Introduction: Sweating is essential for thermoregulation during exercise but causes variable losses of water and sodium chloride (NaCl). Field-based methods to assess these losses are limited by sampling errors and the lack of real-time feedback. This study evaluated the validity and reliability of the BeOne armband for continuous measurement of sweat conductivity, used to estimate sweat [NaCl] equivalent in real time. Methods: Ten armbands were first tested against standard NaCl solutions (5-120 mmol·L-1) to assess the validity of the conductivity measurement. Reliability was then examined in sixteen men who completed two 60-min cycling sessions at 150 W in temperate condition (24 ± 1 °C; 50 ± 10% RH) while wearing armbands on both forearms. Outcomes included sweat onset (defined as the time from exercise onset to first detection of sweat conductivity), sweat [NaCl] equivalent drift, and time-normalized [NaCl] equivalent kinetics. Results: Measured values showed minimal bias (0.28 mmol·L-1, 95% LoA: -1.21 to 1.77), very low CVs (0.5-2.6%), and excellent inter-device agreement (ICC = 1.00) for conductivity measurements. During exercise, sweat onset did not differ between arms or sessions, but showed moderate within-session (ICC = 0.72) and low between-session reliability (ICC = 0.29), indicating substantial day-to-day variability of this specific index. Drift indices and time-normalized [NaCl] equivalent kinetics revealed a reproducible temporal profile, characterized by an initial peak, a rapid decline, a gradual increase, and a plateau toward the end of exercise. Conclusion: The BeOne armband demonstrated excellent analytical validity for sweat conductivity measurement and reliable within-session characterization of conductivity-derived sweat electrolyte dynamics. Although conductivity-based [NaCl] equivalents do not represent a laboratory gold standard and remain sensitive to matrix effects and non-specific ionic contributions, this approach appears sufficiently robust for applied sport-science contexts requiring continuous monitoring.
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