Why might a deep-tissue massage help to relieve Mary's discomfort? Be specific!?

[Smart]

Mary is a 15 year old friend of yours. She calls you one morning, complaining of very sore muscles in her legs. You ask her if anything happened recently, and she tells you that the only thing she can think of is that she was running late for one of her classes yesterday afternoon, so she had to run as fast as she could for about 15 minutes to get there. While running, she felt exhausted, and very out of breath, but finally she made it, dropping into her seat just as the bell rang. But this morning, when she woke up, her legs were killing her! You smile and reassure Mary that everything is fine, and that the soreness will pass in a day or so.