Hypothesis

Aging of the vocal fold lamina propria results not merely from passive accumulation of collagen and loss of elastin, but from a failure of phonation‑induced mechanotransduction to maintain fibroblast homeostasis. In young tissue, cyclic tensile strain during phonation activates integrin‑FAK‑YAP/TAZ signaling in fibroblasts, promoting a contractile, matrix‑remodeling phenotype that balances collagen I/III deposition with elastin synthesis and regulates collagen‑fibrillogenesis proteins (lumican, fibromodulin, decorin) [Collagen regulatory proteins shift during aging]. With age, reduced vibratory amplitude and altered strain patterns diminish this mechanochemical drive, leading to fibroblast senescence or a maladaptive myofibroblast shift that overproduces stiff collagen while suppressing elastin [Collagen accumulation and elastin loss characterize aged vocal folds]. This hypothesis predicts that restoring physiologic strain patterns—either via targeted phonatory exercise or functional electrical stimulation (FES) that induces muscle hypertrophy and restores mucosal wave dynamics [FES produces substantial muscle hypertrophy]—will reactivate YAP/TAZ signaling, normalize collagen regulatory protein expression, and reverse ECM stiffness.

Testable Predictions

1. In vitro strain response: Primary vocal fold fibroblasts isolated from young (3‑month) and aged (24‑month) rats will be subjected to cyclic tensile strain (1 Hz, 10 % elongation) mimicking physiologic phonation. Young cells will show rapid nuclear translocation of YAP/TAZ, up‑regulation of elastin (ELN) and down‑regulation of collagen I (COL1A1), while aged cells will exhibit blunted YAP/TAZ activation and a persistent high COL1A1/ELN ratio. Pharmacologic inhibition of integrin‑β1 (e.g., with AIIB2) will abolish these responses in young cells, confirming mechanotransduction dependence.

2. In vivo rescue: Aged rats receiving daily FES‑induced thyroarytenoid hypertrophy (as in prior work [FES produces substantial muscle hypertrophy] ) for 4 weeks will demonstrate, compared to age‑matched controls, a significant increase in YAP/TAZ nuclear signal in lamina propria fibroblasts (immunofluorescence), a reduction in collagen I/III immunostaining density, and an increase in elastin fiber organization (verified by second‑harmonic generation imaging). Concurrently, luminal proteomics will reveal normalization of lumican, fibromodulin, and decorin levels toward youthful profiles [Collagen regulatory proteins shift during aging];

3. Functional outcome: High‑speed videoendoscopy will show improved mucosal wave amplitude and reduced glottal gap in FES‑treated aged rats, correlating with acoustic measures (lower jitter, higher harmonic‑to‑noise ratio). If the hypothesis is false, FES will increase muscle bulk without altering fibroblast signaling or ECM composition, and acoustic improvement will be absent or marginal.

Falsifiability

The hypothesis is falsifiable if any of the following occurs: (a) aged fibroblasts exhibit YAP/TAZ nuclear localization equal to or greater than young cells under strain, yet still produce a pathological ECM; (b) FES treatment fails to modify YAP/TAZ signaling or collagen regulatory protein expression despite demonstrable muscle hypertrophy; or (c) mechanical strain applied directly to the lamina propria (bypassing muscle) does not rescue the aged ECM phenotype. Demonstrating any of these outcomes would refute the claim that disrupted phonation‑driven mechanotransduction is a primary driver of presbyphonic ECM remodeling.

Significance

Linking muscle hypertrophy to fibroblast mechanotransduction offers a unified explanation for why FES yields disproportionate functional gains relative to its modest hypertrophy [FES produces substantial muscle hypertrophy] and why presbyphonia accounts for a minority of dysphonia in elders [Presbyphonia prevalence is only 17.78% among older adults with voice disorders]. It redirects focus from passive ECM aging to active, strain‑dependent cellular regulation, opening therapeutic avenues that target mechanosensitive pathways (e.g., YAP/TAZ modulators, integrin agonists) alongside or instead of conventional voice therapy.