The muscle cells burn off ATP (Really fast) and they get much tighter to deal with the strain. Make sure that you always stretch and warm up before running as the muscles could strain!
When you breathe in, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal muscles between your ribs...
When you breathe in, oxygen flows down your windpipe into your two lungs. At the same time, they are expanding and your diaphragm moves down towards the floor to allow the lungs to...