Validity and Reliability of a New Wearable Chest Strap to Estimate Respiratory Frequency in Elite Soccer Athletes

Assessing respiratory frequency (<i>f</i>_R) is practical in monitoring training progress in competitive athletes, especially during exercise. This study aimed to validate a new wearable chest strap (wCS) to estimate <i>f</i>_R against ergospirometry as a criterion device in soccer players. A total of 26 elite professional soccer players (mean [standard deviation]: 23.6 [4.8] years; 180.6 [5.7] cm; 77.2 [5.4] kg) from three Italian Serie A League teams participated in this cross-sectional study. The sample included attackers, midfielders, and defenders. <i>f</i>_R was assessed during a maximal cardiopulmonary exercise test (CPET) on a treadmill using (i) a breath-by-breath gas exchange analyzer (Vyntus^® CPX, Vyaire Medical) and (ii) a novel wCS with sensors designed to assess breath frequency following chest expansions. Pearson's correlation coefficient (<i>r</i>), adjusted coefficient of determination (aR²), Bland-Altman plot analysis, and Lin's concordance correlation coefficient (ρ_c) were used for comparative analysis (correlation and concordance) among the methods. The repeated measures correlation coefficient (r_rm) was used to assess the strength of the linear association between the methods. The intraclass correlation coefficient (ICC) and the Finn coefficient (r_F) were used for inter-rater reliability. All statistical analyses were performed within the R statistical computing environment, with 95% confidence intervals (95% CIs) reported and statistical significance set at <i>p</i> < 0.05. A total of 16529 comparisons were performed after collecting the CPET data. The robust time series analysis with Hodges-Lehmann estimation showed no significant differences between both methods (<i>p</i> > 0.05). Correlation among devices was statistically significant and very large (<i>r</i> [95% CI]: 0.970 [0.970, 0.971], <i>p</i> < 0.01; aR² [95% CI]: 0.942 [0.942, 0.943], <i>p</i> < 0.01) with strong evidence supporting consistency of the new wCS (BF₁₀ > 100). In addition, a high concordance was found (ρ_c [95% CI]: 0.970 [0.969, 0.971], bias correction factor: 0.999). Vyntus^TM CPX, as a standard criterion, showed moderate agreement with wCS after Bland-Altman analysis (bias [95% lower to the upper limit of agreement]; % agree: 0.170 [-4.582 to 4.923] breaths·min^-1; 69.9%). A strong association between measurements (r_rm [95% CI]: 0.960 [0.959, 0.961]), a high absolute agreement between methods (ICC [95% CI]: 0.970 [0.970, 0.971]), and high inter-rater reliability (r_F: 0.947) were found. With an RMSE = 2.42 breaths·min^-1, the new wCS seems to be an valid and reliable in-field method to evaluate <i>f</i>_R compared to a breath-by-breath gas exchange analyzer. Notwithstanding, caution is advised if methods are used interchangeably while further external validation occurs.