Purpose: Most healthy individuals show a multisegmental control strategy during challenging standing conditions, whereas others show a rigid ankle-steered strategy, which is assumed as suboptimal. Respiratory-demanding tasks exert a perturbing effect on balance, although the underlying mechanisms remain poorly understood. The purpose of this study was to investigate whether inspiratory resistive loading (IRL) affects postural strategy, back muscle oxygenation, and blood volume during postural control.
Methods: We assessed the acute effects of increased respiratory effort by measuring the center of pressure displacement in 12 healthy individuals during upright standing on an unstable support surface while breathing against an IRL. Simultaneous ankle and back muscle vibration was used to evaluate the proprioceptive strategy (multisegmental vs ankle-steered) during postural control. Back muscles oxygenation and blood volume were assessed using near-infrared spectroscopy (tissue oxygenation index, deoxyhemoglobin, oxyhemoglobin, and combined hemoglobin).
Results: An increased proprioceptive gain at the ankles and an decreased gain at the back were observed after approximately 7 min of IRL. Retrospectively, the group was subdivided on the basis of the participants' dominant proprioceptive use during a baseline postural control. During IRL, the ankle-steered group showed an increased reliance on ankle proprioception compared with a multisegmental group (-5.9 ± 3.1 and 1.0 ± 1.9 cm, respectively, P < 0.05). Tissue oxygenation index, deoxyhemoglobin, oxyhemoglobin, and combined hemoglobin declined progressively in the ankle-steered group during the IRL (from baseline (100%) to -1%, -1%, -45%, and -18%, respectively, P < 0.05), whereas no decline was found in the multisegmental group (from baseline (100%) to 134%, 82%, 129%, and 153%, respectively, P > 0.05).
Conclusion: Individuals who adopted an ankle-steered strategy during IRL showed a progressive decline in back muscle oxygenation and blood volume. In contrast, IRL did not affect back muscle oxygenation and blood volume in individuals who showed a multisegmental strategy in upright standing.