Sweat output on the dorsal aspect of the forearm and foot

We have used intradermal electrical stimulation to elicit a direct local sweating response in a reliable and dose-dependent manner. We applied this model to describe the dose-response characteristics of the forearm and foot. We hypothesized that the sweat volume-response curve would be similar for the foot and forearm. The local sweat rate (LSR) was determined with a small sweat capsule (0.53 cm²) flushed with dry air (100 mL/min). A constant 5-mA electrical stimulus was applied to the intradermal space at 10 different frequencies (0.2, 1, 2, 4, 8, 10, 12, 16, 32, and 64 Hz) to generate the stimulus-response curves. Subjects rested at an ambient temperature of 28°C. The data (means ± 1 SD; n = 15; 8 females and 7 males) revealed that the stimulus-response curves were significantly different (P < 0.001) with the foot sweating being slightly higher than the forearm at low-stimulus frequencies. When sweat output was normalized as a percentage of the maximal area under the curve the analysis revealed a 21% increase in sweat output [AUC (%max)] for the foot between stimulus frequencies 0.2, 1, 2, and 4 Hz. The overall response curve for the foot was shifted upward (P = 0.011) at the lowest four stimulus frequencies, and the EC₅₀ was lower for the foot (7.29 ± 2.79 Hz) than the forearm (8.8 ± 2.39 Hz; P = 0.002). The significantly higher baseline indicates that the foot was slightly more sensitive to mild intradermal electrical stimulation than the forearm.NEW & NOTEWORTHY We noted a greater sensitivity of the foot during direct activation of the sudomotor nerves compared to the forearm. This observation contrasts with earlier findings indicating similar sweating responses from the dorsal aspect of the foot and forearm.