Uncovering the alterations in extrinsic foot muscle mechanical properties and foot posture in fibromyalgia: a case-control study

Objectives The aim of the study was to evaluate foot posture and the mechanical properties of extrinsic foot muscles in fibromyalgia. Methods Patients with fibromyalgia ( n = 86) and age- and gender-matched controls ( n = 41) were included in the study. Foot Posture Index (FPI), Beighton and Brighton criteria were used to evaluate static foot posture, joint hypermobility, and benign joint hypermobility syndrome (BJHS), respectively. Tonus, elasticity, and stiffness of the extrinsic foot muscles including gastrocnemius medialis, tibialis anterior, and peroneus longus were measured by the MyotonPRO ® device. Foot function, quality of life, and physical activity level were assessed by the Foot Function Index (FFI), Short Form-36 (SF-36), and the International Physical Activity Questionnaire-Short Form (IPAQ-SF), respectively. Results The frequency of abnormal foot posture in the fibromyalgia and control groups was 68.6% versus 39%, respectively; and neutral foot posture was approximately twice as prevalent in the control group compared to the fibromyalgia group ( p = 0.006). Pronated foot posture was the most common abnormality in fibromyalgia (61.6%) and observed in 85.5% of the patients with joint hypermobility and in 87.1% of those meeting BJHS criteria. The elasticity of peroneus longus at rest and the elasticity of tibialis anterior in the standing position were significantly different between the fibromyalgia group and the control group [1.08 (0.22) vs. 1.02 (0.25), p = 0.037 and 0.92 (0.29) vs. 0.87 (0.24), p = 0.011, respectively]. Regarding the fibromyalgia group, no difference was detected among foot posture groups in terms of myotonometric data. Conclusions Pronation foot posture is common in fibromyalgia with a much higher frequency in fibromyalgia patients with hypermobility. In daily clinical practice for fibromyalgia, particular attention should be given to foot alignment.