Tymewear Vital Strap Pro: Tidal/Tv Ratio Profiling on Road and TT Bike — Cross-Session Stability, Absolute Variability and Bronchospasm Confound (Ath-01)

Study 4 of the VST Research Series adopts an instrumental case study design conducted under field conditions. It documents the Tidal/Tv ratio profile of the Tymewear Vital Strap Pro (TWP) across three consecutive sessions with Ath-01 (elite male WorldTour cyclist, CP ≈ 390 W): road bike on 10 and 12 March 2026, TT bike on 11 March 2026. Results are compared to Study 2 reference (22–24 February 2026, FEV1 4.85 L, no core module). The directional difference nasal > mouth and handlebar > TT bar (nasal) is preserved without exception across all sessions and conditions, including during documented active exercise-induced bronchoconstriction (EIB; FEV1 4.1 L, −15% vs baseline; conditions: rainfall, 9–10°C). The nasal−mouth Δ on TT flat is near-identical at +13.86 AU/L (Study 2) vs +13.16 AU/L (Study 4, active EIB). A systematic absolute decrease of approximately −20 AU/L is observed across all TT conditions on 11 March, with partial recovery on 12 March. A mechanistically coherent hypothesis — EIB inducing dynamic hyperinflation and reducing thoracic wall excursion — is formulated and supported by concurrent FEV1 and tidal volume data, and by the literature. RF measurement accuracy (percentage error, PE) remains excellent across all conditions (PE ≤ 1.5%). No population-level inference is claimed.