Subacromial Impingement Syndrome (SIS) is the most frequent diagnosis in shoulder pain, accounting for up to 65% of shoulder complaints, and is strongly associated with impairments in posture, scapular control, and cervical spine function. While current rehabilitation emphasizes scapular stabilization and shoulder mechanics, poor cervical posture and reduced deep cervical flexor (DCF) endurance may contribute to altered scapular kinematics and persistent dysfunction. This randomized controlled trial aims to evaluate the additional benefit of DCF endurance training, using a pressure biofeedback device, alongside conventional physiotherapy. Thirty-six patients aged 18–40 years with chronic SIS were randomly assigned to two groups: Group A received conventional therapy (hot packs, IFT, and shoulder strengthening), while Group B received the same plus DCF endurance training performed three times per week for four weeks. Pain (NPRS), disability (SPADI), and cervical endurance (CCFT) were measured pre- and post-intervention. It is hypothesized that DCF training will produce greater reductions in pain and disability and improve cervical endurance compared to conventional therapy alone. The study addresses a gap in SIS management by considering the role of cervical stabilization, potentially offering a more comprehensive rehabilitation approach for patients.