Step-adaptive sound guidance enhances locomotor-respiratory coupling in novice female runners: A proof-of-concept study

Abstract

Introduction: Many runners struggle to find a rhythm during running. This may be because 20-40% of runners experience unexplained, unpleasant breathlessness at exercise onset. Locomotor-respiratory coupling (LRC), a synchronization phenomenon in which the breath is precisely timed with the steps, may provide metabolic or perceptual benefits to address these limitations. It can also be consciously performed. Hence, we developed a custom smartphone application to provide real-time LRC guidance based on individual step rate.

Methods: Sixteen novice-intermediate female runners completed two control runs outdoors and indoors at a self-selected speed with auditory step rate feedback. Then, the runs were replicated with individualized breath guidance at specific LRC ratios. Hexoskin smart shirts were worn and analyzed with custom algorithms to estimate continuous LRC frequency and phase coupling.

Results: LRC guidance led to a large significant increase in frequency coupling outdoor from 26.3 ± 10.7 (control) to 69.9 ± 20.0 % (LRC) "attached". There were similarly large differences in phase coupling between paired trials, and LRC adherence was stronger for the indoor treadmill runs versus outdoors. There was large inter-individual variability in running pace, preferred LRC ratio, and instruction adherence metrics.

Discussion: Our approach demonstrates how personalized, step-adaptive sound guidance can be used to support this breathing strategy in novice runners. Subsequent investigations should evaluate the skill learning of LRC on a longer time basis to effectively clarify its risks and advantages.

Keywords: breathing strategies; breathing techniques; entrainment; locomotor-respiratory coupling; running; synchronization.

Figure 1

Experimental sensor setup with Hexoskin smart shirt, Breathtool app, and Cosmed spirometer. Note that this upper arm placement of the phone was not used for the actual experiment (instead, waist). Permission was obtained for use by the persons in the photo.

Figure 2

Experimental protocol for T1–T4.

Figure 3

Interval plot of T1 vs. T3 attachment for all participants across equal quartiles of the run.

Figure 4

Example LRC ratio, phase synchrogram, and attachment plot for two participants during T1 and T3 outdoor runs. LRC ratio quantified using SR/BR quotient (steps per breath). Phase synchrogram shows relative phase of each step within the breath cycle (0–360 degrees, 0 denotes inhalation, 180, exhalation). Steps nearest inhalation are calculated relative to inhalation (270–360, 0–90), and same with steps closest to exhalation (90–270 degrees). This is done for visual assessment only. Note that this differs from the sine circle map and circular statistics approach, which uses relative phase of breath to step for statistical efficacy. Attachment calculation described in methods section as five or more breath cycles within 5% of prescribed LRC ratio. First column shows participant 6 with T1 % time attached = 9.4%; T3, 87.7%. In the top panel note the highly variable LRC ratio throughout the run, starting between 6-7 steps/breath and then speeding up to around 5 steps per breath as BR increased. In the bottom panel note the strong adherence to the instructed ratio of 5 throughout most of the run. Parallel dot groups indicate consistent relative phase of steps within the breath cycle. Second column shows participant 1 with lower comparative LRC strength in T3 (LRC sounds). T1 % time attached = 24.2 %; T3, 51.8%. Note the large amount of attachments & detachments as well as inconsistent relative phase of steps within breath (random dots dispersion).

Figure 5

Sine-circle map of T1 vs. T3 (A) expiration and (B) inspiration entrainment resultant vectors relative to step cycle. Each marker represents the ensemble average for one participant over the entire run.

Figure 6

Gardner-Altman plot of T3 expiration entrainment stability ρ during attachment vs. unattachment. Each line corresponds to one participant. Note the single participant with small negative difference in entrainment when attached (dotted line).