Introduction:
Moderate to high-intensity resistance exercises result in rapid changes and high values of heart rate, which may impact the accuracy of measurements in devices utilizing photoplethysmography technology. Nevertheless, the validity and reliability of the Mi Band 7 (MB7) have not yet been subjected to analysis, particularly in the context of resistance exercise programs.

Aim of the study:
The aim of this study was to analyze the validity and reliability of the MB7 device for measuring heart rate during resistance exercises in young male adults.

Material and methods:
In a concurrent validity study design, 15 participants performed 3 × 12-15 repetitions (60% 1-RM) of bench press and low pulley seated row exercises, with a 1-minute interval between sets. Heart rate was simultaneously recorded by electrocardiogram (Polar H10) and two MB7 worn randomly on the right and left forearms. A Bland–Altman analysis was used to assess the systematic error. In addition, reliability was determined using the intraclass correlation coefficient (ICC) and the accuracy was also quantified as the mean absolute percentage error (MAPE).

Results:
Heart rate underestimation increased with higher exercise intensity regardless of which forearm the device was on. The mean MB7 error was 15-25 bpm in the bench press (MAPE = −18.5 to −12.5%), and 14-40 bpm in the low pulley seated row (MAPE = −24.4 to −10.1%). Moreover, the forearm on which the MB7 was worn seemed to have no impact on the reliability of the device for measuring heart rate during these exercises.

Conclusions:
The MB7 device demonstrated inadequate indexes of validity and reliability for the measurement of HR during bench press and low pulley seated row exercises.