OBJECTIVE: The aim of present study was to compare abdominal (transversus abdominis (TrA), internal oblique (IO) and external oblique (EO)) and lumbar multifidus muscles (LM) evaluated with ultrasonographic (US) imaging in patients with FM (Fibromyalgia) and asymptomatic individuals and to examine the relationship between these muscle thickness and endurance, pain, fatigue and functional mobility. METHODS: Women with FM group (n: 53, age: 45.96 ± 9.96 years), and asymptomatic control group (n: 49, age: 45.12 ± 7.28), were included in this study. Pain severity, disease activity, physical activity level, fatique, thickness of TrA, IO, EO and LM muscles and endurance, and functional mobility were evaluated with the Visual Analogue Scale (VAS), Fibromyalgia Impact Questionnaire (FIQ), International Physical Activity Questionnaire- Short Form (IPAQ-SF), Fatigue Severity Scale (FSS), US imaging, McGill core endurance tests, and physical fitness tests, respectively. FM patients were classified according to the FSS score. RESULTS: The thickness of the IO (right side) (p = 0.013) and LM (both sides) (p <
0.001) muscles, lumbopelvic muscle endurance (all p <
0.001) and physical fitness tests (all p <
0.001) were lower in FM group compared to the asymptomatic group. No statistically significant differences were found in TrA, IO (left side), EO muscles thickness between the two groups (all p >
0.05). LM muscle thickness was significantly correlated with lumbopelvic muscle endurance (all p <
0.05), physical fitness tests (all p <
0.001) and fatique (p = 0.001). Moreover, significant differences in LM muscle thickness (p = 0.007), trunk flexor muscle endurance (p = 0.016), left trunk lateral flexor muscle endurance (p = 0.045) and 30-s chair stand test (p = 0.025) in favor of the low-fatigue group were detected. CONCLUSION: The thickness of LM muscle, lumbopelvik endurance and functional mobility in FM patients have been affected negatively. These findings should be considered in management of FM.