Question

lo 4.4. what structure in a muscle causes it to shorten? sarcomere tendon fascicle z - line myofibril

Explanation:

Sarcomeres are the basic contractile units of muscle fibers. Muscle contraction occurs when sarcomeres shorten. Tendons attach muscle to bone, fascicles are bundles of muscle fibers, Z - lines are boundaries of sarcomeres, and myofibrils are made up of sarcomeres but it is the sarcomere itself that directly shortens to cause muscle shortening.

Answer:

Sarcomere