The primary objective of this area of work is to quantify changes in musculoskeletal architecture, composition, and material properties in individuals with mobility impairments (age-related decline of mobility) and neurological impairments (stroke, cerebral palsy, spinal cord injury). Using B-mode ultrasound, we make measurements of musculoskeletal architecture (moment arm fascicle length, thickness, pennation angle) and muscle composition by quantifying the echogenicity. Using shear wave ultrasound elastography, we measure the speed of shear wave propagation through the muscle which is related to tension and material properties such as stiffness. We can then relate these changes in musculoskeletal properties to muscle function and severity of impairment through measurements of gait, muscle strength, and clinical tests.