Analysis of muscle architecture variables and their relationship with functional capacity in patients with fibromyalgia: A cross-sectional study

Background Fibromyalgia (FM) is a chronic syndrome characterised by widespread musculoskeletal pain, accompanied by fatigue, functional alterations, and changes in body composition. Objective The main objective was to analyse muscle architecture variables using ultrasound in patients with FM. Design A cross-sectional observational study conducted in accordance with the STROBE guideline. Methods 80 patients with FM (54.06 ± 9.66 years) were assessed. Sociodemographic and anthropometric variables, functional capacity (5-sit-to-stand-test (STST), Handgrip (HGS), Time Up Go (TUG) and 4- metres-walk-test (4MWT) and muscle architecture variables (cross-sectional-area of vastus lateralis (VL-CSA) and cross-sectional-area of rectus femoralis (RF-CSA), thickness of vastus lateralis and pennation angle of vastus lateralis (AP-VL)) were collected. Normality and variance homogeneity were assessed, and Pearson or Spearman correlations were applied as appropriate. Multiple linear regression was performed to evaluate the association between VL-CSA and clinical, functional, and anthropometric variables. Statistical significance was set at p < 0.05. Results Participants reported high pain intensity (VAS: 7.03 ± 1.94). Mean VL-CSA was 11.73 ± 3.76 cm 2 . Ultrasound-derived muscle parameters showed significant positive associations with lean mass and anthropometric variables, with a stronger association between VL-CSA and trochanteric circumference than with body mass index (BMI). The regression model explained 46.6% of VL-CSA variance ( p < 0.001), with BMI, trochanteric circumference, 5-STST, performance and handgrip strength emerged as independent predictors ( p < 0.05), while gait speed was not significant. Conclusions Correlations were found between anthropometric variables and muscle architecture. VL-CSA was significantly and independently related to BMI, 4-meter test, 5 sit-stand test, handgrip strength, and trochanter circumference.