Oxygen uptake ($\dot{V}{O}_2$) is an essential metric for evaluating cardiopulmonary health and athletic performance, which can barely be directly measured. Heart rate ($HR$) is a prominent physiological indicator correlated with $\dot{V}{O}_2$ and is often used for indirect $\dot{V}{O}_2$ prediction. This study investigates the impact of $HR$ placement on $\dot{V}{O}_2$ prediction accuracy by analyzing $HR$ data combined with the respiratory rate ($RESP$) and minute ventilation ($\dot{V}E$) from three anatomical locations: the chest; arm; and wrist. Twenty-eight healthy adults participated in incremental and constant workload cycling tests at various intensities. Data on $\dot{V}{O}_2$, $RESP$, $\dot{V}E$, and $HR$ were collected and used to develop a neural network model for $\dot{V}{O}_2$ prediction. The influence of $HR$ position on prediction accuracy was assessed via Bland–Altman plots, and model performance was evaluated by mean absolute error (MAE), coefficient of determination (R²), and mean absolute percentage error (MAPE). Our findings indicate that $HR$ combined with $RESP$ and $\dot{V}E$ ($\dot{V}{O}_{2\mathrm{H}\mathrm{R}+\mathrm{R}\mathrm{E}\mathrm{S}\mathrm{P}+\dot{\mathrm{V}}\mathrm{E}}$) produces the most accurate $\dot{V}{O}_2$ predictions (MAE: 165 mL/min, R²: 0.87, MAPE: 15.91%). Notably, as exercise intensity increases, the accuracy of $\dot{V}{O}_2$ prediction decreases, particularly within high-intensity exercise. The substitution of $HR$ with different anatomical sites significantly impacts $\dot{V}{O}_2$ prediction accuracy, with wrist placement showing a more profound effect compared to arm placement. In conclusion, this study underscores the importance of considering $HR$ placement in $\dot{V}{O}_2$ prediction models, with $RESP$ and $\dot{V}E$ serving as effective compensatory factors. These findings contribute to refining indirect $\dot{V}{O}_2$ estimation methods, enhancing their predictive capabilities across different exercise intensities and anatomical placements.

Background: Recreational exercise has become increasingly common among Chinese adults. However, population-specific data on sweat rate and electrolyte composition during typical daily physical activities remain limited. Therefore, this study aimed to comprehensively characterize the hydration status, sweat rate, and sweat electrolyte composition among Chinese adults engaging in common daily exercises under controlled environmental conditions, and to examine sex-related differences. Methods: In this cross-sectional study, 285 healthy adults (143 men, 142 women) were assigned to one of three separate activity groups: brisk walking (n = 100), running (n = 90), or cycling (n = 95). Activity protocols were standardized using fixed activity-specific speeds corresponding to 5.2, 8.2, and 4.4 METs for brisk walking, running, and cycling, respectively, based on the Chinese Compilation of Physical Activities. Each participant completed one 60-min exercise session under controlled environmental conditions. Sweat samples were collected from the chest using sweat patches and analyzed for Na⁺, K⁺, Ca²⁺, Mg²⁺, Fe, Zn²⁺, and Cu²⁺ using ICP-MS/OES. Whole-body sweat sodium and potassium concentrations were estimated using validated regression equations. Body mass loss (BML) and sweat rate were calculated from pre- and post-exercise nude body mass. Results: Across all participants and activity types, the overall mean sweat rate was 0.71 ± 0.28 L/h, and the mean percentage of body mass loss (BML%) was 0.78 ± 0.45%. Among the three physical activities, running elicited higher sweat rates (0.92 ± 0.29 L/h) and BML% (1.16 ± 0.36%) than brisk walking or cycling (p < 0.05). The estimated whole-body Na⁺ and K⁺ concentrations across all participants were 34.10 ± 10.31 mmol/L and 3.35 (3.02–3.93) mmol/L, respectively, with 59.3% of participants classified as having moderate Na⁺ levels (30–60 mmol/L). Men exhibited higher sweat rates and Na⁺ concentrations, whereas women showed higher K⁺, Zn²⁺, and Cu²⁺ levels (p < 0.05). Conclusions: Chinese adults engaging in common daily physical activities under temperate environmental conditions demonstrated low-to-moderate sweat rates and sodium concentrations. These findings provide baseline reference data for population-specific hydration education and may inform future validation and application of wearable sweat-sensing technologies in public health monitoring.