This article, the fifth in its series, delves into the underlying cause of subscapularis syndrome, a condition that frequently mimics upper-extremity radiculopathy. The author argues that true clinical success requires looking beyond the dysfunctional muscle to its "upstream" biomechanical drivers. The primary culprit is identified as scapular dyskinesis—an alteration in the scapula's position, motion, or timing. When scapular mechanics are compromised, the subscapularis muscle is forced into a compensatory stabilizing role for the shoulder joint. Over time, this chronic overuse transforms the muscle into a pain generator, producing pseudo-radicular symptoms that can be mistaken for a cervical nerve issue. This process mirrors piriformis syndrome in the lower body. This pattern is especially evident in overhead athletes but is also common in the general population due to poor posture and repetitive tasks. Critically, the article notes that co-existing cervical dysfunction is often a secondary adaptation, not the primary source of pain. Therefore, treating the neck in isolation is unlikely to provide lasting relief. Recognizing scapular dyskinesis as the root cause reframes the issue as a regional control problem, sharpening the differential diagnosis and preventing unnecessary, invasive spinal interventions.