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The Sliding Filament Theory explains the mechanism of muscle contraction in terms of the interaction between two types of protein filaments, actin and myosin, within muscle fibers. According to this theory, during muscle contraction, myosin filaments pull actin filaments closer together, causing the muscle to shorten and generate force. This sliding action allows muscles to contract without the filaments themselves changing length; instead, they slide past each other.

In more detail, the myosin heads attach to binding sites on the actin filament, forming cross-bridges. When these myosin heads pivot, they pull the actin filaments inward, thus bringing the Z lines closer together and shortening the sarcomere, which is the functional unit of a muscle fiber. This process requires energy in the form of ATP, which is necessary for the continuous attachment and release of myosin heads from the actin filaments.

The other options do not accurately describe this fundamental process. Lengthening of filaments during relaxation does not occur; rather, the muscle returns to its original length as the filaments slide back to their initial positions. Muscle cells are dynamic rather than static, actively engaging in contraction and relaxation cycles. Lastly, while myosin and actin interact, it