Validity of Smartphone Heart Rate Variability Pre- and Post-Resistance Exercise

Abstract

The aim was to examine the validity of heart rate variability (HRV) measurements from photoplethysmography (PPG) via a smartphone application pre- and post-resistance exercise (RE) and to examine the intraday and interday reliability of the smartphone PPG method. Thirty-one adults underwent two simultaneous ultrashort-term electrocardiograph (ECG) and PPG measurements followed by 1-repetition maximum testing for back squats, bench presses, and bent-over rows. The participants then performed RE, where simultaneous ultrashort-term ECG and PPG measurements were taken: two pre- and one post-exercise. The natural logarithm of the root mean square of successive normal-to-normal (R-R) differences (LnRMSSD) values were compared with paired-sample t-tests, Pearson product correlations, Cohen's d effect sizes (ESs), and Bland-Altman analysis. Intra-class correlations (ICC) were determined between PPG LnRMSSDs. Significant, small-moderate differences were found for all measurements between ECG and PPG: Base_Pre1 (ES = 0.42), Base_Pre2 (0.30), RE_Pre1 (0.26), RE_Pre2 (0.36), and RE_Post (1.14). The correlations ranged from moderate to very large: Base_Pre1 (r = 0.59), Base_Pre2 (r = 0.63), RE_Pre1 (r = 0.63), RE_Pre2 (r = 0.76), and RE_Post (r = 0.41)-all p < 0.05. The agreement for all the measurements was "moderate" (0.10-0.16). The PPG LnRMSSD exhibited "nearly-perfect" intraday reliability (ICC = 0.91) and "very large" interday reliability (0.88). The smartphone PPG was comparable to the ECG for measuring HRV at rest, but with larger error after resistance exercise.

Keywords: autonomic modulation; fatigue; mobile devices; photoplethysmography; pulse rate variability; recovery.

Figure 1

Bland–Altman plots comparing the log transformation of the root mean square of successive RR differences (LnRMSSD) values from the smartphone application photoplethysmography (PPG) with the criterion electrocardiogram (ECG). The solid lines represent the mean bias, whereas the outside dashed lines represent the 95% limits of agreement. Black diamonds represent “optimal” and “good” signal quality measures, and white circles represent “failed” signal quality.