Two isothermal challenges yield comparable physiological and subjective responses

Ventilated vests are developed to reduce thermal stress by enhancing convective and evaporative cooling from skin tissue underneath the vest. By enhancing convection and evaporation, the exercise-induced increase in body core temperature may be attenuated (Barwood et al. 2009; Reffeltrath 2006). Even without a physiological effect, if a ventilated vest can reduce discomfort by providing a lower heat sensation this may also be considered beneficial. As previous research showed that local discomfort overrides whole-body comfort (Zhang et al. 2004), a ventilated vest could positively influence whole-body subjective comfort or thermal sensation by mitigating local discomfort. The other way around, it is discussed whether a local warm stimulus can be detrimental to performance through psychological stress (i.e., thermal sensation) (Lloyd and Havenith 2019; Van Cutsem et al. 2019). In the experiment of Van Cutsem et al. (2019), twelve trained cyclists or triathletes performed a time to exhaustion test which consisted of a warming-up (cycling for 5 min at 40% VO_2max), followed by cycling until exhaustion at 70% VO_2max in a climate chamber set to 20 °C and 44% relative humidity (RH). Their participants performed the trial twice: one time with and one time without a 30 × 40 cm electric heat pad (~ 40 °C) covering the upper back and both scapulae. Time to exhaustion was found to decrease by 9% without changes in physiological measures. Thus, the performance decrease was attributed to an increased subjective thermal strain (Van Cutsem et al. 2019). However, Lloyd and Havenith (2019) argued that the actual heat load was increased by 4–6% in the experiment of Van Cutsem et al. (2019) so it could have been that there was no independent role of subjective thermal strain in the heat-induced reduced performance. Here we are interested in whether two different local clothing setups, which are comparable with respect to physical thermal strain, would result in comparable physiological, in particular local sweating, and subjective thermal responses.

The human body exchanges heat through conduction, convection, radiation (dry heat exchange) and evaporation (latent heat exchange). In common work situations, the body’s maximal cooling capacity is limited by convective and evaporative heat loss capacity (Kenney et al. 2015). Convective heat loss is a function of the effective air velocity over the body surface area and the temperature gradient between the air and the body surface (Parsons 2014). Evaporative heat loss is a function of the amount of sweat that is produced and the physical ability of sweat to evaporate (Parsons 2014). That is, in the best cooling scenario the sweating response is maximal and all sweat can evaporate. This is most likely to occur in situations with a high vapour pressure difference between skin and air (i.e., with a low RH) and high air speed. As ambient temperature increases, the effectiveness of convection to facilitate body heat loss decreases due to the small temperature gradient between air and skin temperature (T_sk). Consequently, in hot environments evaporation provides the primary defense against overheating (Parsons 2014). A variety of external cooling strategies to lower the exercise-induced increase in body core temperature by enhancing convection and evaporation, including ventilated vests, have been evaluated in the military, occupational settings and elite sports (Barwood et al. 2009; Bongers et al. 2017; Eijsvogels et al. 2014; O’Hara et al. 2016). Prior to using such systems in occupational settings or sports, its performance should ideally be assessed using standard methods (ASTM-F2300-10 2016; ASTM-F2371-16 2010).

From a physiological control point of view, body core temperature is the central controller for the production of sweat and T_sk has a modifying effect (McCaffrey et al. 1979; Nadel et al. 1971a, b). This modifying effect of T_sk is estimated to be a tenth of the core temperature part (Nadel et al. 1971a). The effect is stimulating when T_sk is high and inhibiting when T_sk is low. By applying local ventilation using a ventilated vest, local T_sk may decrease, causing less sweat production underneath the vest. However, recently Hospers et al. (2020) showed that whole-body sweat rate is highly correlated to required evaporation (E_req) and does not depend on mean T_sk independent of E_req to achieve heat balance. From a biophysical perspective, E_req is defined by body heat production minus dry heat loss from skin and respiratory heat loss (Cramer and Jay 2016). Since the aim of our study is to create two locally isothermal situations, no differences in local E_req and consequently local sweat rate (LSR) is expected, as per heat balance theory. Note that sweating per se is not equal to evaporation, and maximal evaporation (E_max) is limited by the vapour pressure difference between skin tissue and the air, and the convective heat transfer coefficient (Cramer and Jay 2016). With respect to a ventilated vest, it is unknown whether local enhancement of evaporation would cause a decrease in local required evaporation on other skin sites (to balance the global E_req). If true, LSR (supply of water) outside the ventilated vest should decrease proportionally to the amount of sweat that is evaporated extra underneath the vest.

Ventilated vests come with the cost of extra insulation. However, by bridging the low conductivity of the vest with ventilation one can compensate for insulation or even exceed it if applying even higher ventilation rates. In the current study, two isothermal systems were tested: one with and one without ventilated vest. The air speed of the ventilated vest was set such that it compensated for the dry thermal resistance of the vest to create two different but isothermal challenges. It was hypothesized that two systems with similar dry thermal resistance—and, therefore, similar E_req—provide similar thermal stress, measured by physiological and subjective responses. If successful, the present study provides a method to calculate the limit of air movement (due to walking) for which a ventilated vest is beneficial compared to wearing no ventilated vest, or the required ventilation for protective equipment in occupational settings. This may be useful to extend the verification procedure for standard settings on ventilated vest performance (ASTM-F2300-10 2016; ASTM-F2371-16 2010; McCullough and Eckels 2009).

This study shows that two clothing systems, one of them with a highly insulating ventilated vest, but both with similar local dry effective thermal resistance (0.185 m² KW⁻¹ for the ventilated vest condition and 0.188 m² KW⁻¹ for no-vest) provided similar thermal stress. No significant differences between conditions were found in evaporated mass, WBSL, T_gi, and HR throughout the sessions in the climate chamber. In addition to these central thermal responses, no significant differences were found in T_neck, T_scapula, T_hand and T_shin between conditions. Likewise, LSR on the central upper back, forehead, upper arms and upper legs was not significantly different between conditions. Subjective responses supported these findings as no significant differences were found between the vest and no-vest condition in RPE, TS and TC.

These findings add to a recent debate discussing whether a local warm stimulus would cause a shorter time to exhaustion in a temperate condition (20 °C, 44% RH) (Lloyd and Havenith 2019; Van Cutsem et al. 2019). One of the main points of discussion was whether the local warm stimulus added true physical heat stress or only psychological heat stress (i.e., thermal sensation) as the authors argued. The authors found a 9% decrease in time to exhaustion without changes in physiological measures. They therefore attributed the performance decrease to an increased subjective thermal strain. However, Lloyd and Havenith (2019) suggested that heat load was increased by 4–6% in their experiment (Lloyd and Havenith 2019). In this paper, we show that in two different but isothermal settings no physiological and subjective differences were observed during a fast-paced walking activity (Figs. 3, 4, 5, 6). While this study did not provide a local warm stimulus and compensated with local cooling at a different site, we do show that in absence of a physical difference in heat stress, no difference in thermal outcome measures can be expected.

The skin temperature sensor on the scapula was the only sensor located underneath the vest, but there was no significant decrease in T_scapula in the vest condition. There was a trend towards lower values: ΔT_scapula = 0.35 ± 0.37 °C from baseline to final with vest and ΔT_scapula = 0.74 ± 0.62 °C (p = 0.096) in the no-vest condition (Fig. 4b). Considering the minor contribution of T_sk to sweating (Nadel et al. 1971a, b) and the limited changes in T_scapula, this most likely explains why LSR was not affected by the ventilated vest either (Fig. 5). In combination with the comparable core temperature (as indicated by T_gi; Fig. 3), the main driver of sweat production (Nadel et al. 1971a, b), it is not surprising that LSR was similar between conditions as per physiological control point of view. Our findings also support the biophysical perspective on the regulation of sweating. Since on the torso the ventilated vest manipulation led to equal thermal strain compared to a no-vest situation (Table 1), presumably no differences in E_req occurred between conditions (Gagnon et al. 2013; Ravanelli et al. 2020). With similar E_req values, the absence of differences in LSR can likely be explained. Interestingly, on skin locations outside the ventilated vest, the underwear (i.e., on the upper arms) was slightly wet. On these locations, the residual local E_req potentially was higher than the local E_max. The absence of a difference in LSR on the upper arm (Fig. 5) may be contributed to achieving the local E_max. On skin locations under the vest, it is unlikely that E_max was achieved as underwear was dry underneath the ventilation channels. Another possible explanation for the similar LSR could be that in addition to thermal factors, sweating is influenced by exercise-induced non-thermal factors including positive stimuli from mechanoreceptors and metaboreceptors (Kondo et al. 2010). Since the same exercise protocol was utilized in the two isothermal conditions, the non-thermal contribution to sweating was most likely similar. Depending on how big the contribution of non-thermal factors was, this could help in explaining our findings.

In the current study, large individual variation between physiological and subjective responses was observed (Figs. 3, 4, 5, 6), which may be explained by differences in personal characteristics such as training background. Fitness level and body composition were assumed to be homogenous considering the small range in body mass index, the inclusion of participants with a certain training frequency (≥ 3 times 1.5 h a week) and appropriate anthropometrics to fit the ventilated vest (i.e., little upper body fat) and were therefore not expected to influence our results. The large regional variation in LSR that was found in the current study is in accordance with existing literature (Smith and Havenith 2011) and the absorbent patch technique used in the current study correlates well with measurements of a ventilated capsule system (Morris et al. 2013). Both systems were commonly used to detect small differences in sweat rate. Additionally, Smith and Havenith (2011) used the absorbent patch technique to detect regional differences in sweat rate over the entire body surface during moderate-intensity exercise, which is comparable to the current study. Therefore, it was expected that our methods were sensitive enough to detect the differences in sweat characteristics.

Apart from the physiological responses, ventilated vests could also be beneficial when reducing heat sensation. These subjective responses may apply to whole-body or local sensations. Previous research showed that local discomfort overrides whole-body comfort (Zhang et al. 2004). Thus, if the ventilated vest, a local manipulation, felt uncomfortable during the experiment, this most likely would have determined the subjective scores. However, no significant differences in subjective scores were found between conditions (Fig. 6), again implying that thermal stress was similar with and without ventilated vest.

Having such a ventilated vest one can put extra layers on top of it with a limited increase of the total thermal insulation (provided that vest inlets and outlets are not obstructed), since the vest circumvents the thermal insulation of the vest itself and layers on top. Additional tests with multiple layers (ballistic and load bearing vest; LCPB) suggest that adding an extra layer (\(I_{\text{LCPB}}\) = 0.218 m² KW⁻¹) could increase static local thermal insulation without ventilated vest to:Whereas with the inclusion of the vest (ventilated at 1.5 ms⁻¹), total measured local thermal insulation was \(I_{{{\text{system,LCPB}},1.5}}\) = 0.242 m² KW⁻¹. Given that \(I_{system,1.5}\) = 0.196 m² KW⁻¹, wearing the LCPB only adds 0.046 m² KW⁻¹ instead of 0 \(I_{\text{system,LCPB}}\) = 0.218 m² KW⁻¹. This means that the ventilated vest roughly compensates 75% of the insulation provided by additional layers. Such a ventilated vest could be beneficial in settings in which several clothing layers are worn on top of each other, for example in occupational settings (police) and the military. To be able to use such a system in contaminated environments, some practical modifications have to be made (the system allows for filters to avoid contamination by particles underneath the vest). Moreover, to verify performance, the system ideally should be evaluated according to ASTM standards (ASTM-F2300-10 2016; ASTM-F2371-16 2010; McCullough and Eckels 2009). The protocols performed in the present study diverge from these standards, especially regarding metabolic rate (~ 415–520 W (ISO8996 2004) here; 250 W prescribed by ASTM), which should be taken into account prior to usage in practice. Nevertheless, the calculations made here can potentially be used to develop a cooling regulation algorithm to predict the limit of air movement (due to walking) for which a ventilated vest is beneficial or to estimate the required ventilation to compensate for insulation of added layers. This information may be useful to extend the current verification procedures for standard settings on ventilated vest performance.

Findings only apply to the specific condition (34 °C, 20% RH) and exercise intensity (walking at 7 km h⁻¹) that was used during the current experiment. Furthermore, as one participant was excluded from the study, the detectable effect size value increased from 0.85 to 0.90 (given α = 0.05, power = 0.8 and n = 9). This denotes we were able to detect a difference between the two conditions once the effect size exceeded 0.90, which is typically classified as a large effect. The study lacks power to detect a smaller difference.

To carry the additional mass of the ventilated vest (1.3 kg), metabolic rate could have been increased. Previously it was observed that by adding 2 kg around the waist, metabolic rate increased by 3% (Dorman and Havenith 2007). Adding 1.3 kg most likely caused a minor increase of ~ 10 W (~ 2%) in metabolic rate (Epstein et al. 1987; Pandolf et al. 1977).

It is further assumed that the underwear supported wicking of sweat, yet despite the dry conditions, at the end of the experiments the underwear was slightly wet, but the channels were dry. Therefore, on locations outside the vest that were covered by the underwear and were wet at the end of the experiment, local E_max was presumably lower than local E_req. As the channels were dry, on the torso E_max > E_req.

Another potential limitation could be that, as an exception due to logistical constraints, thermal insulation of the underwear (\(I_{\text{uw}}\)) was determined by applying wind (speed 1.5 ms⁻¹) from the front. In the experiment, ventilation was applied from the front and back by the ventilated vest. As a consequence, the actual \(I_{\text{uw}}\) could be slightly different. However, because the exact same underwear was worn in both conditions this is not affecting our results. Second, a ventilation speed of 1.5 ms⁻¹ was achieved underneath the ventilated vest on the manikin, but due to breathing and body morphology, the actual ventilation speed during the experiment was most likely slightly lower. Therefore, additional measures with a ventilation speed of 1 ms⁻¹ were performed. This resulted in \(I_{{{\text{system}},r,1.0}}\) = 0.222 m² KW⁻¹. Presumably the dynamic \(I_{{{\text{system}},r}}\) during the experiment was in between 0.222 m² KW⁻¹ (1 ms⁻¹) and 0.185 m² KW⁻¹ (1.5 ms⁻¹). The \(I_{{{\text{system}},r}}\) value of the no-vest condition (0.188 m² KW⁻¹) falls within this range. Both issues may have caused an offset in our calculations.

Finally, LSR underneath the vest was not assessed in the current study for practical reasons, yet no difference underneath the vest is assumed because there was no difference in measured whole-body evaporated mass and LSR outside the vest (Fig. 3, 5). Future studies should take into account the additional measures of LSR under the vest and include recordings of wet discomfort and clinginess as they may reveal differences between both conditions.

This paper shows that in two different isothermal settings, no physiological and subjective differences are observed during a fast-paced walking activity. Two clothing systems with comparable dry thermal resistance (local thermal resistance: 0.185 m² KW⁻¹ for the vest and 0.188 m² KW⁻¹ for the no-vest condition) provided equal thermal stress and local ventilation did not alter local sweating responses on body parts outside the vest. The latter can be explained by equal required evaporation, a dominant central drive for sweating and/or exercise-induced non-thermal factors. The calculations made in the current study can be used to develop a cooling regulation algorithm, predicting the limit of air movement for which a ventilated vest is beneficial and/or estimating required ventilation to compensate for the insulation of added layers, or even exceed it if applying higher ventilation rates. This could potentially be used to extend the standard verification procedures for ventilated vest performance. Such a ventilated vest could be beneficial in the military and for the police since they wear multiple clothing layers on top of each other and the ventilated vest circumvents the insulation that comes with the extra clothing.