A Mechanical Hypothesis of Deep Sclerotic Fibrous Tissue in the Dorsocervical Hump as the Dominant Passive Constraint in Forward Head Posture: Multi-Pathway Origins and Testable Predictions

The traditional framework for forward head posture (FHP) attributes its maintenance to muscle imbalance and behavioral habits. The deep sclerotic fibrous tissue of the dorsocervical hump—rather than its superficial fat pad—is usually treated as a secondary consequence or a co-localized accompanying phenomenon, with its mechanical contribution considered secondary to that of muscle and behavioral factors. We propose a falsifiable mechanical hypothesis: in individuals with long-standing FHP, deep sclerotic fibrous tissue at the cervicothoracic junction—investing fascial fibrosis with increased stiffness—may have evolved from a secondary consequence into a dominant passive constraint and may constitute the main mechanical resistance to returning the head to a neutral position. The hypothesis targets sclerosis (increased stiffness), not increased volume or fat accumulation. Dominance is operationalized by two criteria: a mechanical threshold criterion, which is necessary but not sufficient, and a joint model criterion, which directly compares the explanatory power of passive constraint, muscle function, and behavioral factors. Deep sclerotic tissue is an operational designation for a mechanical functional unit and does not presuppose equal degrees of fibrosis across the investing fascia, fibrous septa, and nuchal ligament attachment sites. Literature-based support includes load-induced fascial fibrosis, posture-related nuchal fascial stiffening, coexistence of posterior cervical structural changes with FHP, quantitative shear-wave elastography (SWE) of cervical fibrosis, layered anatomy of the posterior cervical fat pad, high SWE reliability, PIEZO2 expression in trapezius fascia, and a fibrosis-dominant pathological subtype with collagen content >38.4%. The hypothesis comprises three falsifiable components—weight, independence, and removal effect—and yields four testable predictions with a three-tier validation pathway. If validated, intervention logic may shift in part from posture correction alone toward relief of passive constraint; before rigorous controlled validation, this hypothesis should not be read as evidence for any specific treatment as a routine management option for FHP.