English: Muscle Contraction and relaxation is regulated by the release and storage of Calcium ions within muscle fibers. Calcium ions are released into the cell by a structure called the sarcoplasmic reticulum, causing the muscle fibers to contract. This releasing mechanism, green openings on top, are then closed and calcium ion pumps, yellow openings at bottom, then pump all excess calcium back into the reticulum for relaxation.
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As muscles contract, Calcium ions release from the sarcoplasmic reticulum by release channels. These channels close and calcium pumps open to relax muscles. After extended exercise the release channels can begin to leak and cause for muscle fatigue.