Study design

This case-control study was conducted based on the STROBE checklist for observational studies [20]. The Ethics Committee of Necmettin Erbakan University approved the study protocol (Approval Number: 2023/4699, Date: December 15, 2023). This study was conducted in regarding the rules of the Helsinki Declaration. It was performed in the Rheumatology Department of Necmettin Erbakan University Hospital between January 2024 and May 2024. All participants were informed about the detail of study and written informed consent was obtained.

Participants

A total of 53 female FM patients aging from 18 to 65 years who presented at the Rheumatology Department of Necmettin Erbakan University Hospital and age-gender matched 49 asymptomatic control subjects participated in this study. Patients were included who had been diagnosed with FM by a rheumatologist at least one year prior regarding to the 2016 American College of Rheumatology diagnostic criteria [21]. Prior to the study, a detailed medical history of the patients was obtained and patients were excluded if they had neurological and/or other inflammatory rheumatic disorders, previous orthopedic or spinal surgery, serious spine deformity such as diagnosed scoliosis and/or any pathology involving the spine, any lumbal dysfunction (such as facet joint syndrome disc stenosis, spondylosis, herniation), serious cardiopulmonary problems such as chronic obstructive pulmonary disease, chronic heart failure, musculoskeletal problems related to lower extremities, vestibular system disorder, psychiatric disease, presence of malignancy, being pregnant, doing regular exercise (at least 150 min of moderate-intensity aerobic activity or 75 min of vigorous-intensity aerobic activity and at least two days of muscle-strengthening activities per week) [22], and not being volunteer. Asymptomatic individuals who having no low back pain, were not diagnosed with any musculoskeletal, neurologic, and/or rheumatologic disorders, not being pregnant, not doing regular exercise and being volunteer were participated in the control group.

Outcome measures

All participants completed physical and demographic parameters as age, body mass index and smoking history. Then, disease-related characteristics of the patients were assessed by using Visual Analogue Scale (VAS) for pain, Fibromyalgia Impact Questionnaire (FIQ) for disease activity, International Physical Activity Questionnaire- Short Form (IPAQ-SF) for physical activity level, Fatigue Severity Scale (FSS) for fatique. In addition, detailed medical history such as current opioid use and FM duration were obtained. Finally, US measurements, lumbopelvic muscle endurance and physical fitness tests were evaluated for all participants. All these evaluations were performed by the same researcher and took 30–45 min. No negative situations was happened during the assessment process. Moreover, the patients were asked to not take any acute medication in the day before the evaluation.

Primer outcomesUltrasound measurements

The thickness of the TrA, IO, EO and LM muscles were evaluated using US, performed by a qualified radiologist with a Siemens Acuson S3000 ultrasound system (Siemens Healthineers, Germany). A 9 MHz linear probe was employed to evaluate the TrA, IO and EO muscles. Participants were instructed to lie in a supine position on a stretcher and expose their abdomen. TrA, IO and EO muscles were imaged from both the right and left side, approximately 10 cm lateral and inferior to the umbilicus, using a 4–9 MHz linear probe, held at an oblique angle during scanning (Fig. 1a). During these measurements, participants were asked to avoid any actions that could influence the results, such as coughing or holding their breath, and were asked to maintain normal breathing. For the LM muscle, subjects were positioned in prone position. A 1.5-6 MHz convex probe was first placed transversely over the L4-S1 region to detect the L4-L5 intervertebral space. Once the L4-L5 level was fixed, the probe was aligned longitudinally along the spine (Fig. 1b). Thickness of the LM muscle on both the right and left sides was measured using this method [10].

Fig. 1

(a) Thickness of transverse abdominis, and external and internal oblique muscles, (b) Thickness of lumbar multifidus muscle

Seconder outcomesLumbopelvic muscle endurance

The endurance of the lumbopelvic muscles was assessed using the McGill core endurance tests, which consist of trunk flexion, extension, and right/left lateral flexion endurance tests. Previous researches have demonstrated excellent reliability for these tests, with intraclass correlation coefficients (ICC) reported as 0.97, 0.97, and 0.99 for trunk flexor, extensor, and right/left lateral flexor muscle endurance, respectively [23]. Participants were instructed on how to perform each position, followed by a trial to familiarize themselves with the postures. They were then encouraged to maintain each test position isometrically for as long as possible. The time held in the correct position was recorded in seconds.

Physical fitness tests

Timed Up and Go (TUG) test was performed to evaluate functional mobility. It has been performed in previous study involving the women FM population, showing excellent reliability (ICC = 0.935) [24]. Participants were asked to stand up from a chair without the use of armrests, walk a distance of 3 m as quickly as possible without running, turn and sit down again without utilizing the armrests. Time was recorded using a manual stopwatch by one of the researchers.

30-s chair stand test was performed to obtain lower body muscular strength. This test has been previously used in FM patients [13, 18]. It involves counting the number of times in 30 s that participant may rise from a sitting position to a full stand.

Pain severity

Self-reported pain severity of patients was evaluated using VAS. This scale consists of a line, which is defined 0: No pain, 10: Unbearable pain. Patients described their pain severity on a line [25].

Disease activity

Disease activity of patients was assessed using FIQ. It consists of ten distinct parameters, which include daily activities, difficulties in performing occupational tasks, fatigue, morning stiffness, pain levels, as well as anxiety and depression. With higher scores reflecting greater disease impact and activity, the total score ranges from 0 to 100 [26].

Physical activity level

Physical activity level of the patients were assessed by using IPAQ-SF. The IPAQ-SF assesses energy expenditure over the past week in terms of total metabolic equivalent. Moderate and vigorous activity and the total frequency (days) and duration (minutes) of walking are taken into account in the calculation of the total score. For an activity to be contributed in total score, each activity should be done for a minimum duration of 10 min at a time. According to total score, patients are classified as the follows; low physical activity level: 600 MET-min/week or lower, moderate physical activity level: 601–3000 MET-min/week and high physical activity level: more than 3000 MET-min/week [27].

Fatigue

FSS, which is a 9-item, was performed to assess the fatigue severity of patients. FSS consists of likert-type scoring, and each item is rated on a scale from 1 (completely disagree) to 7 (completely agree). The total score is calculated by summing all individual scores and dividing the result by 9. A cut-off score of 4 has been established; a total score of 4 or higher presents a high level of fatigue, while a score below 4 presents a low level of fatigue [28]. In this study, patients were divided into two groups: those with a fatigue score of 4 or higher were assigned to the high-fatigue group, while the remaining patients were categorized into the low-fatigue group.

Statistical analysis

Statistical analyses were conducted using SPSS Version 22.00 (SPSS Inc., Chicago, IL) and Graphpad Prism version 8.3.0 (Graphpad Software). Visual assessments (histograms/probability graphs) and analytical tests (Kolmogorov-Smirnov/Shapiro-Wilk) were used to determine whether the variables were normally distributed. Continuous variables are presented as mean and standard deviation (SD) or as median (minimum–maximum), while categorical variables are expressed as frequencies and percentages. Comparisons between groups were conducted using either the Independent Samples t-test or the Mann-Whitney U test, depending on the data distribution.

Correlation analyses were conducted using Spearman method, with interpretation as follows: negligible (0–0.29), low (0.30–0.49), moderate (0.50–0.69), high (0.70–0.89), and very high (0.90–1.00) [29]. Cohen’s guidelines were performed to evaluate the magnitude of a correlation for interpretation purposes. Based on Cohen’s classification, a d value below 0.2 signifies a negligible effect, whereas a value greater than 0.2 suggests a small effect. Similarly, a d value exceeding 0.5 denotes a moderate effect, while a value greater than 0.8 indicates a large effect. Lastly, a d value exceeding 1.13 represents a very large effect [30]. P value < 0.05 was accepted for statistical significance level.

In the absence of comparable researches on this issue, a pilot study was conducted with ten participants from each group. The effect size of this preliminary study, based on the IO muscle thickness measurements, was calculated to be 0.66. Consequently, a total of 98 participants, with at least 49 in each group, was found sufficient to achieve 90% statistical power, assuming an effect size of d = 0.66, α = 0.05 (type I error), and β = 0.10 (type II error). The required sample size was determined using the G*Power 3.1.9.2 software.