Sweat from gland to skin surface: production, transport, and skin absorption.

By combining galvanic skin conductance (GSC), stratum corneum hydration (HYD) and regional surface sweat rate (RSR) measurements at the arm, thigh, back and chest, we closely monitored the passage of sweat from gland to skin surface. Through a varied exercise-rest protocol, sweating was increased slowly and decreased in 16 male and female human participants (25.3 ± 4.7 yr, 174.6 ± 10.1 cm, 71.3 ± 12.0 kg, 53.0 ± 6.8 ml·kg-1·min-1). ∆GSC and HYD increased before RSR, indicating pre-secretory sweat gland activity and skin hydration. ∆GSC and HYD typically increased concomitantly during rest in a warm environment (30.1 ± 1.0°C, 30.0 ± 4.7% relative humidity) and only at the arm did ∆GSC increase before an increase in HYD. HYD increased before RSR, before sweat was visible on the skin, but not to full saturation, contradicting earlier hypotheses. Maximal skin hydration did occur, as demonstrated by a plateau in all regions. Post exercise rest resulted in a rapid decrease in HYD and RSR but a delayed decline in ∆GSC. Evidence for reabsorption of surface sweat into the skin following a decline in sweating, as hypothesized in the literature, was not found. This suggests that skin surface sweat, after sweating is decreased, may not diffuse back into the dermis, but is only evaporated. These data, showing distinctly different responses for the three measured variables, provide useful information about the fate of sweat from gland to surface that is relevant across numerous research fields (e.g., thermoregulation, dermatology, ergonomics and material design). NEW & NOTEWORTHY After sweat gland stimulation, sweat travels through the duct, penetrating the epidermis before appearing on the skin surface. We found that only submaximal stratum corneum hydration was required before surface sweating occurred. However, full hydration occurred only once sweat was on the surface. Once sweating reduces, surface sweat evaporation continues, but there is a delayed drying of the skin. This information is relevant across various research fields, including environmental ergonomics, dermatology, thermoregulation, and skin-interface interactions.

Female thermal sensitivity to hot and cold during rest and exercise.

Regional differences in thermal sensation to a hot or cold stimulus are often limited to male participants, in a rested state and cover minimal locations. Therefore, magnitude sensation to both a hot and cold stimulus were investigated during rest and exercise in 8 females (age: 20.4 ± 1.4 years, mass: 61.7 ± 4.0 kg, height: 166.9 ± 5.4 cm, VO2max: 36.8 ± 4.5 ml·kg(-1)·min(-1)). Using a repeated measures cross over design, participants rested in a stable environment (22.3 ± 0.9°C, 37.7 ± 5.5% RH) whilst a thermal probe (25 cm(2)), set at either 40°C or 20°C, was applied in a balanced order to 29 locations across the body. Participants reported their thermal sensation after 10s of application. Following this, participants cycled at 50% VO2max for 20 min and then 30% VO2max whilst the sensitivity test was repeated. Females experienced significantly stronger magnitude sensations to the cold than the hot stimulus (5.5 ± 1.7 and 4.3 ± 1.3, p<0.05, respectively). A significant effect of location was found during the cold stimulation (p<0.05). Thermal sensation was greatest at the head then the torso and declined towards the extremities. No significant effect of location was found in response to the hot stimulation and the pattern across the body was more homogenous. In comparison to rest, exercise caused a significant overall reduction in thermal sensation (5.2 ± 1.5 and 4.6 ± 1.7, respectively, p<0.05). Body maps were produced for both stimuli during rest and exercise, which highlight sensitive areas across the body.

Thermal comfort following immersion.

Unlike thermal comfort in air, little research has been undertaken exploring thermal comfort around water sports. We investigated the impact of swimming and cooling in air after swimming on thermal comfort. After 10 min of swimming-and-resting cycles in 28°C water, volunteers wearing two types of garments or in swim briefs, faced winds in 24°C air, at rest or when stepping. Thermal comfort was significantly higher during swimming than resting. Post-immersion, following maximum discomfort, in 45 of 65 tests thermal comfort improved although mean skin temperature was still cooling (0.26 [SD 0.19] °C·min(-1) - max was 0.89°C·min(-1)). When thermal comfort was re-established mean skin temperature was lower than at maximal discomfort in 39 of 54 tests (0.81 [SD 0.58] °C - max difference was 2.68°C). The reduction in thermal discomfort in this scenario could be due to the adaptation of thermoreceptors, or to reductions in cooling rates to levels where discomfort was less stimulated. The relief from the recent discomfort may explain why, later, thermal comfort returned to initial levels in spite of poorer thermal profiles.

Thermal sensitivity to warmth during rest and exercise: a sex comparison.

PURPOSE: The study aimed to compare thermal sensation in response to a fixed warm stimulus across 31 body locations in resting and active males and females. METHODS: Twelve males (20.6 ± 1.0 years, 78.1 ± 15.6 kg, 180 ± 8.9 cm, 34.4 ± 5.2 ml kg(-1) min(-1)) and 12 females (20.6 ± 1.4 years, 62.9 ± 5.5 kg, 167 ± 5.7 cm, 36.5 ± 6.6 ml kg(-1) min(-1)) rested in a thermoneutral (22.2 ± 2.2 °C, 35.1 ± 5.8 % RH) room whilst a thermal probe (25 cm(2)), set at 40 °C was applied in a balanced order to 31 locations across the body. Participants reported their thermal sensation 10 s after initial application. Following this, participants began cycling at 50 % [Formula: see text] for 20 min, which was then lowered to 30 % [Formula: see text] and the sensitivity test repeated. RESULTS: Females had significantly warmer magnitude sensations than males at all locations (4.7 ± 1.8 vs 3.6 ± 2.2, p < 0.05, respectively). Regional differences in thermal sensation were evident but were more prominent for females. Thermal sensation was greatest at the head then the torso and declined towards the extremities. In comparison to rest, exercise caused a significant reduction in thermal sensation for males (∆thermal sensation; 0.86 ± 0.3, p < 0.05), but only at select locations in females (0.31 ± 0.56, p > 0.05). CONCLUSION: The data provide evidence that the thermal sensation response to warmth varies between genders and between body regions and reduces during exercise. These findings have important implications for clothing design and thermophysiological modelling.

The role of decreasing contact temperatures and skin cooling in the perception of skin wetness.

Cold sensations are suggested as the primary inducer of the perception of skin wetness. However, limited data are available on the effects of skin cooling. Hence, we investigated the role of peripheral cold afferents in the perception of wetness. Six cold-dry stimuli (producing skin cooling rates in a range of 0.02-0.41°C/s) were applied on the forearm of 9 female participants. Skin temperature and conductance, thermal and wetness perception were recorded. Five out of 9 participants perceived wetness as a result of cold-dry stimuli with cooling rates in a range of 0.14-0.41°C/s, while 4 did not perceive skin wetness at all. Although skin cooling and cold sensations play a role in evoking the perception of wetness, these are not always of a primary importance and other sensory modalities (i.e. touch and vision), as well as the inter-individual variability in thermal sensitivity, might be equally determinant in characterising this perception.