Effects of air-perfused rucksack on physiological and perceptual strain during low-intensity exercise in a hot environment

ABSTRACT

The upcoming Tokyo Olympic and Paralympic Games may be held amid extremely high wet-bulb globe temperature conditions. Many studies have focused on countermeasures to prevent the reduction in exercise performance in the heat. However, cooling strategies for managing heat stress of staff and spectators remain poorly understood. The present study investigated the effects of a lightweight fan cooling device, namely a commercially available air-perfused rucksack, on physiological and perceptual responses during low-intensity exercise in a hot environment. Ten males walked (5.5 km/h, 2.0% gradient) for 60 min in hot conditions (35°C, 50% relative humidity). All participants performed two trials with and without the air-perfused rucksack, respectively. Air was blown onto the upper back and neck via two fans attached on either side of the rucksack. Rectal temperature, neck skin temperature, heart rate, and physiological strain index were significantly lower during walking (P < 0.05) with the rucksack. Additionally, the ratings of perceived exertion, thermal sensation, and thermal comfort were significantly lower (P < 0.05) with the rucksack. These data suggest that the air-perfused rucksack may be effective for managing heat stress of staff and spectators at the Tokyo Olympic and Paralympic Games.

Abbreviations

CON: control trial; ES: effect sizes; FAN: fan cooling trial; HR: heart rate; _mT_sk: mean skin temperature; pre: pre-exercise; PSI: physiological strain index; RPE: rating of perceived exertion; SD: standard deviation; TC: thermal comfort; T_neck: neck skin temperature; T_re: rectal temperature; TS: thermal sensation

Introduction

Worldwide, heat stress has increased with global warming and climate variation. The upcoming Tokyo Olympic and Paralympic Games may be held amid extremely high wet-bulb globe temperature conditions [1]. Indeed, the wet-bulb globe temperature of Tokyo in summer is expected to be higher than in any previous host cities (i.e., Rio de Janeiro, London, and Beijing) [1]. In such a situation, the management of heat stress for athletes, as well as staff and spectators, is important. As cooling strategies for athletes, previous studies have examined whether interventions such as water immersion or ice slurry ingestion can prevent the reduction in exercise performance in the heat [2]. However, cooling strategies for managing heat stress of staff and spectators have not been fully investigated.

Microclimate cooling systems such as air- or liquid-perfused garments have been developed for cooling individuals who are required to wear protective clothing in a hot environment (e.g., military personnel, firefighters, and athletes). These garments increase heat loss through conduction, convection, and/or evaporation, and thus effectively attenuate core temperature elevation [3–12] and maintain thermal sensation (TS) [5,8,10,11] when protective clothing is worn in a hot environment. Several studies have reported that the use of air-perfused vests reduces heat stress during low-intensity exercise in a hot environment [3,7–9]. For example, the use of air-perfused vests (1.2 kg) when wearing a standard battle dress uniform and a ballistic vest (10 kg) attenuates the increases in rectal temperature, heart rate (HR), and physiological strain during walking in a hot environment [9]. Additionally, using such vests (approximately 2.3 kg) when wearing the Army Combat Uniform, interceptor body armor, and a Kevlar helmet (approximately 8 kg) attenuates the increases in rectal temperature and HR during walking in hot conditions [7].

However, these traditional cooling devices are relatively large and heavy. A small, light, and easy-to-use cooling device is required for managing heat stress of staff and spectators at the Tokyo Olympic and Paralympic Games. An air-perfused rucksack with battery-driven fans was recently developed. Air is blown continuously onto the back and neck via two fans attached to either side of the rucksack. Cooling the neck effectively improves thermal comfort (TC) at rest in the heat [13], and improves TS during exercise in a hot environment [14–16]. The back is the most profuse site of sweating on the body [17]. Thus, applying fan cooling to the back is expected to lead to significant evaporative heat loss. The present study investigated the effect of a commercially available air-perfused rucksack on core temperature, HR, and TS during low-intensity exercise in a hot environment. We hypothesized that the improvement in evaporative heat loss on the back and neck by fan cooling would attenuate the core temperature elevation and improve TS during walking in a hot environment.

Methods

Participants

Ten young males volunteered to participate in this study (mean ± standard deviation [SD]; age = 22.5 ± 1.7 years, height = 1.71 ± 0.05 m, body mass = 70.3 ± 8.9 kg). All participants were healthy and nonsmokers, with no history of cardiovascular disease. Participants were instructed to keep their normal diet, hydration state, and physical activity level during the study period. Participants refrained from strenuous activity, and the ingestion of alcohol and caffeine, in the 24-h period prior to testing. The study was approved by the Research Ethics Committee of Chukyo University in conformity with the Declaration of Helsinki.

Fan cooling device

The fan cooling device used in the present study was an air-perfused rucksack with a hood (Sports Fan Ruck [SUMMER RUNNER]; Worker, Okazaki, Japan) (Figure 1). There are two battery-driven fans on the either side of the rucksack. The side that makes contact with the back has a mesh structure with an area of approximately 595 cm². The speed of the air flow onto the upper back is 4–5 m/s, as measured in our laboratory. The top of the rucksack has a hole through which air blows onto the neck. A hood is used to direct air onto the back of the neck. The weight, length, and width of the rucksack are 648 g, 40 cm, and 11–29 cm, respectively. According to the manufacturer, the flow rates of the fan units (AC150; RYOBI, Hiroshima, Japan) attached to the rucksack are 53 l/s at 9 V.

Figure 1.

Photographs of front side (a) and back side (b) of the air-perfused rucksack.

Experimental session

All participants performed two trials with (FAN) and without (CON) the fan cooling device, respectively, in a randomized, counterbalanced order, separated by at least 5 days to minimize carryover effects because of exercise in a hot environment. Trials were conducted at the same time of day to minimize the effect of circadian rhythm. Participants wore an identical 100% polyester T-shirt, running shorts, and running shoes during the two trials. The rucksack was placed over the T-shirt in the FAN trial. All experimental sessions were performed in a climate chamber (35°C, 50% relative humidity [RH]) (TBR-12A4PX; ESPEC, Osaka, Japan). The participants walked on a treadmill (BM-1200; S&ME, Tokyo, Japan) at 5.5 km/h (2.0% gradient) for 60 min. Upon arrival at the laboratory, participants were instructed to empty their bladder. They then drank a sports drink (200 ml, 4 °C) to avoid dehydration, and their body mass was measured. After placement of rectal and skin thermistors and the HR sensor, the participants entered the climate chamber and rested on a chair for >10 min. The participants then put on the air-perfused rucksack, and the two fans were turned on just before the start of walking. The participants walked at a constant intensity (5.5 km/h, 2.0% gradient) for 60 min. At 30 min and 60 min after the beginning of walking, the participants drank a sports drink (200 ml, 4 °C). After exercise, they rested on a chair for 20 min. Their body mass was measured after the rectal and skin thermistors and the HR sensor were removed.

Measures

Rectal temperature (T_re) was measured using a flexible rectal thermistor (LT-ST08-11; Gram, Saitama, Japan) with the probe cover inserted 10 cm beyond the anal sphincter. Skin temperature was measured using skin thermistors (LT-ST08-00; Gram) attached to the chest (T_chest), upper arm (T_{upper arm}), thigh (T_thigh), calf (T_calf), and neck (T_neck). According to the manufacturer, the accuracy of the thermistors was 0.01°C. Rectal and skin temperature data were recorded using a data logger (LT-8 Series; Gram) every 30 s throughout an experimental session. The weighted mean skin temperature (_mT_sk) was calculated as [18]:

$${ }_{\mathrm{m}}{\mathrm{T}}_{\mathrm{s}\mathrm{k}}=0.3\times \left({\mathrm{T}}_{\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{t}}+{\mathrm{T}}_{\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{r}\mathrm{m}}\right)+0.2\times \left({\mathrm{T}}_{\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}}+{\mathrm{T}}_{\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{f}}\right).$$

HR was measured every 1 s using a HR monitor (POLAR A300; Polar, Kempele, Finland) throughout an experimental session. The physiological strain index (PSI) was calculated as [19]:

$$\mathrm{P}\mathrm{S}\mathrm{I}=5\times \left({\mathrm{T}}_{\mathrm{r}\mathrm{e}}\mathrm{t}-{\mathrm{T}}_{\mathrm{r}\mathrm{e}}0\right)/\left(39.5-{\mathrm{T}}_{\mathrm{r}\mathrm{e}}0\right)+5\times \left(\mathrm{H}\mathrm{R}\mathrm{t}-\mathrm{H}\mathrm{R}0\right)/\left(180-\mathrm{H}\mathrm{R}0\right),$$

where T_ret and HRt are the values measured at a certain time point, T_re0 and HR0 are the values measured at the beginning of exercise, and 39.5 and 180 are the mean maximum values of core temperature and HR, respectively [19]. PSI was calculated every 10 min during walking. The rating of perceived exertion (RPE) was measured using the Borg scale [20] every 10 min during walking. TS was measured using a 9-point scale (−4, very cold; −3, cold; −2, cool; −1, slightly cool; 0, neutral; 1, slightly warm; 2, warm; 3, hot; and 4, very hot). TC was measured using a 7-point scale (−3, very uncomfortable; −2, uncomfortable; −1, slightly uncomfortable; 0, neutral; 1, slightly comfortable; 2, comfortable; and 3, very comfortable). TS and TC were recorded every 10 min throughout an experimental session. Whole body sweat volume was calculated by subtracting the post-exercise body mass and the consumed sports drink volume (400 ml) during exercise from the pre-exercise body mass, under the assumption that 100 g of body mass is equivalent to 100 ml of body sweat.

Data and statistical analyses

T_{re, m}T_sk, and HR were averaged over 30 s. All variables measured during the experiment are presented as mean ± SD. T_{re, m}T_sk, T_neck, HR, PSI, RPE, TS, and TC (n = 10) were compared using two-way (trial × time) repeated-measures analysis of variance. Post hoc analyses were performed using the Bonferroni correction when a significant interaction was detected. Additionally, whole body sweat volumes (n = 10) were compared using a two-tailed paired t-test. All statistical analyses were performed using statistical software (SPSS v23; SPSS, Chicago, IL, USA). Effect sizes (ES) were calculated according to Cohen’s d as small (0.2–0.5), moderate (0.5–0.8), or large (>0.8) [21]. Statistical significance was set at P < 0.05.

Results

No significant difference (P > 0.05) was found for the pre-exercise values of T_{re, m}T_sk, T_neck, HR, TS, TC, and body mass between FAN and CON. A significant interaction between trial and time (P < 0.05) was detected for T_{re, m}T_sk, and T_neck. T_re was significantly lower (P < 0.03, ES = 0.51–1.00: medium–large) in FAN than in CON at 10–80 min during an experimental session (Figure 2). T_re increased by 0.71 ± 0.22°C in FAN and 0.82 ± 0.26°C in CON during walking, reached 37.73 ± 0.23°C in FAN and 37.98 ± 0.36°C in CON at 60 min (P < 0.01, ES = 0.83: large), and reached 37.40 ± 0.24°C in FAN and 37.69 ± 0.33°C in CON at 80 min (P < 0.01, ES = 0.99: large). _mT_sk was significantly lower (P = 0.03, ES = 0.57: medium) in FAN (34.05 ± 0.42°C) than in CON (34.26 ± 0.34°C) at the beginning of walking (Figure 3(a)). T_neck was significantly lower (P < 0.04, ES = 0.54–2.18: medium–large) in FAN than in CON throughout the exercise session (Figure 3(b)). T_neck reached 35.13 ± 0.50°C in FAN and 36.06 ± 0.35°C in CON at 30 min (P < 0.01, ES = 2.17: large), and reached 35.01 ± 0.71°C in FAN and 35.76 ± 0.66°C in CON at 60 min (P = 0.02, ES = 1.09: large).

Figure 2.

Changes in rectal temperature during the experimental session in FAN (●) and CON (○). Values are presented as means ± SD (n = 10). Significant difference between trials for a measured point: * P < 0.05 and ** P < 0.01. pre: pre-exercise values.

Figure 3.

Changes in mean skin temperature (a) and neck skin temperature (b) during the experimental session in FAN (●) and CON (○). Values are presented as means ± SD (n = 10). Significant difference between trials for a measured point: * P < 0.05 and ** P < 0.01. pre: pre-exercise values.

A significant interaction between trial and time (P < 0.05) was detected for HR and PSI. HR was significantly lower (P < 0.04, ES = 0.61–0.96: medium–large) in FAN than in CON at 10–80 min (except at 70 min) during an experimental session (Figure 4). HR reached 108 ± 10 bpm in FAN and 119 ± 14 bpm in CON at 30 min (P < 0.01, ES = 0.89: large), and reached 113 ± 13 bpm in FAN and 124 ± 18 bpm in CON at 60 min (P = 0.01, ES = 0.74: medium). PSI was significantly lower (P < 0.03, ES = 0.98–1.75: large) in FAN than in CON at 10–60 min, and reached 2.9 ± 0.5 in FAN and 3.8 ± 1.1 in CON at 60 min (P < 0.01, ES = 1.20: large) (Figure 5).

Figure 4.

Changes in heart rate during the experimental session in FAN (●) and CON (○). Values are presented as means ± SD (n = 10). Significant difference between trials for a measured point: * P < 0.05 and ** P < 0.01. pre: pre-exercise values.

Figure 5.

Changes in physiological strain index during the experimental session in FAN (●) and CON (○). Values are presented as means ± SD (n = 10). Significant difference between trials for a measured point: * P < 0.05 and ** P < 0.01. pre: pre-exercise values.

A significant interaction between trial and time (P < 0.05) was detected for TS. A significant main effect of trial (P < 0.05) was detected for RPE and TC, respectively. RPE was significantly lower (P = 0.02) in FAN than in CON during walking, and reached 10.8 ± 1.1 in FAN and 11.1 ± 1.5 in CON at 60 min (Figure 6(a)). TS was significantly lower (P < 0.03, ES = 1.49–2.15: large) in FAN than in CON at 0–80 min during an experimental session, and reached 1.4 ± 1.0 in FAN and 3.1 ± 0.6 in CON at 60 min (P < 0.01, ES = 2.15: large) (Figure 6(b)). TC was significantly lower (P = 0.01) in FAN than in CON during an experimental session, and reached −0.9 ± 0.9 in FAN and −2.0 ± 0.7 in CON at 60 min (Figure 6(c)). No significant difference (P = 0.21, ES = 0.44: small) was found for whole body sweat volume between FAN (670 ± 130 g) and CON (725 ± 120 g).

Figure 6.

Changes in (a) rating of perceived exertion, (b) thermal sensation, and (c) thermal comfort during the experimental session in FAN (●) and CON (○). Values are presented as means ± SD (n = 10). Significant difference between trials for a measured point: * P < 0.05 and ** P < 0.01. Significant main effect between trials: # P < 0.05. pre: pre-exercise values.

Discussion

Practical cooling strategies for staff and spectators at the Tokyo Olympic and Paralympic Games have not been fully investigated. The present study examined the effect of a fan cooling device on physiological and perceptual response during walking (5.5 km/h, 2.0% gradient) for 60 min in a hot environment (35°C, 50% RH). The results show that the use of an air-perfused rucksack attenuated increases in rectal temperature, HR, and PSI during walking in a hot environment. In addition, RPE, TS, and TC improved with fan cooling. Thus, the air-perfused rucksack may be useful as a cooling strategy for staff and spectators at the Tokyo Olympic and Paralympic Games.

Our study showed that a smaller and lighter fan cooling device (i.e., air-perfused rucksack; 648 g) than those used in previous studies [3,7–9] significantly attenuated the core temperature elevation (T_re) in a hot environment (Figure 2). The difference in T_re elevation after walking for 60 min was 0.11°C (FAN: 0.71 ± 0.22°C; CON: 0.82 ± 0.26°C). However, because the difference in T_re between trials showed a trend toward increasing over time (Figure 2), its physiological relevance may increase with continued exercise. In a previous study, an air-perfused vest (approximately 2.3 kg) was reported to attenuate the increases in T_re by approximately 0.15°C during walking (1.34 km/h) for 120 min in a hot condition (40°C, 20% RH) [7]. Additionally, such large and heavy vests reduce the _mT_sk elevation because they apply fan cooling to the whole torso [7–9]. In the present study, although fan cooling significantly attenuated the increases in T_neck (ES = 0.54–2.18) during walking, it did not attenuate the increases in _mT_sk except at 0 min (Figure 3), because it was aimed at the upper back and neck. It is likely that intensive air blowing (4–5 m/s) around the neck (and presumably the upper back) led to great evaporative heat loss and attenuated the elevation in core temperature.

We also found that the air-perfused rucksack significantly reduced the increase in HR during walking in a hot environment (Figure 4). The difference in HR between the trials was approximately 11 bpm at 30 min (FAN: 108 ± 10 bpm; CON: 119 ± 14 bpm) and 11 bpm at 60 min (FAN: 113 ± 13 bpm; CON: 124 ± 18 bpm). Previous studies also reported that the use of air-perfused vests reduced HR when protective clothing was worn during walking in a hot environment [7–9]. For example, Chinevere et al. (2008) showed that an air-perfused vest significantly attenuated the increases in HR during walking in three climate conditions of 30°C (50% RH), 35°C (75% RH), and 40°C (20% RH) [7]. Further, use of an air-perfused vest was reported to lower HR by approximately 11 bpm after walking for 100 min in a hot environment (35°C, 60% RH) [9]. These data suggest that fan cooling devices such as an air-perfused vest and rucksack can effectively attenuate cardiovascular strain.

The present study showed that an air-perfused rucksack significantly reduced PSI (ES = 0.98–1.75: large) during walking in a hot environment (Figure 5). PSI is based on T_re and HR, and reflects the combined strain of the thermoregulatory and cardiovascular systems [19]. Although the body area cooled by fan cooling in our study is not precisely known, the results show that applying fan cooling to the upper back (595 cm²) and neck by an air-perfused rucksack effectively attenuates physiological strain in the heat. The body area cooled by an air-perfused rucksack is smaller than that with traditional cooling devices such as air-perfused vests [3,7–9]. In a recent study of personal cooling as part of social adaptation to heat [22], developing better cooling garments was suggested to be important for coping with a hotter future climate, and that cooling garments may be widely accepted by integrating with fashion design. Thus, a light and small device such as the air-perfused rucksack may provide a suitable countermeasure to these conditions.

RPE, TS, and TC were significantly improved by fan cooling applied to the upper back and neck during walking in a hot environment (Figure 6). In particular, the ES in TS was large (1.49–2.15) throughout the experimental session (Figure 6(b)). A previous study also reported that RPE, TS, and TC were significantly improved by the use of an air-perfused vest when protective clothing was worn during walking in a hot environment [8]. Further, TS and TC in the heat are improved when skin and core temperatures are reduced by fan cooling [23]. Additionally, previous studies showed that cooling the neck effectively improves TC at rest in the heat [13], and improves TS during high-intensity cycling exercise in a hot environment [14–16]. Our results are consistent with these studies.

There are some limitations in the present study. The effects of the extra load (648 g) imposed by wearing the rucksack is unknown, because we did not conduct a trial using the rucksack without blowing of air. Further, it remains unknown which device most effectively reduces heat stress, because we did not compare the effects of the air-perfused rucksack with those of other devises such as air-perfused vests. Such a comparison was beyond the scope of our study. As we did not measure skin temperature on the upper back, it remains unclear whether the evaporative heat loss on the upper back was actually increased. Because the ability of the rucksack to evaporate sweat is affected by some factors such as sweating rate and saturation water vapor pressure (i.e., ambient temperature and relative humidity), future studies should investigate whether this rucksack reduces heat stress at various experimental settings (e.g., during high-intensity exercise and/or in different environmental conditions). Finally, we did not provide a headwind that corresponded to the walking speed during exercise. Headwind may influence T_re, HR, RPE, TS, and TC. Nevertheless, for managing the heat stress of a person who is standing still (e.g., spectators or people in a crowded area), an air-perfused rucksack seems to be effective.

Conclusion

The present study demonstrated that a lightweight fan cooling device, namely an air-perfused rucksack with battery-driven fans, attenuated the increases in T_re, T_neck, HR, and PSI, and improved RPE, TS, and TC, during walking for 60 min in a hot environment. The use of an air-perfused rucksack attenuated physiological and perceptual strain during low-intensity exercise in a hot environment. Thus, this device may be effective for managing heat stress of staff and spectators at the Tokyo Olympic and Paralympic Games.

Funding Statement

This study was partially funded by Worker Co., Ltd., Okazaki, Japan. However, the opinions and assertions expressed in this article are those of the authors, and do not reflect the policy or position of the company.

Disclosure statement

No potential conflicts of interest were disclosed.