Association of Muscle Strength, Psychological Factors, and Central Sensitization With Movement-Evoked Pain in Patients With Nonspecific Chronic Low Back Pain: The BACKFIT Project

Abstract

Background:

People with chronic pain might evade certain movements to prevent their experience of pain. Movement-evoked pain (MEP) might induce lower functionality during daily activities.

Hypothesis:

(1) MEP after physical fitness tests would vary depending on the main musculature involved in the test; (2) physical and psychological factors would be associated with MEP in patients with NSCLBP.

Study Design:

Cross-sectional design.

Level of Evidence:

Level 3.

Methods:

A total of 104 (69 women) patients aged 51.0 ± 10.3 years with NSCLBP participated. MEP was measured with a visual analog scale (VAS) at baseline and immediately after performing each physical fitness test, that is, the Biering-Sørensen, prone bridging, handgrip, chair-stand, and 8-foot time-up-and-go tests, measuring back extensor, back flexor, upper- and lower-body muscle strength, and motor agility, respectively. Global muscle strength was calculated with normalized index (z-score) procedure. Depression, anxiety, pain catastrophizing, and central sensitization were assessed with the Beck Depression Inventory II, State Trait Anxiety Inventory I, Pain Catastrophizing Scale, and Central Sensitization Inventory (CSI), respectively.

Results:

Patients showed greater pain after completion of the Biering-Sørensen (mean difference 95% CI, 0.02, 1.11), prone bridging (0.15, 1.21), lower pain after handgrip (-1.46, -0.52), and the 8-foot time-up-and-go (-1.43, 0.46) tests (all P ≤ 0.04). Lower global muscular strength (β between -0.18 and -0.30), and greater pain catastrophizing (β = 0.16), and CSI scoring (β between 0.18 and 0.27) were associated with greater MEP (all P ≤ 0.04).

Conclusion:

Greater MEP was observed after tests measuring core musculature strength than after tests measuring distal (limbs) strength and agility. Greater MEP was overall associated with lower muscle strength, greater pain catastrophizing, and central sensitization.

Clinical Relevance:

Fitness testing might be implemented as a complementary tool for the monitoring of NSCLBP in clinical settings.

Low back pain (LBP) is among the most common health problems seen in primary care. Two-thirds of patients living with high-impact chronic pain report LBP. ³⁴ Globally, LBP accounted for approximately 568.4 million cases in 2019. ¹¹ It is estimated that 70% to 85% of the population will have LBP at some time in their life, ²⁰ with 10% to 40% becoming chronic. ³⁰ Chronic LBP (CLBP) is defined as pain located between the lower limit of the ribs and the sacral region that lasts >12 consecutive weeks. ³⁰ In most cases, no specific cause can be attributed and consequently it is classified as nonspecific chronic low back pain (NSCLBP).

Musculoskeletal pain experienced during and/or after active/passive movement is commonly referred to as movement-evoked pain (MEP). ¹⁹ People with chronic pain might evade certain movements to prevent their experience of pain, ¹⁶ which might explain why people suffering from chronic pain are less active than healthy age-matched controls. ³⁸ MEP ratings can provide a more realistic representation of a person’s daily pain experience than a static pain measure. ¹⁶ MEP seems to be a distinct pain-related phenomenon not generally reflected by pain recall,^16,19 having different responses to treatment than resting pain, ²⁵ and consequently should be used as a measure of CLBP severity above and beyond traditional self-report questionnaires. ³¹ MEP can be measured through either isolated (eg, hip flexion) or functional (eg, walking) movements, simulating the daily pain experience of people with chronic pain (ie, it hurts when I move). ²⁵ Furthermore, MEP has been shown to be greater than resting pain for a range of pain conditions. ¹⁹

MEP can induce lower functionality during daily activities such as getting dressed, sitting, standing, walking, etc. ¹⁹ But, reciprocally, lower physical fitness might be associated with greater pain. Recent literature suggests resistance exercise focused on improving muscle strength as a first- approach treatment for patients with NSCLBP. ¹⁸ In fact, trunk muscle strength has the potential to be an objective risk factor for NSCLBP. ¹² The inclusion of muscle strength measures (not only the trunk but also upper and lower body) might determine whether muscle strength is associated with MEP. ¹³

Negative psychological factors (eg, depression and pain catastrophizing) have been associated previously with the development and maintenance of chronic pain. ²⁶ For instance, pain catastrophizing - a maladaptive coping process whereby a person engages in catastrophic thinking about pain - has consistently been associated with recalled resting pain, ²⁶ and influences LBP in patients during completion of a functional task.^32,33 Further evidence suggested that both depression and pain catastrophizing are associated with greater MEP in persistent musculoskeletal pain conditions.^16,24,33,41 Furthermore, previous research has suggested a link between MEP and central sensitization and self-reported functioning in patients with CLBP.^1,31

The inclusion of MEP measures in research may help in our understanding of the bidirectional associations of pain and movement.^16,29 In fact, following recent directions in the literature, possible factors (physical, cognitive, psychological and/or social) that influence the experience of MEP are warranted in future research. ¹⁹ In this sense, information on when movement provokes versus alleviates pain is essential. ¹⁶ Consequently, we aimed to (1) examine the presence of MEP after completing standardized physical fitness tests and (2) test potential factors associated with MEP in patients with NSCLBP.

Methods

Participants

Patients were recruited from the Virgen de las Nieves University Hospital Rehabilitation Unit. The recruitment process lasted from February 1 to March 31, 2022. All potential participants were informed of the study procedure during a consultation session. Participants were required to be previously diagnosed with NSCLBP by a healthcare professional according to criteria previously established, ²⁰ be capable of walking and moving without external help, be able to communicate possible problems emerging during the evaluation tests, and be ≥18 and ≤65 years old. Exclusion criteria included (1) spondylolysis, spondylolisthesis, canal stenosis, degenerative disc disease, and/or disc herniation, tumor, trauma or fracture of the lumbar and lower limbs, cauda equina syndrome, and radicular leg pain (given that spine degeneration issues are commonly present in asymptomatic persons and increase with age, ⁷ only serious lumbar structural disorders were considered); (2) lumbar surgery as source of pain; (3) acute or terminal illness; (4) physical injury, (5) physical or mental illness; and (6) medical prescriptions that prevented participation in the study. Participants who satisfied inclusion and exclusion criteria, and agreed to take part in the study were informed about the procedure of the evaluations and were asked to sign a written informed consent (n = 106). The Research Ethics Committee of the province of Granada, Spain approved the study (code: 2255-N-21).

Procedures

Participants attended 2 appointments. Assessments were carried out in the Faculty of Sport Sciences at the University of Granada. In the first appointment, an initial anamnesis was administered via interview, and anthropometry and body composition, muscular fitness (back extensor, upper-body, lower-body muscle strength, motor agility, and back flexor muscle strength tests, in that order), and pain were measured. Participants received questionnaires to fill out at home and return to the research team 9 days later (second appointment).

Measurements

Sociodemographic and Clinical Data

Data were collected using a self-reported questionnaire including age, marital status (married/ not married), education level (university/nonuniversity), and occupational status (working/housekeeper/not working).

Anthropometry and Body Composition

Weight (kg) and total body fat percentage were measured using a portable 8-polar tactile-electrode impedance analyser (InBody R20). Participants were asked not to shower, not to practice intense physical activity, and not to ingest large amounts of fluid and/or food in the 2 hours before the measurement session. Patients were required to remove all clothing (except underwear) and metal objects during the assessment. The validity and reliability of this instrument have been reported elsewhere.^27,39

Height (cm) was measured to the nearest 0.1 cm using a stadiometer (Seca 22) with the patient standing barefoot with feet flat, heels in contact, arms in anatomic position, and head in Frankfort horizontal plane. Body mass index (BMI) was calculated as weight (kg) divided by height (m) squared. Waist circumference was measured to the nearest 0.1 cm with a measuring tape (Holtain 110P) at the midpoint between the lower border of the rib cage and the iliac crest. ³⁷

Acute Pain

A visual analog scale (VAS) was used to measure subjective changes in back pain intensity and unpleasantness. Testing was performed at arrival while the participant was at rest sitting (VAS-pain at baseline) and immediately after performing each physical fitness test during assessment (MEP). Patients had to indicate the intensity of pain they felt on a 10-cm long straight line with proved validity and reliability. ⁵ The range went from 0 (“no pain”) to 10 (“the worst pain imaginable”). The score was measured to the nearest millimeter. To compare with previous literature, a global MEP was also calculated as the average of all MEP measurements. ⁴²

Physical Fitness

Back-extensor muscle strength was measured with the Biering-Sørensen test (erector spinae, multifidus and quadratus lumborum static and isometric endurance).^10,15 Patients lay down in a prone position with the iliac crest placed at the edge of the examination couch. Two straps were tied around the waist and the knees of the patient. Two sticks in parallel with the couch supported a strip positioned on the patients’ back that allowed correct position feedback. The hands were crossed on the sides of the head (alternatively on the chest if the patient was unable due to a specific reason) and the trunk was in the air. When the patient adopted the above position correctly, the chronometer was started, and the participant held this position as long as possible. A notice was given to correct the position if necessary and when the patient could not maintain or correct the proper position the test ended.

Back-flexor muscle strength was measured with the prone bridging test (abdominal static and isometric endurance).^6,10 Participants were positioned in prone forearm plank where both elbows were under the glenohumeral joint and both humeri joints were perpendicular to the floor; with the head being aligned with the cervical spine (neutral position); trunk and pelvic floor were in neutral position; knees were extended and toes contacting with the ground. When the patient adopted the above position correctly, the chronometer was started, and the participant had to maintain this position as longer as possible. If the patient could not maintain this position properly, a notice was given as a reminder to correct the position. When the participant was not able to maintain or correct the proper position the test was finished. When this position was maintained for 4 minutes, the test was stopped.

Upper-body muscle strength was measured with a hand grip dynamometer (TKK 5101) (upper-body muscle isometric strength).^10,36 The patient was in a standing position with shoulder abduction ~10 cm separated from the body, extended elbow, and the wrist in neutral position. The participant had to press the dynamometer with maximum contraction for 3 to 5 seconds. Participants performed the test twice, allowing a 1-minute rest between attempts. The best score from both hands was averaged. The grip was adjusted as described elsewhere. ³⁶

Lower-body muscle strength (quadriceps, hamstrings, and gluteus muscles endurance) was measured with the 30-second chair-stand test.^8,10,35 Participants started in a seated position in the center of a chair (44-47 cm height) stabilized against a wall, with back straight and feet flat on the floor. The arms were crossed at the wrists and held against the chest. When the test started the participant had to raise to full standing and then return to the sitting position described previously as many times as possible for 30 seconds. Before starting the test, the participant performed a pair of trial repetitions to ensure correct performance.

To create a variable for global muscle strength, we calculated a normalized index (z-score) of each strength fitness test. The z-score is calculated as (value - mean)/standard deviation. We calculated the weighted average of all these z-scores together.

Motor agility was measured with the 8-foot up-and-go test. The patient had to stand up from a chair, walk 2.44 m to and around a cone, and return to the chair in the shortest period of time. The best time from 2 trials was recorded.^8,10,35

Pain Catastrophizing

Pain catastrophizing was assessed with the Pain Catastrophizing Scale (PCS). ⁴⁵ This scale consists of 13 items, with a 0 to 4 Likert scale. The total PCS score was calculated by summing the values of the 13 items and ranges from 0 to 52. Higher scores indicate a greater tendency to catastrophize pain symptoms.

Depression and Anxiety

The Beck Depression Inventory-II (BDI-II) was used to assess depression severity. ³ The BDI-II contains 21 items with a 0 to 3 Likert scale and the range of the total score is 0 to 63, with higher score indicating greater depression.

The State Trait Anxiety Inventory-I (STAI) was used to assess the level of current anxiety. ⁴⁴ STAI is a 20-item self-administered questionnaire with a 1 to 4 Likert scale; the range of score is 20 to 80, with higher scores indicating a greater state of anxiety.

Central Sensitization

The severity of the symptoms related to central sensitization were assessed with part A of the Central Sensitization Inventory (CSI), ¹⁷ which is a reliable, consistent, and valid measure. ²⁸ The CSI contains 25 statements related to current health symptoms. Each item is measured on a 5-point temporal Likert scale: never (0), rarely (1), sometimes (2), often (3), and always (4). The range of score is 0 to 100: subclinical (0-29), mild (30-39), moderate (40-49), severe (50-59), and extreme (60-100). ²⁸

Statistical Analysis

Descriptive continuous data are shown as means and standard deviations, whereas categorical data are presented as n (%). To test the differences between clinical and sociodemographic variables by sex, t test or chi-square test was used for continuous or categorical variables, respectively. Due to the relatively large sample size, we assumed normal distribution according to the central limit theorem. Nonetheless, analyses were replicated using nonparametric tests and statistically significant results remained unchanged. To test the differences between VAS-pain after performing each physical fitness test and VAS-pain at baseline, a paired t test was performed. To assess the magnitude of the difference (effect size) Cohen’s d coefficient was used.

Finally, to test the association between possible factors (age, sex, time since diagnosis, marital status, education, fat percentage, global muscular fitness, motor agility, catastrophizing, depression, anxiety, and central sensitization) and MEP, we used multivariate linear regression analysis. MEP scores after completing each physical fitness test (Biering-Sørensen, prone bridging, handgrip, chair-stand, and 8-foot up-and-go tests) were introduced individually as dependent variables in all models. VAS-pain at baseline (ie, resting pain) was included with the “enter” method as an independent variable. In addition, age, sex, time since diagnosis, marital status, education, fat percentage, global muscular fitness, motor agility, catastrophizing, depression, anxiety, and central sensitization were introduced simultaneously as independent variables using the “stepwise” method. The presence of multicollinearity (variance inflation factor >4) was tested. Visual inspection of skewness and kurtosis and possible outliers were checked. Since no clear differences between sex were observed, all analyses were performed across the whole sample. The Statistical Package for the Social Sciences (IBM SPSS Statistics for Windows, Version 22.0) was used. Statistical significance was set at P < 0.05.

Results

One participant dropped out from the study during the assessment period. After the assessment, 1 participant had incomplete data. The final sample included in the analyses comprised 104 patients (69 women). Clinical and sociodemographic characteristics of these patients and differences by sex are displayed in Table 1. Women showed lower waist perimeter (mean difference, -6.0; 95% CI, -0.02, -11.7; P = 0.04), greater fat percentage (11.0; 14.6, 7.3; P < 0.01), pain catastrophizing (7.4; 12.7, 2.2; P < 0.01), and CSI scoring (13.8; 20.3, 7.4; P < 0.01), and worse performance in all physical fitness tests (all P ≤ 0.01), except in the Biering-Sørensen test (P = 0.11). Also, educational level was higher in favor of women compared with men (university studies, 49.3 vs 25.7%; P = 0.02).

Table 1.

Clinical and sociodemographic characteristics of participants, n = 104

	All		Women (n = 69)		Men (n = 35)
Variable	Mean	SD	Mean	SD	Mean	SD	P
Age, y	51.0	10.3	51.7	9.6	49.0	10.6	0.20
Body mass index, kg/m2	28.6	5.9	28.5	6.4	27.9	4.2	0.62
Waist perimeter, cm	95.8	14.1	93.1	15.2	99.1	10.8	0.04
Fat percentage, %	34.9	10.3	37.7	9.4	26.7	7.5	<0.01
Catastrophizing	22.0	13.1	24.8	13.6	17.3	10.9	<0.01
Depression	16.5	12.2	18.0	12.9	13.5	10.2	0.08
Anxiety	26.3	13.2	27.6	13.7	23.7	12.0	0.16
CSI	42.4	16.8	46.4	16.7	32.6	13.2	<0.01
Sorensen test, s	33.0	25.2	29.9	26.4	38.6	24.4	0.11
Hand grip test, kg	27.0	12.5	21.0	6.3	41.7	11.4	<0.01
Chair-stand test, rep	12.3	3.6	11.9	3.6	13.7	2.9	0.01
Time up-and-go, s	5.5	1.5	5.7	1.6	4.9	0.9	<0.01
Prone bridging test, s	49.9	45.6	39.1	31.9	77.4	56.5	<0.01
Variable	n	%	n	%	n	%	P
Marital status							0.14
Married	61	58.7	37	53.6	24	68.6
Not married	43	41.3	32	46.4	11	31.4
Educational level							0.021
Nonuniversity	61	58.7	35	50.7	26	74.3
University	43	41.3	34	49.3	9	25.7
Time since diagnosis, y							0.62
≤5	44	42.3	28	40.6	16	45.7
>5	60	57.7	41	59.4	19	54.3

CSI, Central Sensitization Inventory.

Differences between MEP and resting VAS-pain in patients with NSCLBP are shown in Table 2. Patients showed greater pain after the Biering-Sørensen (mean difference, 0.56; 95% CI, 0.02, 1.11; P = 0.04) and the prone bridging tests (0.68; 0.15, 1.21; P = 0.01) compared with resting pain. Patients showed lower pain after the handgrip (-0.99; -1.46, -0.52; P < 0.01) and the 8-foot up-and-go tests (-0.94; -1.43, 0.46; P < 0.01) compared with resting pain. No differences between MEP after the chair-stand test completion and resting pain were observed (-0.32; -0.88, 0.25; P = 0.27).

Table 2.

MEP (posttest minus baseline) in patients with NSCLBP, n = 104

	Baseline		Postest		Mean Difference	95% CI	t	P	Cohen’s d
Variable	Mean	SD	Mean	SD
VAS-pain, Biering-Sørensen test	4.0	2.9	4.6	3.1	0.56	0.02; 1.11	2.04	0.04	0.20
VAS-pain, prone bridging test	4.0	2.9	4.7	3.1	0.68	0.15; 1.21	2.53	0.01	0.25
VAS-pain, handgrip test	4.0	2.9	3.1	2.8	–0.99	–1.46; –0.52	–4.17	<0.01	0.40
VAS-pain, chair-stand test	4.0	2.9	3.7	3.0	–0.32	–0.88; 0.25	–1.12	0.27	–
VAS-pain, 8-foot time up-and-go test	4.0	2.9	3.1	2.8	–0.94	–1.43; –0.46	–3.85	<0.01	0.38
VAS-pain, average muscular fitness tests	4.0	2.9	3.8	2.6	–0.20	–0.64; 0.24	–0.90	0.37	–
VAS-pain, average all tests	4.0	2.9	4.0	2.6	–0.02	–0.46; 0.43	–0.07	0.95	–

MEP, movement-evoked pain; NSCLBP, nonspecific chronic low back pain; VAS, visual analog scale.

Exploratory factors associated with MEP in patients with NSCLBP are shown in Table 3. Greater CSI scoring was associated with greater MEP after the Biering-Sørensen (B, 0.05; 95% CI, 0.02, 0.08), and handgrip (0.03; 0.00, 0.06) tests (all P ≤ 0.03). Greater global muscular fitness was associated with lower MEP after the chair-stand (-1.31; -2.10, -0.53) and prone bridging (-0.81; -1.54, -0.09) tests (all P ≤ 0.03). Greater age was associated with lower MEP after the chair-stand test (-0.06; -0.11, -0.01; P = 0.03). Greater catastrophizing score was associated with greater MEP after the 8-foot up-and-go test (0.03; 0.00, 0.07; P = 0.04). There was no evidence of multicollinearity in any of the models mentioned above.

Table 3.

Factors associated with MEP in patients with NSCLBP, n = 104

Variable	β	B	95% CI	t	P	Partitioning R2
MEP, Biering-Sørensen test						0.36
VAS-pain, baseline	0.47	0.52	0.34; 0.70	5.76	<0.01
CSI	0.27	0.05	0.02; 0.08	3.28	<0.01
MEP, prone bridging test						0.35
VAS-pain, baseline	0.53	0.57	0.40; 0.75	6.46	<0.01
Global muscular fitness	–0.18	–0.81	–1.54; –0.09	-2.22	0.03
MEP, handgrip test						0.42
VAS-pain baseline	0.58	0.58	0.43; 0.73	7.46	<0.01
CSI	0.18	0.03	0.00; 0.06	2.27	0.03
MEP, chair-stand test						0.32
VAS-pain baseline	0.42	0.45	0.27; 0.62	4.95	<0.01
Global muscular fitness	–0.30	–1.31	–2.10; -0.53	–3.33	<0.01
Age	–0.20	–0.06	–0.11; -0.01	–2.24	0.03
MEP, 8-foot time up-and-go test						0.39
VAS-pain baseline	0.58	0.57	0.41; 0.72	7.34	<0.01
Catastrophizing	0.16	0.03	0.00; 0.07	2.04	0.04

B, unstandardized coefficient; β, standardized coefficient; CSI, Central Sensitization Inventory; MEP, movement-evoked pain; NSCLBP, nonspecific chronic low back pain; VAS, visual analog scale.

Discussion

The data presented indicate that MEP was observed after completion of activities involving greater back extension and flexion musculature in patients with NSCLBP. Interestingly, MEP decreased after performing activities involving greater distal (limbs) or walking activities and did not change after activities requiring sitting and standing. In fact, our findings suggest that MEP might be more related to muscle contraction of certain musculature than movement itself. Greater muscle strength was associated with lower MEP, whereas higher pain catastrophizing and central sensitization were associated with greater MEP.

A recent review uncovered that measurement of MEP varied across studies and no standardized methods were recognized. ¹⁹ To assess MEP, some authors have used an average/maximum MEP score (ie, representing the pain experienced by patients during a specific movement task), while others included a MEP index score (ie, maximum or average pain, corrected for baseline pain). ²⁴ Recent literature has suggested that a MEP index score should preferably be used because it informs about real pain increase/decrease provoked by movement compared with resting pain. ²⁴ Completion of activities that required a main strength activity of core muscles induced MEP; however, activities where core muscles were not greatly implicated in the task (ie, handgrip and 8-foot up-and-go tests) reduced MEP. Notably, completion of the chair-stand test, where lumbopelvic movement is included but back flexor and extensor musculature play a secondary role, did not elicit MEP. Interestingly, a previous study demonstrated that exercise activated pain inhibitory pathways among healthy adults. ²³ However, among persons with myalgia, only exercising nonpainful muscles activated these inhibitory mechanisms, consequently reducing pain sensitivity, while exercising painful muscles failed to produce pain inhibition. ²³ This is in concordance with results of the current study, where MEP was observed after activation of main muscles (back extensor and flexor) affected by NSCLBP. Therefore, MEP might develop when performing activities that do not necessarily show external movement, but where internal muscle forces are entangled, and specifically in musculature mainly affected by NSCLBP, ie, the core muscles. ⁴⁶

As motor control of our patient cohort is compromised, affecting their motor agility, the 8-foot up-and-go test was also included in the regression models. The 8-foot up-and-go test is predictive of future disability and adverse health outcomes in older adults, and has shown a negative association with MEP in patients with NSCLBP. ²² However, no association between the 8-foot up-and-go test and MEP was found in the current study. It might be suggested that muscular fitness and global sensitization presented stronger associations with MEP, thus masking associations observed with other factors in previous literature. ²² To date, it is unclear how physical function translates into worse MEP and/or vice versa. Patients with CLBP usually present kinesiophobia (fear of physical movement), thus expecting a worsening of their condition when they engage in physical activity/movement behaviors. ³³ Recent research showed that patients with NSCLBP had reduced strength in abdominal, paravertebral, hamstring, transversus abdominis, internal oblique, and gluteus maximus muscles compared with controls. ⁴⁶ Also, a recent meta-analysis showed lower muscle strength of hip abductor/extensors and knee extensors in patients with LBP in comparison with healthy controls. ⁴³ In fact, improvement of lumbar extension strength has been suggested as a target for interventions in CLBP.^4,14 The mechanisms regulating such weakness could relate to deconditioning, psychological factors (eg, fear of movement), and the inhibitory role of the pain experience itself. ⁴⁶ It is difficult to compare the results of the current study with previous research, since methodologies to assess MEP are disparate, and studies failed to include muscular fitness tests in their protocol, using other physical functioning tests or movement tasks to examine associations with MEP. ³¹ Nonetheless, previous literature seem to agree with the current findings and showed that MEP and physical functioning are associated.^16,22,31

Multiple disciplines have recommended pain catastrophizing to be a primary focus of CLBP treatment and using pain catastrophizing as an a priori covariate.^22,41 Previous studies suggested that, apart from pain catastrophizing,^24,33 other negative psychological factors such as depression and anxiety might be associated with MEP.^24,31,41 The current study agreed with previous literature confirming that psychological factors such as pain catastrophizing are associated positively with MEP. However, aligning with previous research proposing that the relationship of depression and anxiety with pain intensity was completely mediated by central sensitization, ⁴⁰ the current study showed a lack of association between depression and anxiety with MEP. This observation may be attributed to the associations identified between central sensitization and MEP. Therefore, in the management of (chronic) pain, psychosocial approaches (eg, pain education) tackling pain mechanisms and maladaptive psychological factors might be recommended, specifically in groups where involvement of central mechanisms (predominance of nociplastic pain) can be found. ²⁴

Clinical Implications

Deficits in strength showed by previous literature and the association of muscle strength with MEP presented in the current study highlight the potential role of physical fitness in NSCLBP. Fitness testing might be implemented as a complementary tool for the monitoring of NSCLBP in clinical settings, as has been suggested in other pain conditions.^9,47 Fitness testing is safe and requires relatively minimal equipment and space, which makes it easy to administer. Furthermore, exercise programs designed to specifically address strength enhancement in patients with NSCLBP are requested. Clinical guidelines for persons with NSCLBP recommend exercise, and a state-of-the-art systematic review determined that Pilates, strength, and core-based approaches are more beneficial than other exercise modalities. ¹⁸ Interestingly, a recent study showed that improving isolated lumbar extension strength correlated with VAS-pain; therefore, it should be considered as a mechanism of action affecting symptom improvement. Interestingly, positive changes not only in low-back but also in upper-body strength have been associated with LBP. ² Focus on increasing overall muscular fitness might help maintain patient autonomy in daily life activities and ability to work, thus leading to lower need for medical care. ²¹ Therefore, exercise therapy should be considered as the first-choice treatment for MEP in clinical practice. ²⁵ Since the evidence is sparse, and specifically in patients with NSCLBP, future studies should focus on assessing the effect of physical exercise therapy on MEP. ²⁵ Also, as the perception of pain is influenced by biological, psychological, and movement system factors, as confirmed by associations of pain catastrophizing and central sensitization with MEP, multimodal interventions seem to be a better treatment option in this population. ²⁴ Future studies including combinations of resistance training, mindfulness, and pain neuroscience education therapy might have greater impact on MEP and patient recovery than traditional interventions.

Limitations and Strengths

The study was cross-sectional and observational in nature, so causal-effect relationships cannot be ascertained. Therefore, to enable the development of tailored interventions in this population, prospective data are needed to elucidate the direction of associations. The study was underpowered to detect significant associations. Nonetheless, the sample size might be considered acceptable compared with other MEP literature.

The inclusion of a MEP index is a strength of the current study. Furthermore, the inclusion of individual MEP measurements after each physical fitness test completion and not only a global measure, as in previous research, allowed us to discover that some movements can trigger pain, while others might diminish it. Previous studies are based mainly on simple correlations; however, we performed hierarchical linear regressions to ascertain those factors that really account for MEP variance. Our study contributes to the standardization of a methodology to elicit MEP with valid physical fitness tests in patients with NSCLBP, which may improve the reproducibility and generalizability of this methodology and results.

Conclusion

Movement might trigger, diminish, or not generate pain in patients with NSCLBP. MEP might develop performing activities that do not necessarily show external movement, but rather internal muscle forces, and specifically in the core musculature. The potential importance of muscular fitness, together with central sensitization and pain catastrophizing, is emphasized in the current study. Interventions targeting increases in muscle strength might focus on noncore strengthening activities at first and progress toward a main focus on core eventually, as participants gain strength, since tasks activating mainly noncore musculature were not associated with greater MEP. Multimodal interventions including strengthening exercise and mind-body therapies might be highly welcome to ascertain whether addressing physical and psychological factors can help improve MEP in this population more efficiently than unimodal therapies.