Final answer:
Skeletal muscle fibers are composed primarily of thick actin and thin myosin proteins. Myofibrils contain thick filaments made of myosin and thin filaments made of actin, which slide past each other during muscle contraction.
Step-by-step explanation:
Skeletal muscle fibers are composed primarily of thick actin and thin myosin proteins. Myofibrils, which are the contractile units of muscle fibers, are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments made of myosin and thin filaments made of actin. These filaments work together during muscle contraction, with myosin and actin sliding past each other to generate force.