1. A muscle's contractile units lie within individual muscle cells, which are also called muscle fibers. Each muscle fiber contains numerous bundles of protein filaments. These bundles, called myofibrils, are organized into repeating contractile units called sarcomeres. 2. A sarcomere contains two types of protein filaments: actin filaments and myosin filaments. Each actin filament is attached to a Z line, which is found at either end of a sarcomere. The thick myosin filaments lie between the Z