The goal of this observational study is to determine whether there is decrease in muscle mass and muscle strength in Fibromyalgia Syndrome (FMS) patients in comparison to controls. And to determine whether these parameters are correlated with clinical ones. Briefly the main questions investigators aims to answer are: * Is there a significant difference in muscle morphology between FMS and controls? * Is there a significant difference in muscle strength between FMS and controls? * Is there a relationship between muscle thickness and pennation angle of the Quadriceps, gastrocnemius medialis, gastrocnemius lateralis and Tibialis anterior muscles and disease activity, pain and functionality? * Is there a correlation between muscle strength in FMS and disease activity, pain and functionality?