This is when our calcium concentration increases. This is known as the contraction period, and this is known as our relaxation period. So now let's move on to our cardiac muscle contraction. So remember, the action potential of skeletal muscles looks like this. But the action potential for cardiac muscle looks like this, because we have this extended period known as our plateau phase. So basically, in this case, our comparison between the graph or the action potential on the membrane and the muscle contraction looks something like this, which is slightly different.