Mechanistic sensitivity analysis of musculoskeletal factors affecting dropped-head syndrome-like sagittal alignment using a human finite-element model

Abstract Dropped-head syndrome (DHS) is characterised by excessive head and neck flexion, impaired horizontal gaze and sagittal imbalance, particularly in older adults. Although rehabilitation targeting muscle function has shown clinical benefits, the underlying biomechanical mechanisms remain unclear. We conducted a mechanistic sensitivity analysis using a human finite-element model with a muscle controller to quantify how changes in passive muscle and tendon elastic moduli and hip-muscle activation affect sagittal alignment. Adult and elderly model conditions were compared. Reductions in passive elastic moduli increased the chin–brow vertical angle (CBVA) and decreased the C2–7 angle most prominently without active muscle force, whereas these effects were limited when muscle activation was retained. Reduced hip-muscle activation decreased the C7–S1 sagittal vertical axis and sacral slope. However, CBVA generally remained below the clinical DHS range when muscle activation was retained. The simulations represented transient responses to gravity rather than stable DHS standing posture. These findings identify model-predicted biomechanical responses and generate hypotheses concerning interactions among cervical muscle activity, passive musculoskeletal properties and trunk–pelvic alignment. Further experimental and clinical validation is required before these hypotheses can inform rehabilitation strategies.