Final answer:
During the plateau phase, calcium enters the sarcoplasmic reticulum and triggers the release of additional calcium ions, which are necessary for muscle contraction.
Step-by-step explanation:
During the plateau phase, calcium enters the sarcoplasmic reticulum (SR) through calcium channels in the membrane. This influx of calcium ions triggers the release of additional calcium ions from storage in the SR. The released calcium ions then bind to the regulatory protein troponin, removing the inhibition that prevents muscle contraction. Approximately 20 percent of the calcium required for muscle contraction is supplied by the influx of calcium ions from the extracellular fluid (ECF) during the plateau phase, while the remaining calcium is released from the SR.