Myofibrils are long, thread-like structures found within muscle cells, specifically in skeletal and cardiac muscles. They are made up of smaller units called sarcomeres, which are the basic contractile units of muscle. When a muscle contracts, these myofibrils shorten, allowing the muscle to generate force and movement. Each myofibril contains two main types of protein filaments: actin and myosin. These proteins interact in a process called the sliding filament theory, where myosin heads pull on actin filaments, causing the muscle to contract. This intricate system is essential for all voluntary movements in the body.