• Muscle contraction occurs when muscle fibers get shorter.
  • To understand how this happens, you need to know more about the structure of muscle fibers.
  • A muscle fiber either contracts fully or it doesn’t contract at all.
  • The number of fibers that contract determines the strength of the muscular force.
  • When more fibers contract at the same time, the force is greater.
  • The most widely accepted theory explaining how muscle fibers contract is called the sliding filament theory.
  • According to this theory, myosin filaments use energy from ATP to “walk” along the actin filament with their cross bridges.
  • The movement of the myosins pulls the Z lines closer together, thus shortening the sarcomere.
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