Final answer:
The sliding filament theory is the most widely accepted theory explaining how muscle fibers contract. The two protein filaments involved in muscle contraction are the thick myosin filaments and the thin actin filaments.
Step-by-step explanation:
The sliding filament theory is the most widely accepted theory explaining how muscle fibers contract. According to this theory, myosin filaments use energy from ATP to "walk" along the actin filaments with their cross bridges, pulling the actin filaments closer together. The two protein filaments involved in muscle contraction are the thick myosin filaments and the thin actin filaments.