The mechanisms behind PNS for the treatment of HSP are not known. First, individual variation may contribute to response to PNS, thus we will explore the association of subject-specific clinical and demographic information and pain relief from PNS. Secondly, our team and others have found that in the chronic stage central sensitization mechanisms may also have a role in perpetuating pain, as it does in other forms of chronic shoulder pain. These mechanisms will be explored via measures of sensory and pain perception. Finally, our approach to delivering PNS for HSP is different from other treatments in which PNS is delivered through skin surface electrodes (e.g., transcutaneous electrical nerve stimulation or TENS) in that our treatment produces repeated muscle contraction over the course of weeks. Treatments such as TENS generates tingling sensations (paresthesias) over the painful area, and pain relief following stimulation is short-lived, seldom lasting more than a few hours. We postulate that long-term pain relief for HSP after PNS treatment is due to the repeated muscle contraction that occurs daily over the course of treatment, thus we will explore the dose-response association of muscle-contraction from PNS and pain reduction, completion of activities of daily living (ADLs), and improvement in quality of life.