It's caused by a buildup of lactic acid, but is really a signal your muscle is using to let your brain know it's running out of energy. Eventually, you'll build up enough endurance that your muscles won't freak out so soon.
A person who’s running lands on one foot with all of his or her body weight, multiplied two to three times by the forces of gravity and forward momentum. With this kind of hammering, it’s no surprise that injuries occur.
