The relationship between psychological factors and performance on the Biering-Sørensen back muscle endurance test

The Spine Journal 11 (2011) 849–857

Clinical Study

The relationship between psychological factors and performance on the Biering-Sørensen back muscle endurance test Anne F. Mannion, PhDa,*, David O’Riordan, BSca, Jiri Dvorak, MDa, Youssef Masharawi, PhDb a Spine Center, Schulthess Klinik, Lengghalde 2, 8008 Z€urich, Switzerland Spinal Research Laboratory, Department of Physiotherapy, The Stanley Steyer School of Health Professions, Sackler Faculty of Medicine, Tel Aviv University, Lebanon St., Ramat Aviv, Tel Aviv 69978, Israel Received 14 January 2011; revised 19 May 2011; accepted 4 August 2011

b

Abstract

BACKGROUND CONTEXT: Many studies report an association between low back pain (LBP) and reduced back muscle endurance and consider this to indicate muscular dysfunction. However, few have investigated the potentially confounding influence of psychological factors on performance during such endurance tests. PURPOSE: This study examined whether psychological factors were associated with ‘‘underperformance’’ on the Biering-Sørensen (BS) test (ie, not performing as well as one is physiologically capable of). STUDY DESIGN/SETTING: Cross-sectional study of the baseline data of patients with chronic (O3 months) nonspecific LBP (cLBP) before participation in a clinical trial of exercise therapy. PATIENT SAMPLE: One hundred forty-eight patients with cLBP (43% men; age, 45610 years). OUTCOME MEASURES: The time for which the modified BS isometric endurance test could be performed to exhaustion minus the time that would have been predicted based on the rate of decline in median frequency of the surface electromyographic (EMG) signal recorded bilaterally from the erector spinae at L3 and L5. METHODS: Back pain and disability, psychological disturbance, catastrophizing, fear-avoidance beliefs, back beliefs, and exercise self-efficacy were measured using validated questionnaires. Patients performed the BS test to exhaustion while physiological muscle fatigability was measured from continuous surface EMG recordings. RESULTS: Multivariable regression analysis controlling for gender revealed that greater psychological disturbance (p5.003) and more negative back beliefs (p5.015) were unique predictors of the extent of ‘‘underperformance,’’ accounting for 22.3% variance in expected endurance time minus actual time. CONCLUSIONS: It is important that the underlying nature (psychological or physiological) of performance deficits be identified during such tests because this may influence the interpretation of prospective studies reporting risk factors for LBP and dictate the particular treatment or interventional approach required to remedy the situation in individuals with LBP. Ó 2011 Elsevier Inc. All rights reserved.

Keywords:

Back muscle endurance; Muscle fatigue; Electromyography; Psychological disturbance; Chronic low back pain

Introduction The relationship between back muscular endurance and low back pain (LBP) has been examined both prospectively—

FDA device/drug status: Not applicable. Author disclosures: AFM: Nothing to disclose. DOR: Nothing to disclose. JD: Nothing to disclose. YM: Nothing to disclose. * Corresponding author. Spine Center, Schulthess Klinik, Lengghalde 2, 8008 Z€ urich, Switzerland. Tel.: (41) 44-385-7584; fax: (41) 44-3857590. E-mail address: [email protected] (A.F. Mannion) 1529-9430/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2011.08.004

investigating endurance as a predictor of first-time LBP [1–3]—and on a cross-sectional basis, examining differences between patients with LBP and controls (eg, Refs. [4–9]). Muscular endurance is commonly defined as the ‘‘ability to sustain a given force or power output’’ [10], with performance being measured by the time for which the load can be sustained. The ‘‘given force output’’ is typically set as a percentage of the maximum voluntary contraction, or as a standard absolute load, or in relation to body dimensions. The use of a maximum voluntary contraction can be problematic as it may pose a risk of injury [11] and may also, because of the fear of injury or

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Context Muscle endurance or other performance tests are often used in cross-sectional and longitudinal studies to (1) compare patients with lower back pain to asymptomatic control group patients and (2) determine functional capacity. Although psychological factors have been shown to adversely affect patient performance on physical capacity tests among patients with low back pain, the potential confounding effects of such factors on muscle endurance test results have not been explored. Contribution The authors found that psychological disturbance (measured by combined scores of the modified somatic perception questionnaire and the modified Zung depression questionnaire) and more negative beliefs about back trouble (measured by the Back Beliefs Questionnairre) were predictive of ‘‘underperformance’’ on the Biering-Sørensen back muscle endurance test among 148 patients with chronic nonspecific low back pain. Pain intensity, frequency, and duration were not predictive, whereas female gender and older age were associated with underperformance. Implication Results from muscle endurance and other performance tests cannot be assumed to be due only to physical factors such as muscle function or specific physiological characteristics of the back muscles. The validity of socalled functional tests for determining disability or predicting prognosis is limited in patients with a significant underlying psychological disturbance. —The Editors

pain, lead to suboptimal ‘‘maximal’’ efforts and hence underestimation of the load to be used in subsequent submaximal testing. Because the endurance time during a fatiguing task is dependent on the relative force output [12,13], this has important implications for the comparison between individuals. The use of a fixed absolute submaximal load for the endurance test [14] obviates the hazards of maximal testing but clearly places smaller and weaker individuals at a disadvantage, which also challenges the validity of interindividual comparisons. Consequently, tests, such as the Biering-Sørensen (BS) test [3], that use one’s own body weight to create the resistance, have become popular [15]. Because, in general, an individual’s strength relates reasonably well to his/her body mass, the test aims to present all individuals with a similar proportional load in relation to their strength. Obviously, individual differences in relative loading will arise when subjects are ‘‘atypically proportioned’’—for example, an overly heavy upper body

in relation to back strength, or vice versa—but overall the test is probably one of the most appropriate of its type, especially for use in clinical populations. However, even if an appropriate test load can be set, muscular endurance tests suffer from further methodological problems that complicate the comparison between individuals: the endurance time may be determined not only by the intrinsic fatigability of the muscle but also by the factors, such as motivation, tolerance of the discomfort of fatiguing muscles, and, especially in the clinical situation, pain, or fear of pain. In a previous study of back-healthy individuals performing the BS test, the relationship between psychological disturbance and performance was examined: for each individual, the time completed on the endurance test was compared with the endurance time that would have been predicted for them on the basis of the rate of change in electromyographic (EMG) median frequency (MF) recorded from their back muscles [16]. Such EMG changes represent a valid objective indicator of intrinsic muscle fatigue [17,18] that is strongly related to mechanical [12,13] and metabolic [19,20] indices of the same phenomenon. In the previous study [16], the expected time was determined from a regression equation that linked endurance time to EMG changes in the group of subjects examined, in such a manner that ‘‘underperformance’’ or ‘‘overperformance’’ was determined relative to the other volunteers in the group. The extent to which a subject’s endurance time exceeded or fell short of the predicted time was significantly correlated with their score for psychological disturbance [21]. No such studies have been carried out in patients with LBP (in whom psychological distress is often pronounced), yet the test is commonly used to make inferences about ‘‘muscle function’’ and the ‘‘degree of deconditioning’’ in these patients [4–9]. Furthermore, the influence on ‘‘underperformance’’ of other psychological factors and beliefs has not been investigated to date. The present study sought to examine whether in patients with chronic LBP ‘‘underperformance’’ on the BS test, determined using the method described above, was related to various psychological factors, while taking into account potential confounders, such as gender and age. We hypothesized that an LBP patient’s willingness to drive himself/herself to a limit commensurate with their true muscular capacity might depend not only on psychological disturbance but also on factors such as, self-efficacy, catastrophizing, and fear-avoidance beliefs (FABs). Hence, these factors were also taken into consideration in predicting ‘‘underperformance.’’

Methods Patients One hundred forty-eight patients with chronic nonspecific LBP, diagnosed according to the diagnostic triage reported in the current European guidelines [22], took part

A.F. Mannion et al. / The Spine Journal 11 (2011) 849–857 Table 1 Sociodemographic and baseline characteristics of the study group (n5148) Parameter

Mean or percent

Age (mean6SD) Gender (% male:female) Highest education level (%) Junior high school/comprehensive school High school/sixth-form college/day release University

45.169.9 43:57 56 34 10

Work status (%) Full time Part time Retired/unemployed/homemaker

47 31 22

Heaviness of work load (%) Office work/sedentary Light manual handling Heavy manual handling

47 49 4

Involvement in disability claim (%)* LBP duration (mean6SD) (y) LBP intensity: highest (VAS) (mean6SD) LBP intensity: average (VAS) (mean6SD)

9.5 10.969.5 6.562.0 4.261.9

LBP frequency (%) Permanent Often Sporadic

50.0 40.5 9.5

Disability (Roland and Morris Questionnaire) (mean6SD)

7.864.6

SD, standard deviation; LBP, low back pain; VAS, visual analog scale. * Regardless of whether claim being considered or already submitted, granted, or turned down.

in the study. Their sociodemographic and clinical characteristics are shown in Table 1. They were participating in a randomized trial of different exercise therapies, and the present study concerns the data from their baseline assessments [23]. Most were attracted to the study to benefit from the subsequent treatment that they were to receive for their back pain (at no cost to them or their insurance). The recruitment methods used and the specific inclusion and exclusion criteria for participation in the study have been described in detail previously [24,25]. Briefly, the study was advertised in the local media, and patients (N5255) attended the hospital for examination of their suitability according to the admission criteria; 148 satisfied these criteria (with most of the rest presenting with acute/subacute rather than chronic LBP) and were recruited. The main inclusion criteria were less than 65 years old; more than 3 months’ LBP with/without referred pain (nonradicular) serious enough to require medical attention or work absence. The exclusion criteria were constant or persistent severe pain, pregnancy, previous spinal surgery, current nerve root entrapment accompanied by neurological deficit, spinal cord compression, tumors, severe structural deformity, severe instability, severe osteoporosis, inflammatory disease of the spine, spinal infection, severe cardiovascular/metabolic disease, and acute infection.

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The patients gave their signed informed consent to participate, and the study was approved by the University Ethics Committee. Questionnaire assessments Questionnaires inquired about the following factors:  Sociodemographic characteristics: age, gender, education level, work status, heaviness of work, involvement (past, current, or intended) in a disability claim.  Characteristics of LBP: intensity of highest pain and average pain over the last 6 weeks (visual analog scale), duration, frequency (never, sporadic, often, permanent).  LBP-related disability (Roland and Morris Questionnaire [26,27]. This is a 24-item questionnaire used to assess disability caused by LBP in relation to various daily functions.  Psychological disturbance, determined from the combined scores of the Modified Somatic Perception Questionnaire (MSPQ) [28] (inquires about the frequency of somatic symptoms experienced in the last week) and the modified ZUNG Depression Questionnaire [29], as previously validated [30].  Catastrophizing, using the six items on the catastrophizing subscale from the Coping Strategies Questionnaire [31].  Beliefs about physical/work activity being a cause of the patient’s back trouble and fears about the dangers of such activities when experiencing an episode of LBP (Fear-Avoidance Beliefs Questionnaire [32,33]).  Beliefs about back trouble (Back Beliefs Questionnaire [34]). This assesses beliefs about the ‘‘inevitability’’ of the future as a consequence of having back pain.  Exercise self-efficacy [35] with 12 items enquiring about how confident the patient is that he or she will carry out his/her exercise program despite various distractions [36]. All the questionnaires were available in German or had been adapted for the German language before the study. Biering-Sørensen isometric endurance test A modification of the original BS test [3] as described previously [16] was used to assess back muscle endurance. The patient was placed in a prone position on an examination couch with the lower body, from the superior border of the iliac crest downward, strapped to the couch. With hands touching the ears, elbows out to the side and level with the trunk, and the head in a neutral position, the patient was required to maintain the unsupported upper body in a horizontal position until, because of fatigue, he or she was no longer able to overcome the force of gravity. Verbal encouragement was given throughout the test (on an ‘‘as needed’’

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basis in the expert opinion of the assessors), and the endurance time was recorded to the nearest whole second. The tests were administered by two assessors working together from a pool of six physiotherapists/exercise scientists, with one and the same assessor (one of the authors) being in the pair for most tests. Surface EMG Surface EMG was used to quantify the rate of development of back muscle fatigue. Before placement of the EMG electrodes, the skin was abraded with special skin preparation pads (Biolect Cardioprep pads; MBS, Marlborough, Wiltshire, UK) and cleaned with alcohol. Pairs of surface electrodes were positioned bilaterally over the erector spinae muscle belly at the levels of L3 and L5 (2–4 cm lateral to the midline of the spine) with a center-to-center distance of 25 mm. A reference electrode (one accompanying each pair of recording electrodes) was attached approximately 10 cm lateral to the recording electrodes. Electromyographic signals were recorded using a bipolar EMG system (MC-1M; DBC International Ltd, Vantaa, Finland and MEGA Electronics, Kuopio, Finland) interfaced to a personal computer. Each of the four recording channels was equipped with a preamplifier that was attached to the reference electrode accompanying the recording electrode pair. The raw EMG signals were recorded at a sampling rate of 1,000 Hz; band-pass filtered (7–500 Hz); amplified (differential amplifier, common mode rejection ratio O130 dB; total gain 1,000; noise !1 uV); analog to digital converted (12-bit); and stored on a computer. The EMG power spectral density was computed using a Fast Fourier Transform algorithm for 1.0-second sampling periods with a 0.5-second sliding overlap. The MF was defined as that which divided the spectrum into two regions containing equal power. Data analysis The slope of the linear regression of EMG MF on time (MFgrad, normalized to the intercept [MFint]; %.second 1) gave a measure of fatigability [17] for each recording site for each individual. For the whole group, endurance time was then regressed on the MFgrad for the most fatigable region (ie, the region with the steepest MFgrad). This variable (expressed as log values) was the best predictor of endurance time within the group, consistent with previous reports [16]. Adequate test-retest reliability (intraclass correlation coefficient O0.7–0.8) for MFgrad (which reflects all potential sources of measurement error, from electrode placement to test positioning) has been reported in many previous studies [16,37–39]. Using the regression equation derived from the group data, the ‘‘expected’’ endurance time for a given individual’s MFgrad was determined (Figure). This was deducted from the ‘‘actual’’ endurance time to give an indication of ‘‘underperformance’’ or ‘‘overperformance’’ with respect to

Figure. Plot of the endurance time versus the EMG log greatest MF decline (EMG fatigability) for all patients. Using the regression equation depicting the relationship between these two measures for the group, it was possible to predict the expected endurance time associated with a given back muscle fatigability. For example, the blue vertical dotted line indicates an MF decline of approximately 0.75%/s accompanied by an actual endurance time (solid blue circle) of approximately 110 seconds (lower horizontal dotted line); the predicted endurance time for such an MF decline would be approximately 162 seconds (upper horizontal dotted line). The difference between these two values indicates this individual’s ‘‘underperformance’’ (approximately 52 seconds) compared with his/her peers. EMG, electromyographic; MF, median frequency.

the rest of the group. These values were also dichotomized to categorize patients as ‘‘underperformers’’ or ‘‘overperformers’’ depending on whether their endurance time fell short of or exceeded the endurance time predicted from the MFgrad. Unpaired t tests were used to examine the significance of the difference between underperformers and overperformers in their mean scores for the psychological questionnaires and various potential confounding factors. Forward stepwise multiple regression analysis was used to identify the factors that made a significant contribution to explaining the variance in performance (actual minus expected endurance time, as a continuous variable). Only the variables that were significant in bivariate analyses were included. To confirm the findings of the multiple regression model, a forward conditional logistic regression analysis was also carried out, with the dependent variable being performance dichotomized as ‘‘underperformance’’ and ‘‘overperformance.’’ Statistical significance was accepted at the 5% level.

Results Table 2 shows the mean values for the clinical variables (pain and disability), performance (endurance test time and EMG fatigability), and psychological attributes for the ‘‘underperformers’’ and ‘‘overperformers.’’ There was no significant difference between the groups for body mass index, average pain, pain frequency, pain duration, EMG fatigability, and FABs about work (each pO.05); FAB about physical activity showed a difference that was of borderline significance

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Table 2 Difference in psychological characteristics and other potential confounding factors between ‘‘overperformers’’ and ‘‘underperformers’’ in the BS test Variable Gender (% female) Body mass index Age (y) Greatest MF slope (any site) during BS test (%/s) BS test time (s) Average pain (0–10) Pain frequency (0–3) Pain duration (mo) Roland Morris disability (0–24) FABQ physical activity (0–24) FABQ work (0–42) BBQ (9–45) Psychological disturbance (ZUNG & MSPQ) (0–99) Catastrophizing (0–36) Exercise self-efficacy (12–84)

Overperformers (N575)

Underperformers (N573)

p Value for group differences

39

75

!.0001 .14 !.0003 .88 !.0001 .70 .92 .20 !.0001 .055 .54 .01 !.0001 .01 .001

23.4 42.2 0.361 159.3 4.2 2.4 119 6.3 12.8 15.2 30.0 14.1 10.3 72.8

(2.8) (9.9) (0.125) (48.4) (1.9) (0.7) (107) (4.1) (5.8) (12.3) (6.1) (8.6) (6.3) (9.6)

24.3 48.0 0.357 81.9 4.3 2.4 143 9.4 14.5 16.3 27.4 22.0 13.0 68.8

(4.1) (9.2) (0.148) (33.6) (1.8) (0.7) (120) (4.5) (4.8) (10.0) (5.6) (10.1) (6.4) (10.0)

MF, median frequency; BS test, Biering-Sørensen test; FABQ, Fear-Avoidance Beliefs Questionnaire; BBQ, Back Beliefs Questionnaire; ZUNG, ZUNG Depression Questionnaire; MSPQ, Modified Somatic Perception Questionnaire. Greatest MF slope during BS test, maximum MF slope at any site during the BS test; BS test time, BS test endurance time; FABQ, score on FABQ (about physical activity and work; higher scores indicate greater FABs); BBQ, score on BBQ (higher scores indicates healthier back beliefs); Psychological disturbance, measured by the sum scores of the ZUNG and the MSPQ; catastrophizing, catastrophizing subscale of the Coping Strategies Questionnaire.

(p5.055). The ‘‘underperformers’’ group comprised more females, was older, had a higher self-rated disability, more ‘‘negative’’ back beliefs, greater psychological disturbance, greater catastrophizing, and lower exercise self-efficacy compared with the ‘‘overperformance’’ group (each p!.05). For the psychological variables that showed a significant group difference, the effect sizes (mean group difference/standard deviation for the group as a whole) ranged from 0.41 (for catastrophizing) to 0.78 (for ZUNG and MSPQ). Multiple regression analysis of all the variables that were significant in bivariate analyses revealed that female gender, a greater level of psychological disturbance, and more negative back beliefs were each unique significant predictors of the degree of ‘‘underperformance’’ in the BS test (ie, the expected minus actual endurance time), together accounting for 22.3% variance (Table 3). Similar findings were obtained in multiple logistic regression analysis predicting the dichotomized BS test performance (underperformance vs. overperformance): greater

age, female gender, a greater level of psychological disturbance, and lower exercise self-efficacy were all unique significant predictors of underperformance (Table 4), with odds ratios ranging from 0.959 (95% confidence interval, 0.921–0.998) for self-efficacy to 1.081 for psychological disturbance (95% confidence interval, 1.033–1.130).

Discussion The present study sought to examine whether various psychological factors were able to predict underperformance during the BS test, when controlling for confounders, such as gender and age. The main findings were that a greater level of psychological disturbance and more negative back beliefs were significant predictors of the extent of underperformance; when performance was dichotomized as simply ‘‘under/overperformance,’’ psychological disturbance and exercise self-efficacy were identified as

Table 3 Results of the final step of the forward conditional multiple regression analysis to predict the degree of underperformance (predicted time minus actual endurance time; positive values5underperformance) during the Biering-Sørensen test

Independent variables (Constant) Gender (05F, 15M) Psychological disturbance Back beliefs

Unstandardized regression coefficients

95% CI for B

Standardized coefficients

Significance (p value)

B

CI low

CI high

59.3 25.96 1.18

8.9 41.01 0.42

109.8 10.91 1.95

0.267 0.248

.021 .001 .003

1.51

2.71

0.29

0.187

.015

Percent of explained variance Adjusted R2

Beta

22.3 CI, confidence interval.

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Table 4 Results of the multiple logistic regression analysis to identify predictors of membership of the underperformance group (05overperformance and 15underperformance) Unstandardized regression coefficients Independent variables Constant Age Gender (15M) Psychological disturbance Exercise self-efficacy

B 0.685 0.059 0.964 0.077 0.042

Significance

Odds ratio

95% CI for odds ratio

Standard error

p value

(Exp (B))

Lower

Upper

1.781 0.021 0.404 0.023 0.021

.701 .004 .017 .001 .042

0.504 1.061 0.381 1.081 0.959

1.019 0.173 1.033 0.921

1.105 0.841 1.130 0.998

CI, confidence interval.

significant predictors. Effect sizes for the significant group differences between the underperformers and overperformers ranged from 0.41 (for catastrophizing) to 0.78 (for ZUNG and MSPQ), which are considered medium to very large [40] and are comparable to or even greater than the effect sizes for the difference in endurance time between patients and controls [9], between men who do and men who do not develop LBP [3], and between cLBP patients before and after a program of rehabilitation [23,41]. In the multiple linear regression analysis predicting the extent of under/overperformance time, 22% of the variance was accounted for by all variables together (ie, an effect size of 0.28, where the effect size [f2] is given by R2/1 R2), which would also be considered a medium to large effect (where f2 values of 0.02, 0.15, and 0.35 are considered to indicate small, medium, and large effects, respectively [42]). With the limitations of any biological measurement (and its associated measurement error), it is often possible to identify only 25% to 50% of the variance of a relationship [43], and hence, within this context, we consider the findings to be not only statistically significant but also of at least some clinical relevance. Interestingly, body mass index, the rate of decline in MF per second (ie, the fatigability of the muscle per se [17,18]), pain intensity, the duration of LBP, and the frequency of LBP showed no association with under/overperformance, suggesting that these were not confounding factors. In contrast, gender was consistently associated with underperformance, and age was too, for some of the analyses. To the authors’ knowledge, no previous studies in the literature have examined this type of gender difference, but we postulate that it might reflect a lesser inclination of women and older individuals to drive themselves to the limit of their physiological capacity, in keeping with their declared preference for moderate rather than vigorous or strenuous physical activity [44]. It is important to point out that the identification of under/overperformance in a given individual rests on the establishment of a significant relationship between the EMG MF changes of the back muscles (recorded from four sites on the erector spinae) and the test endurance time. Although this relationship has been established in numerous studies in the past [12,13,16,45–47], it is not a perfect correlation and it is clear that back muscle fatigue is not the

only physiological determinant of endurance time. There is evidence that the hip extensor muscles also contribute to the maintenance of the upper body in the horizontal test position [39,48–50], and hence, interindividual differences in the fatigability of this muscle group may obscure the relationship between back muscle fatigue and test endurance time. However, fatigue in these muscles during the test is not as marked [39,48–50] and is a less good statistical predictor of test endurance time than is fatigue of the back muscles [50,51], suggesting that hip extensor fatigue is not likely to represent a major limiting factor to performance. Furthermore, it would appear that an individual with fatigable back muscles also has more fatigable hip extensors [51], and hence, the relative influence of this confounder would not be expected to be large. It is important to remember that the expected endurance time was based on a regression equation that linked endurance time to EMG changes in the same patient population, such that ‘‘over or underperformance’’ was only relative to the other patients in this study. For these purposes, it was considered to represent a valid measure to examine the influence of the psychological parameters within a given group, as a phenomenon per se. However, the equation cannot simply be applied to other individuals to ascertain from their MFgrad whether they are under- or overperforming. The findings of the present study confirm the earlier results in normal back-healthy individuals, in terms of both the regression equation describing the relationship between EMG fatigability and endurance time [16], and the influence of psychological disturbance as a predictor of underperformance [21]. In the latter study, the extent to which the endurance time either exceeded or fell short of the predicted time showed a significant correlation with psychological ‘‘disturbance,’’ which itself was one of the most significant predictors of first-time significant LBP [1]. This brings one to question whether, in risk factor studies in which back muscle endurance has been considered in isolation (ie, without corresponding assessment of psychological factors) [2,3], it may have earned its role in predictive models by virtue of its relationship with some of the important psychological factors. In other words, it is also serving as a sort of proxy or surrogate measure of psychological status rather than just an indicator of muscle function per

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se. In this sense, the importance in LBP of ‘‘fatigability,’’ as a physiological characteristic of the back muscles may have been overestimated. Indeed, in the study of Adams et al. [1], in multivariable analyses that included the psychological variables, the relationship between endurance and the development of LBP—that was apparent on bivariate analysis—was no longer evident. These findings highlight the fact that performance in the BS, as measured by endurance time alone, reflects more than just the intrinsic physiological fatigability of the trunk musculature, and this must be borne in mind when interpreting the results of studies based on this measure. Many previous studies, although not all [52,53], have demonstrated a statistically significant influence of psychological factors on performance, when the latter concerns differences in absolute capacity between individuals within a group. For example, Smeets et al. [54] reported that depression was a consistent (although not major) determinant of performance on a number of physical capacity tests, and Hirsch et al. [55] reported that cLBP patients who exhibited excessive illness behavior had significantly lower strength and motion during lumbar dynamometry tests. Similarly, Alschuler et al. [56] reported that depressive symptoms were negatively correlated (albeit weakly) with the amount of weight lifted on the Progressive Isoinertial Lifting Evaluation. However, none of these studies was able to distinguish between a genuine physiological incapacity to perform, for example, because of disuse effects on the size/quality of the muscles, as opposed to ‘‘underutilization’’ of the available physiological capacity. Real differences in physiological capacity may have masked any underlying influences of the psychological attributes; ideally, the authors should have examined the relationship with, for example, strength normalized to the unit muscle cross-sectional area or kilogram lean body mass rather than absolute strength (in Newton or Newton meter), or covaried for these size factors in the multivariable analysis, in an attempt to adjust for ‘‘available capacity.’’ Separating these influences to examine the unique effects of the psychological variables was the specific and novel focus of the present study. Our findings complement those of two studies in the literature that have taken a similar kind of approach in relation to the back muscles [57] or other muscles [58]. Hutten et al. [57] showed that a small group of cLBP patients with inconsistent (interpreted as ‘‘submaximal’’) performance during lumbar dynamometry tests reported a higher degree of psychological distress than did those patients whose performance was comparable to controls or less good than controls but still consistent and therefore maximal. Using a more valid measure of submaximal performance, the twitch superimposition technique [59]. Verbunt et al. [58] reported a similar phenomenon, namely that the cLBP patients with greater psychological distress showed lowered central activation of their skeletal muscles (quadriceps), resulting in underperformance during strength tests. It is

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interesting to speculate why patients with high levels of distress should underperform during physical capacity tasks. Hirsch et al. [55] suggested that poor performance may simply be a form of abnormal illness behavior, where the latter refers to an individual’s interpretations of the appropriate response to symptoms and is influenced by pre-existing belief systems (shaped by the social, cultural, and experiential context in which an individual resides), by dialog with others, and by societal norms and values [60]. Alschuler et al. [56] proposed that deficits in the ability of the depressed chronic pain patient to put forth maximal effort during physical activities and their decreased interest or motivation in engaging in effortful activity may reflect the known deficits in sustained attention prevalent in persons with depression [61]. Their own studies confirmed that depression influences function at least in part through reduced physiological effort during activity [56]. They also postulated that persons with depression may have more negative views about the outcome of their efforts or be more fearful that activity will cause more pain and discomfort. However, interestingly, neither FABs nor catastrophizing made a significant contribution to explaining submaximal performance in either the present study or that of Verbunt et al. [58]. In the present study, the mean level of self-rated disability (7.864.6) was only ‘‘moderate’’ and slightly lower than that typically reported in the literature for patients with nonspecific cLBP [62]. However, it was not vastly different from the values reported in previous studies of cLBP patients from the same geographical area (8.964.7 [63], 10.865.9 [64], 9.665.4 [65], and 8.964.8 [66]). In the present study, the participants were recruited from the local community and were not necessarily at the ‘‘peak of a bad episode’’ when they were recruited (as patients referred to treatment otherwise tend to be); this might explain their slightly lower Roland Morris scores. The psychological disturbance recorded in these patients was also relatively low. In patients consulting for treatment with a higher self-rated disability and greater psychological disturbance, the observed effects could be expected to be even greater. We strongly encourage further study in such patient groups. In conclusion, the present study revealed that during performance of the BS test, endurance time for a given level of back muscular fatigability is significantly influenced by psychological disturbance. It is important that the underlying nature (psychological or physiological) of performance deficits be identified because this may influence the interpretation of prospective studies reporting risk factors for LBP and dictate the particular treatment or interventional approach required to remedy the situation in individuals with LBP. Future studies seeking to examine muscular endurance as a risk factor for the development of LBP or as a dependent variable in assessing the effectiveness of exercise therapy programs should always examine whether the observed effects are still evident when psychological factors are controlled for, to avoid

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potentially erroneous conclusions regarding the role of muscular (dys)function.

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