## 4 Regional Sweat Patches  
When investigating the effects of HA on electrolyte preservation, most studies examine these adaptations using regional sweat patch measurement, which may demonstrate differences in sweat responses depending on regional body area [65]. Traditional HA protocols are typically characterized by either endurance-based cycling or running exercises. Although both are endurance-based types of exercise, running utilizes more whole-body movement compared to cycling. Thus, regional sweat rates may differ between exercise types, with more sweat potentially being lost in the lower extremities during cycling compared to running since most movement occurs in the leg musculature [66]. However, there seem to be limited to no differences in sweat electrolyte adaptation between HA protocols of cycling versus running [40; 42; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59]. From one of the earlier studies that investigated sweat electrolyte adaptations and HA, [PERSON] and [PERSON] (1986) found increases in sweat Na\({}^{+}\) reabsorption via sweat patches at the chest site, explained by the authors as eccrine gland responsiveness to plasma aldosterone, following a 10-day cycling HA protocol [49]. Significant reductions in sweat Na\({}^{+}\) were noted after the 10 days during both the first and second hour of exercise [49]. In addition, plasma aldosterone concentrations were reduced following HA, which partially demonstrates the sweat Na\({}^{+}\) adaptation. As the chest regional site has been shown to be one of the best sites for predicting whole-body sweat, deductions from this study can be deemed accurate for the determination of sweat electrolyte concentration adaptations. Other investigations have also chosen the chest site for sweat patch placement [47]. Following an 8-day HA cycling protocol, young males, highly fit old males, and normally fit old males collectively experienced sweat Na\({}^{+}\) reductions at several regional sites (chest, back, forearm, and thigh) [47]. The younger males experienced these adaptations at all four sites, whereas the highly fit older males did not experience these reductions at the forearm and thigh sites, nor did the normally fit older males experience these reductions at the back and forearm sites [47]. Several investigations have determined sweat electrolyte adaptation from HA using the upper back and upper arm regional sites [48; 50; 51; 54; 55; 57; 60], with all but one demonstrating positive adaptation [60]. Although significant differences were not seen overall, the researchers did note that a pre-post-training reduction in sweat Na\({}^{+}\) was observed in 13 of the total 17 participants, suggesting some changes had occurred [60]. Additionally, another study utilized regional sweat patches for electrolyte assessment and observed conserved Na\({}^{+}\) following HA; however, the authors did not specify where the patch was regionally located [59]. In field or laboratory settings when skin sites might be restricted due to clothing, a forearm single-site measurement is typically attained. [PERSON] et al. (2010) measured sweat electrolyte loss with sweat patches at a single site, the upper forearm [56]. Following a short-term 4-day HA protocol, male cricket club players exhibited substantial decreases in sweat Na\({}^{+}\), K\({}^{+}\), and Cl\({}^{-}\) concentrations [56]. Although this study involved only 4 days of HA, reductions were still noted, providing evidence that sweat electrolyte adaptations may begin within the first couple of days. Similarly, reduced sweat Na\({}^{+}\) was noted following 3 consecutive days of cycling-based heat training from sweat collected at the forearm site [53]. Thus, the forearm is likely a valid location for sweat electrolyte assessment in settings where clothing hinders other sites. Non-traditional forms of heat training have also resulted in positive electrolyte adaptations via regional patch measurement [61; 62].