activation of the lumbar erector spinae (LES) and lumbar multifidus (LM) muscles in healthy males to investigate the effective method for selective activation of the LM. [Subjects] Twenty-two healthy males were recruited. Surface EMG data were collected from the right LES and LM muscles during three exercises: 1) trunk extension, 2) hip extension, and 3) the arm lift. [Results] The ratio of LM to LES EMG activity during hip extension was higher than those during trunk extension and the arm lift. [Conclusion] Hip extension in a prone position may be effective for selective activation of the lumbar multifidus muscles in healthy males.Key words: Lumbar erector spinae, Lumbar multifidus, Selective activation     

慢性腰痛患者站立位躯干屈伸运动时腰背肌的功能变化研究

目的:研究慢性腰痛(CLBP)患者在站立位矢状面屈伸运动时腰背肌功能状况的变化,揭示其改变的可能机制及临床意义。方法:慢性非特异性腰痛患者25例和健康人25例,在站立位进行躯干的屈曲伸展运动,运用表面肌电图(sEMG)仪和摄像系统同步采集记录双侧L2/3、L4/5水平最长肌、多裂肌在站立位、前屈运动、完全屈曲及回到直立位不同运动时相的sEMG值。结果:两组受试对象站立位时,最长肌及多裂肌均方根(RMS)值差异无显著性意义(P>0.05);前屈运动时,最长肌及多裂肌RMS值增大,完全屈曲时,最长肌及多裂肌RMS值减小,腰痛组在前屈运动和完全屈曲时,最长肌和多裂肌RMS值较健康对照组增大,差异具有显著性意义(P<0.05);由屈曲位回至直立位时,腰痛组最长肌和多裂肌的RMS值明显小于健康对照组,差异具有显著性意义(P<0.05);腰痛组最长肌和多裂肌的屈曲-放松比较健康对照组降低,差异具有显著性意义(P<0.05);运动时相对两组受试对象的最长肌和多裂肌RMS值影响均有显著性意义(P<0.05)。结论:在躯干屈伸运动中,健康人腰背肌存在屈曲-放松现象(FRP),腰痛患者腰背肌功能发生疼痛适应性改变,表现为屈曲-放松反应缺如和主动活动机能不足。     

Measurement of lumbar multifidus muscle contraction with rehabilitative ultrasound imaging

Rehabilitative Ultrasound Imaging (RUSI) has been validated as a noninvasive method to measure activation of selected muscles. The purpose of this study was to determine the relationship between muscle thickness change, as measured by ultrasonography, and electromyography (EMG) activity of the lumbar multifidus (LM) muscle in normal subjects. Bipolar fine wire electrodes were inserted into the LM at the L4 level of five subjects. Simultaneous EMG and RUSI data (muscle thickness) were collected while subjects performed increasingly demanding postural response tasks thought to activate the LM muscle. To determine the relationship between muscle thickness change and EMG activity, the normalized EMG data were correlated to normalized RUSI data. To determine if the tasks increased the demand on the LM, the mean EMG data were compared over each of the four tasks. Muscle thickness change as measured by RUSI was highly correlated with EMG activity of LM in asymptomatic subjects (r=.79,P<.001). Mean EMG data showed increasing levels of activation across tasks (19-34% of maximum voluntary isometric contraction (MVIC)). The results of the repeated measures ANOVA demonstrated theses differences were significant (F(3,12)=25.39,P<.001). Measurement of muscle thickness change utilizing RUSI is a valid and potentially useful method to measure activation of the LM muscle in a narrow range (19-34% of MVIC) in an asymptomatic population.     

Different ways to balance the spine in sitting: Muscle activity in specific postures differs between individuals with and without a history of back pain in sitting

BackgroundPrevious research explored muscle activity in four distinct sitting postures with fine-wire electromyography, and found that lumbar multifidus muscle activity increased incrementally between sitting with flat thoracolumbar and lumbar regions, long thoracolumbar lordosis, or short lordosis confined to the lumbar region. This study used similar methods to explore whether people with a history of low back pain provoked by prolonged sitting used different patterns of trunk muscle activity in specific postures.MethodsFine-wire electromyography electrodes were inserted into the right lumbar multifidus (deep and superficial), iliocostalis (lateral and medial), longissimus thoracis and transversus abdominis muscles. Superficial abdominal muscle activity was recorded with surface or fine-wire electrodes. Electromyography amplitude was compared between postures for the back pain group and observations were contrasted with the changes previously reported for pain-free controls. For comparison between groups normalised and non-normalised electromyography amplitudes were compared.FindingsIndividuals with a history of back pain demonstrated greater activity of the longissimus thoracis muscle in the long lordosis compared with the flat posture [mean difference (95% CI): 46.6 (17.5–75.7)%, normalised to sitting posture peak activity], but pain-free participants did not [mean difference: 7.7 (minus 12–27.6)%]. Pain-free participants modulated lumbar multifidus activity with changes in lumbar curve, but people with a history of pain in prolonged sitting did not change multifidus activity between the long and short lordotic postures.InterpretationIn clinical ergonomic interventions that modify spinal curves and sagittal balance in sitting, the muscle activity used in those postures may differ between people with and without a history of back pain.