Computational, Molecular, and Biomechanical Evidence Reaffirms the Unidirectional Contraction of the Frontalis Muscle: Response to the Letter by Landau and Cotofana et al.

This response addresses the comments by Landau and Cotofana regarding our recently published study on the deformation and resistance behaviour of the frontalis region. Their letter misinterprets both the methodology and the established molecular principles of skeletal muscle contraction. We clarify that our study employed computational biomechanics, not artificial intelligence, and that skin thickness and resistance parameters were derived directly from our cohort through inverse finite element modelling. Molecular and structural data from cryo-electron microscopy to genetic and single-molecule studies confirm the uniaxial contraction of skeletal muscle, refuting any bidirectional mechanism. Complementary biomechanical evidence shows that forehead folding arises from passive elastic buckling once regional resistance thresholds are reached. Together, computational, molecular, and biomechanical findings reaffirm that the frontalis operates through a single active cranial vector, and that the "line of convergence" represents a mechanical, not anatomical, threshold. Level of Evidence V This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266.