Tidal Signal of the Tymewear Vital Strap Pro During Cycling: Role of Breathing Route, Body Position and Bicycle Type — A Complete Bland-Altman Analysis with Hierarchical Biomechanical Modelling

Background. The Tymewear Vital Strap Pro (TWP) generates a dimensionless Tidal signal reflecting thoracic excursion amplitude per respiratory cycle. Converting this signal to absolute tidal volume (VT) requires an individual Tidal/VT ratio whose sensitivity to measurement condition has not been systematically characterised in field cycling. A companion ergometer study (Ricci, 2025) established near-perfect RR accuracy (r=0.9996, LoA ±0.6 bpm) and a +18.8% nasal vs. oral Tidal signal increase at VT1 via four hypothesised mechanisms (H1-H4). The present study extends this analysis to outdoor field conditions across two bicycle types and five postural configurations.Methods. One elite competitive cyclist (Ath-01, anonymised; VST-conditioned; VO2max 91-93 ml/kg/min) was recruited and tested by the author (C. Ricci, operator and researcher). A total of n=67 simultaneous TWP/VO2 Master Pro measurement pairs were recorded across five outdoor field conditions: road oral (n=36), road nasal (n=5), TT handlebar nasal (n=5), TT TTbar nasal (n=15), TT TTbar oral (n=6). Full Bland-Altman analysis, Lin CCC, Passing-Bablok regression, and Cohen's d (Hedges-corrected) were performed per condition.Results. Global RR accuracy: bias=+0.300 bpm, LoA [-1.08;1.67], PE=10.2%, CCC=0.994. TT bike conditions: PE=5.3%, 96% within +/-1 bpm. Breathing route on TT bar is a large effect (d=1.06, +8.7 AU/L, RR-matched). A posture gradient emerges under nasal breathing: ergocycle 72.8 < TTbar 108.5 < handlebar 111.2 < road 114.9 AU/L.Conclusion. Nasal breathing significantly amplifies the Tidal signal even in aerodynamic TT bar position (d=1.06). A four-level hierarchical model (breathing route > position > bicycle type > intensity) provides condition-specific Tidal/VT conversion coefficients. All results are individual (n=1 athlete); population generalisation is not claimed.VST Research Series — Study 3 of 3.