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Muscle fiber relaxation after contraction is primarily triggered by the active transport of calcium back into the sarcoplasmic reticulum (SR). During muscle contraction, calcium ions are released from the SR into the cytosol, where they bind to troponin, leading to a conformational change that allows for actin-myosin cross-bridging and thus muscle contraction.

Once the contraction is signaled to stop, calcium ions need to be removed from the cytosol to allow the muscle fibers to relax. This is achieved through the active transport mechanism, specifically the calcium ATPase pump, which moves calcium ions back into the SR against their concentration gradient. This reduction in intracellular calcium concentrations causes troponin to return to its original shape, resulting in a disengagement of actin and myosin and ultimately leading to muscle relaxation.

In contrast, although ATP accumulation plays a role in muscle fiber function, it is not the primary trigger for relaxation. Similarly, increased intracellular calcium concentration is a key factor for contraction, not relaxation. Lastly, cessation of action potentials does contribute to the end of stimulation but does not directly trigger the mechanics of muscle fiber relaxation; it's the decrease in calcium that is crucial for that process.