When the action potential is taking place, that basically leads to the sarcoplasm reticulum of the skeleton muscle cell to open up, open its calcium channels, and release the calcium ions into the cytoplasm of that cell. And that takes time. So, as the action potential is taking place, during this latent period of the muscle contraction, we basically have the increase in concentration of the calcium inside the cytoplasm. And once the concentration increases to a very high value, basically when this time elapses, then only then do we have our contraction actually taking place. So, in skeletal muscle, the action potential leads to the opening of the calcium channels on the sarcoplasm reticulum. And as the calcium ions begin to flow into our sinoplasm, that increases the concentration.