Partitioning secreted sweat in the measure of sweating efficiency during exercise.

Sweating efficiency (SE), defined as the proportion of total secreted sweat that evaporates from the body, is a critical determinant of evaporative heat loss and fluid requirements during exercise. Despite its physiological importance, direct assessments of evaporated sweat loss (ESL) and SE during upright, clothed exercise are rare, owing to substantial methodological and logistical challenges. Here, we describe a modernized direct system for quantifying ESL, total sweat loss (TSL), and SE during exercise while simultaneously partitioning the end-exercise fate of all secreted sweat. The method combines precision measurement of mass change in a subject-ergometer-oil pan system to quantify ESL with gold-standard nude body mass measurements to determine TSL, after correction for nonsweat losses. Unevaporated sweat is further partitioned into dripped sweat, sweat retained in clothing, and sweat remaining on the skin. Equipment specifications, experimental workflow, validation procedures, and calculations are detailed to ensure that ESL and TSL correspond to the same exercise interval. This approach enables rigorous investigation of environmental, physiological, and apparel-related modifiers of sweating efficiency relevant to studies of human heat and fluid balance.NEW & NOTEWORTHY We present a direct, mass-balance methodology for determining evaporated sweat loss and sweating efficiency during upright, clothed exercise. The approach resolves key technical barriers limiting direct assessment of sweating efficiency and enables a complete accounting of secreted sweat by partitioning it into evaporated sweat, dripped sweat, sweat retained in clothing, and sweat remaining on the skin. This framework supports mechanistic evaluation of factors that promote and limit sweating efficiency and sweat evaporation during exercise-heat stress.