Fixed-point spirometry biases the evaluation of posturo-respiratory coupling: A comparison with optoelectronic plethysmography.

Posturo-respiratory coupling (PRC) reflects the interaction between breathing and postural control, and its accurate assessment is essential for understanding breathing-related balance disorders. This study investigates whether the use of a fixed-point spirometer to assess breathing profile biases PRC evaluation compared to a contactless method-optoelectronic plethysmography (OEP). Thirty healthy participants underwent two conditions in a random order: natural breathing and breathing through a spirometer. Segmental oscillations and center of pressure (CoP) displacements were recorded using contactless motion capture and force platform. Respiratory emergence (REm) and Time Lock Averaging (TLA) were used to assess the effect of breathing on the multi-segmental balance. Breathing through a spirometer significantly reduced the amplitude and velocity of segmental oscillations and CoP displacement compared to natural breathing, while breathing frequency remained unchanged. The spirometer condition induced higher incidence of neck flexion during inspiration (TLA), and altered the REm, particularly by increasing REm at the shoulder level. These findings suggest that using a fixed-point, in-contact spirometer introduces a significant bias in PRC measurements. Two mechanisms of PRC alteration are suggested: (1) due to the physical constraints associated with subject's connection with the device; (2) of cortical origin in response to mouth breathing. In conclusion, although spirometry remains a gold standard for breathing assessment, it is unsuitable for PRC evaluation due to its disruptive influence on natural postural-respiratory dynamics. OEP, by preserving natural breathing and posture, offers a more reliable alternative for studying PRC, especially in ecological or clinical contexts such as respiratory rehabilitation or musical performance.