Surrounding each layer of muscle tissue is a thin layer of tissue called myofascia that is subconsciously controlled by the nervous system. This layer of tissue will tighten up around muscle that is chronically tight. This inhibits the muscle from releasing tension. It's been found that applying manual pressure to the fascia will prompt it to release, allowing the tension in the muscle to also release. The therapeutic benefit of releasing tension in chronically tight muscles is vast and expansive. The following link is to a research paper that describes this in more detail.