Sweat secretion from palmar and dorsal surfaces of the hands during passive and active heating

Introduction: It is generally accepted that the palmar (volar) and dorsal surfaces of human hands display different sudomotor responses to mental or thermal stimuli. We tested the hypothesis that, during thermal stimulation, secretion from the dorsal surfaces would always exceed that from the volar aspect of the hand. Methods: Sweat secretion from 10 hand sites and the forehead was examined (ventilated capsules) in 10 subjects during passive heating (climate chamber: 36°C, 60% relative humidity, water-perfusion suit: 40°C) immediately followed by incremental cycling to volitional fatigue. Results: This treatment significantly increased core temperature (39.3°C), heart rate (178 bpm), and sweat rate at all sites. Mean sweat secretion during exercise was greater at the forehead (2.90 mg·cm ^-2·min^-1; ± 0.19) than the hand (1.49 mg·cm^-2·min^-1; ± 0.27). While no significant differences in sweating were observed among dorsal sites, a nonuniform secretion pattern was observed across the volar surface, with sweating at the palm being the lowest, and that from the volar aspect of the distal phalanges being equivalent to the dorsal hand. These differences became more evident as exercise progressed. Mean hand sweat rate during exercise was 41.7 ml·h^-1, with sweating from the palm accounting for only about 6% of sweat secretion. Conclusion: Sweat secretion from both the palmar and dorsal surfaces of the hand increases during exercise in the heat, although this occurs in a nonuniform fashion. It is possible that a greater sweat gland density on the fingers may account for variations across the volar surface. However, higher dorsal sweating with lower gland counts (high glandular flow) may be attributable to either larger sweat glands, or to a greater cholinergic sensitivity of these glands.