The recruitment and examination of participants were halted prematurely due to depleted funds.

This prospective, cross-sectional, observational study will include subjects with fibromyalgia (FM), psoriatic arthritis (PsA) and asymptomatic controls. Participants will undergo a research ultrasound (US) exam of the enthesis of the Achilles' tendon, the medial collateral ligament at the femoral epicondyle and the common extensor tendon at lateral epicondyle of the elbow in resting conditions. The research US exam will consist of shear wave elastography (SWE) and radiofrequency (RF) data acquisitions. SWE technology will allow quantification of the shear wave speed (SWS) reporting the elastic stiffness of the tissues under investigation. RF data will be used to estimate quantitative ultrasound (QUS) parameters characterizing the mean intensity μ (akin to B-mode echogenicity), acoustic inhomogeneity (1/alpha), and structural spatial organization of echoes (κ) in the tissue.

In this prospective, cross-sectional, observational study consecutive eligible subjects with FM, PsA, and a group of age- and sex-matched asymptomatic volunteers will undergo a research US examination of the dominant Achilles' tendon, knee medial collateral ligament and common extensor tendon of the elbow. Two operators under the supervision of a musculoskeletal radiologist will perform the US exam. B-mode and Power Doppler ultrasound of each tendon and ligament will be performed for morphological characterization. Then, shear wave elastography (SWE) and radiofrequency (RF) will be acquired. The shear wave speed (SWS) will be measured in the longitudinal plane using shear wave measurements. These features will be compared between groups. The HDK parameters (μ, 1/alpha, κ) will be computed offline, using RF sequences recorded in the longitudinal plane. Demographic features (participant age, sex, body mass index), the Leeds Enthesis Index (LEI) score assessed by the referring clinicians, and the self-administered Health Assessment Questionnaire (HAQ), Brief Pain Inventory (PBI) and Fibromyalgia Impact Questionnaire (FIQ) scores will be available for multivariate analyses.

Inclusion Criteria Male and female subjects of all races and ethnic backgrounds aged between 18 - 60 years, able to provide informed consent and willing not to engage in sports activities for two days before the ultrasound (US) evaluation, will be considered eligible for this study.

Subjects with fibromyalgia:

• The diagnosis of fibromyalgia is based on the 2011 ACR criteria and confirmed by a pain medicine physician working at a tertiary care academic center (CHUM) having experience with patients with fibromyalgia and other chronic pain syndromes.
• Fibromyalgia is the primary reason for the patient's symptoms, without any other comorbid condition that could fully account for the symptoms of pain.
• No skin condition (psoriasis, eczema and others) requiring regular topical treatment.

Subjects with psoriatic arthritis:

• Presenting with psoriatic arthritis
• The diagnosis of psoriatic arthritis will be based on the ClASsification for Psoriatic Arthritis (CASPAR) criteria and established by a rheumatologist working at a tertiary care academic center (CHUM).
• No comorbid fibromyalgia as determined by the rheumatologist.

Asymptomatic subjects:

• No history of chronic pain syndromes, malformations, injuries, infections or tumors limiting activities of daily living and work.
• Not engaged in professional sports.
• No skin condition (psoriasis, eczema and others) requiring regular topical treatment.

Exclusion Criteria

• Pregnant and breast-feeding women
• History of spinal surgery or surgery to the extremities.
• Previous severe back or extremity fracture.
• Spinal or joint corticosteroid injections in the past 3 months.
• Major chronic illness (e.g., active cancer, inflammatory rheumatic disease other than psoriatic arthritis, or other chronic uncontrolled condition such as neurological, kidney, liver, cardiac, respiratory, or endocrine disease).
• Subjects unable to answer questionnaires in French or English.
• Subjects with psychiatric disorders that affect communication.
• Subjects who refused to participate.

Subjects with fibromyalgia will be recruited through various outreach approaches such as local newspapers, digital advertising on the Centre hospitalier de l'Université de Montréal (CHUM) platforms, and the collaboration of healthcare providers.

Subjects with psoriatic arthritis will be recruited from those patients attending the rheumatology clinics at the CHUM.

Asymptomatic volunteers will be recruited from the hospital community through local advertisements.