Answer;
The sarcoplasmic reticulum.
The organelle that can chemically regulate the shortening of the muscle is the sarcoplasmic reticulum.
Step-by-step explanation;
-Sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells.
-it constitutes the main intracellular calcium store in striated muscle and plays an important role in the regulation of excitation-contraction-coupling (ECC).
-Also regulates the intracellular calcium concentrations during contraction and relaxation