Sweat lactate threshold heart rate monitoring based on multi-parameter electrochemical sensing and lactate excretion rate analysis.

Lactate is an important biomarker for assessing anaerobic metabolism and exercise intensity, and its continuous non-invasive monitoring is valuable for sports medicine and rehabilitation. However, sweat-based lactate sensing still faces two major challenges: the physiological interpretation of sweat signals remains under discussion, and sensor stability is often limited under dynamic conditions. In this work, we developed a flexible enzymatic electrochemical sensing platform based on toluidine blue O (TBO) and a Nafion cation-exchange membrane. The electrostatic interaction between the sulfonate groups of Nafion and TBO formed a stable electron-transfer interface and effectively reduced mediator leaching. The lactate sensor showed a linear range of 0.1-40 mM and a limit of detection (LOD) of 0.062 mM. At the system level, the sensor was integrated with a multiparametric fusion strategy to calculate sweat lactate excretion rate in real time, which helped overcome the limitation of using concentration alone. Human studies showed that the sweat lactate threshold heart rate (SLTHR), corrected by sweat rate, agreed closely with the blood lactate threshold, with a maximum deviation of 3 bpm. The system also tracked changes in lactate threshold over multiple days of training and captured an increase in threshold heart rate from 147 bpm to 158 bpm in one subject. This demonstrates the potential of the sensing platform for measuring fitness improvements and exercise performance. The platform has potential for long-term physiological monitoring and exercise prescription.