Shoulder pain is extremely common after stroke and occurs in 30-70% of patients. The pain may begin as early as one week after stroke, although peak onset and severity occurs around four months, and persists into the chronic stage. Chronic post stroke shoulder pain (PSSP) interferes with motor recovery, decreases quality of life, and contributes to depression. PSSP is thought to be caused mainly by damage to the myofascial tissues around the shoulder joint. Interestingly, an MRI study in patients with PSSP showed that the degree of structural damage to the muscles did not correlate with the degree of pain. Thus, the pathophysiology of myofascial dysfunction and pain in PSSP has not been elucidated leading to missed opportunities for early diagnosis and variable success with pain management. The accumulation of hyaluronic acid (HA) in muscle and its fascia can cause myofascial dysfunction. HA is a glycosaminoglycan (GAG) consisting of long-chain polymers of disaccharide units of glucuronic acid and N-acetylglucosamine and is a chief constituent of the extracellular matrix of muscle. In physiologic quantities, HA functions as a lubricant and a viscoelastic shock absorber, enabling force transmission during contraction and stretch. Reduced joint mobility and spasticity result in focal accumulation and alteration of HA in muscle. This can lead to the development of stiff areas and taut bands, dysfunctional gliding of deep fascia and muscle layers, reduced range of motion (ROM), and pain. However, the association of muscle HA accumulation with PSSP has not been established. The investigators have quantified the concentration of HA in muscle using T1rho (T1ρ) MRI and found that T1ρ relaxation time is increased in post stroke shoulder pain and stiffness. Furthermore, dynamic US imaging using shear strain mapping can quantify dysfunctional gliding of muscle that may generate pain during ROM. Myofascial dysfunction can result in non-painful reduction in ROM (latent PSSP), which may become painful due to episodic overuse injury producing greater shear dysfunction (active PSSP). Hence, shear strain mapping may differentiate between latent versus active PSSP. Thus, quantitative Motor Recovery (MR) and US imaging may serve as useful biomarkers to elucidate the pathophysiology of myofascial dysfunction.