Electrocardiogram (ECG) devices are commonly used to monitor heart rate (HR) and heart rate variability (HRV), but their signal quality under non-upright or torso-dominant activities may suffer due to motion artifact and interference from surrounding musculature. We compared ECG signal quality during treadmill walking, circuit training, and an obstacle course using three chest-worn commercial devices (Polar H10, Equivital EQ-02, and Zephyr BioHarness 3.0) and a multi-lead ECG monitor (BIOPAC). Signal quality was quantified using the beat signal quality index (SQI), and HR data were rejected if SQI fell below 0.7 or if values were physiologically implausible. Signal rejection rate was calculated as the proportion of low-quality observations across device and activity type. Significant effects of both device (p < 0.001) and activity (p < 0.001) were observed, with greater signal rejection during the obstacle course and circuit training compared to treadmill walking (p < 0.01). The Zephyr exhibited significantly higher rejection rates than the Polar (p = 0.018) and BIOPAC (p = 0.017), while the Polar showed lower average rejection rates across all activities. These findings underscore the importance of including dynamic, non-upright tasks in ECG validation protocols and suggest that certain commercial devices may be more robust under realistic conditions.

Objective. To develop analytical formulas which can serve as quantitative guidelines for the selection of the sampling rate for the electrocardiogram (ECG) required to calculate heart rate (HR) and heart rate variability (HRV) with a desired level of accuracy. Approach. We developed analytical formulas which relate the ECG sampling rate to conservative bounds on HR and HRV errors: (i) one relating HR and sampling rate to a HR error bound and (ii) the others relating sampling rate to HRV error bounds (in terms of root-mean-square of successive differences (RMSSD) and standard deviation of normal sinus beats (SDNN)). We validated the formulas using experimental data collected from 58 young healthy volunteers which encompass a wide HR and HRV ranges through strenuous exercise. Main results. The results strongly supported the validity of the analytical formulas as well as their tightness. The formulas can be used to (i) predict an upper bound of inaccuracy in HR and HRV for a given sampling rate in conjunction with HR and HRV as well as to (ii) determine a sampling rate to achieve a desired accuracy requirement at a given HR or HRV (or its range). Significance. HR and its variability (HRV) derived from the ECG have been widely utilized in a wide range of research in physiology and psychophysiology. However, there is no established guideline for the selection of the sampling rate for the ECG required to calculate HR and HRV with a desired level of accuracy. Hence, the analytical formulas may guide in selecting sampling rates for the ECG tailored to various applications of HR and HRV.

Acute mental stress is a common experience in daily life, significantly affecting both physiological and psychological well-being. While traditional pharmacological interventions can be effective, they often accompany undesirable side effects. Non-pharmacological alternatives, such as non-invasive transcutaneous peripheral neuromodulation, have promise in mitigating acute stress-induced arousal, possibly with fewer side effects. Median nerve stimulation (MNS) and auricular vagus nerve stimulation (AVNS), in particular, have demonstrated notable potential. However, efficacy and mechanism of action pertaining to MNS and AVNS remain largely unknown. This paper comparatively investigated MNS and AVNS in terms of efficacy and mechanism of action in the context of mitigating acute stress-induced arousal. Using an experimental dataset collected from 19 healthy participants who experienced acute mental stressors as well as MNS and AVNS, we showed that (i) MNS and AVNS are both effective in mitigating acute stress-induced cardiovascular arousal with MNS modestly superior to AVNS in terms of a synthetic multi-modal variable derived from physio-markers representing heart rate, blood pressure, stroke volume, cardiac output, and peripheral vasoconstriction: 74% vs. 71% in explainability; 86% vs. 69% in stimulation consistency; 0.77 vs. 0.40 in stimulation sensitivity; and 34% vs. 19% in stimulation effectiveness, respectively; and that (ii) MNS and AVNS mitigate acute stress-induced cardiovascular arousal in a distinct mechanism of action: MNS primarily mitigates the arousal pertaining to the physio-markers representing heart rate and peripheral vasoconstriction, while AVNS mitigates the arousal pertaining to the physio-markers representing heart rate, blood pressure, stroke volume, cardiac output, and peripheral vasoconstriction. These findings may help to support future device development for addressing acute mental stress-induced arousal through MNS or AVNS, and they pave the way toward a better understanding of how to quantify the efficacy of such interventions.