Fibromyalgia (FM) is a rheumatic condition characterized by chronic, diffuse musculoskeletal pain, often associated with other manifestations. Patients with FM commonly experience reduced functional capacity, impairment in performing daily living activities, and worse quality of life (QoL). Compared to healthy control individuals, they show lower muscle strength and functional capacity, which may increase the risk of impairment in the muscular quality index (MQI). The MQI has been proposed as a clinically relevant marker to identify individuals at risk of functional disability, as well as an important prognostic factor for mortality. The MQI can be measured using two approaches: laboratory measurement and field measurement. The field MQI, being easy to implement and low-cost, is a valid alternative for assessing muscular quality, especially in contexts with limited laboratory resources.

To investigate the associations between field and laboratory MQI measurements and QoL in women with FM.

This is a quantitative, cross-sectional, and exploratory study. Women over 18 years old with a confirmed diagnosis of FM according to the criteria established by the American College of Rheumatology (2016) were included. QoL was assessed using the Short Form-36 (SF-36) questionnaire, Brazilian version, which covers eight domains: functional capacity, physical aspects, pain, general health status, vitality, social aspects, emotional aspects, and mental health. The MQI was obtained from the following calculations: the ratio between lower limb (LL) strength and lean mass of LL (laboratory measure); and the ratio between LL strength and body mass index (BMI) (field measure). LL strength was measured using the chair stand test (five repetitions). Lean mass of LL was measured by dual-energy X-ray absorptiometry (DEXA). BMI was calculated by the ratio of weight (kg) to height (m²). The association between the variables was analyzed using Spearman’s correlation test.

The study included 47 women with an average age of 53.45 ± 7.32 years. Significant correlations were found between both laboratory and field MQI and the domains of pain (r = 0.528, p = 0.000; r = 0.452, p = 0.001, respectively); functional capacity (r = 0.359, p = 0.013; r = 0.433, p = 0.002); and physical aspects (r = 0.433, p = 0.002; r = 0.479, p = 0.001). Additionally, a strong correlation was found between field MQI and laboratory MQI (r = 0.94, p = 0.000).

A higher MQI is associated with greater functional capacity, better physical condition, and reduced pain in women with FM. Furthermore, both laboratory and field MQI proved to be relevant indicators of muscular quality, with field MQI representing a valid alternative for clinical contexts due to its accessibility and practicality.

The results of this study may contribute to the development of more specific interventions to improve the QoL of women with FM. Additionally, they reinforce the applicability of field MQI as a viable clinical tool for assessing muscular quality, enabling the monitoring and implementation of management strategies, even in contexts with limited laboratory resources.