Muscles lose their smallest, most basic building blocks after stroke After suffering a stroke, patients often are unable to use the arm on their affected side. Sometimes, they end up holding it close to their body, with the elbow flexed. In a new study, Northwestern University and Shirley Ryan AbilityLab researchers have discovered that, in an attempt to adapt to this impairment, muscles actually lose sarcomeres -- their smallest, most basic building blocks. Stacked end to end (in series) and side to side (in parallel), sarcomeres make up the length and width of muscle fibers. By imaging biceps muscles with three noninvasive methods, the researchers found that stroke patients had fewer sarcomeres along the length of the muscle fiber, resulting in a shorter overall muscle structure.