Assessing Muscle Quality in Women with Fibromyalgia and Associations with Biopsychological Factors: A Cross-Sectional Study

Introduction: Fibromyalgia (FM) is a chronic condition affecting predominantly women, characterized by widespread pain, fatigue, and impaired functionality. Poorer clinical outcomes are linked to changes in muscle strength and body composition. The Muscle Quality Index (MQI) has been proposed as a relevant measure to detect the risk of functional disability. However, the assessment of MQI in women with FM and its association with biopsychosocial factors remains unclear. Methods: Forty-seven women with FM (53.45 ± 7.32 years) were assessed for quality of life (SF-36), FM impact, depressive symptoms, sleep quality, oxidative stress markers, and functional capacity. MQI was calculated using laboratory-based (five times sit-to-stand test [5xSTS]/lower limb lean mass by DEXA) and field-based (5xSTS/body mass index) methods. Results: The pain domain of the SF-36 showed significant associations with both laboratory-based MQI (r=0.534; p<0.001) and field-based MQI (r=0.461; p=0.001), explaining 27.8% and 20.4% of their variance, respectively. MQI also correlated with SF-36 physical functioning, role-physical, and functional capacity. Field- and laboratory-based MQI were strongly correlated (r=0.93; p<0.0001). Conclusion: Pain emerged as the primary factor negatively affecting MQI. The field MQI proved to be a practical, valid, and clinically applicable alternative for functional assessment in women with FM