Wearable equipment is increasingly used in daily life, and the exoskeleton is one of them. In manual work scenarios, workers are exposed to the risk of work-related musculoskeletal disorders (WMSDs). In the United States alone, the annual economic loss caused by WMSDs is as high as $
50 billion USD [1
]. Although automation and intelligent robots reduce the need for manual operations, due to the flexibility of manual operations, many manual tasks still require the experience and skills of workers. Due to the advantages of small size and wearability, the exoskeleton as a support device has attracted widespread attention and research in the industrial field in recent years. These exoskeletons can be used for maintenance, assembly or handling in logistics [2
]. The power forms of the industry exoskeleton include passive and active forms. The passive exoskeleton does not require an external energy supply, which only relies on its mechanical damping to provide support for the workers, so it is low-cost and lighter than the active type. At present, passive exoskeletons are mostly used to support the upper limbs and spine during manual work.
Industrial exoskeletons innovatively provide supporting intervention from the work characteristics of workers to alleviate the workload. Existing studies of exoskeletons have shown that they can reduce muscle fatigue, perceived efforts, and thus have benefits for preventing WMSDs [4
]. However, these advantages to the human body can easily make people ignore the impact of environmental factors on the ergonomics in exoskeleton application. Since working environments such as construction sites and warehouses are open-air or lack the capability of temperature adjustment, the application environment of the industrial exoskeleton is complicated. Some automobile factories have observed that it is difficult to control the workshop temperature below 25 °C even with cooling equipment in hot weather, which greatly affects production efficiency [7
]. In high latitude areas, cold exposure is a common problem faced by work [9
]. In a recent report, workers were asked to use a supporting exoskeleton for overhead assembly in the automotive manufacturing factory, and the questionnaire found that thermal discomfort becomes a major factor in disuse. Workers requested to improve the thermal comfort of the exoskeleton for long-term use [2
]. ASHRAE Standard 55 stipulates the indoor acceptable operative temperature with the applicable prerequisites of the physical activity (1.0 to 1.3 met, 58 W/m2
metabolic rate converted to 1 met) [10
]. However, the level of physical activity is higher in the exoskeleton usage scenario, and people working in the open air are often exposed to hot or cold environments.
Workers and the environment carry out continuous heat exchange to maintain the core body temperature of the human body. Changes in the environmental temperature affect the human body’s thermal response, including physiology and psychology. The ASHRAE uses the scales to quantify people’s thermal sensation and thermal comfort [10
]. When the human body feels hot, the thermal sensation deviates from “neutral” and shifts to the “hot” direction. At the same time, the acute circulatory system response of the human body increases the heart rate, blood flow and sweating [11
]. Cardiovascular activity promotes blood flow to the surface of the body to increase peripheral blood volume. When the ambient temperature is overheated or physical activity increases, sweat evaporates at the ambient temperature to reduce the skin temperature [12
]. In the study of physiological parameters and subjective responses, it was found that there is a strong correlation between skin temperature and thermal sensation [14
]. Therefore, the mean skin temperature (MST) calculated by local skin temperatures and the corresponding weighting factors is usually measured as a physiological parameter related to thermal comfort [15
]. Contrary to a hot environment, when the human body is exposed to a cold environment, the heat transfer from the core of the body to the shell. The contraction of blood vessels enhances the insulation capacity of the skin and subcutaneous tissues [17
]. When the heat dissipation of the human body in a cold environment makes it difficult to maintain the core temperature, the sympathetic nervous system becomes active and increases the metabolic rate of cells to generate heat [19
]. The stimulation of β-adrenergic receptors by the sympathetic nervous system activates brown adipose tissue (BAT), resulting in metabolic heat production. The activity level also affects the thermal response. In previous studies, under a higher metabolic rate condition (e.g., mechanical work), the metabolic rate has a stronger effect on thermal comfort than ambient temperature [20
Occupational heat exposure affects work productivity, physical and cognitive ability and even brings safety risks to workers [21
]. The increase in physical activity makes the metabolic activity more active and increases the heat sensation and sweating rate, which further increases the physical stress of workers [23
]. The factory can alleviate the negative impact of heat exposure on workers by reducing the work intensity and optimizing the work–rest schedule [26
]. Previous studies have shown that exoskeletons have positive results in the biomechanical evaluation. It has been proven to significantly reduce the muscle activity of the main muscles involved in handling or overhead work [27
]. The passive back exoskeleton has been shown to reduce spinal muscle activity by 20 to 25% in real car assembly operations [29
]. In a comparative experiment involving eight workers, a passive assisted exoskeleton significantly reduced the back muscle activity under the repeated lifting task [30
]. When using the upper limbs exoskeleton in the overhead work, the muscle activity of the biceps brachii and the medial deltoid muscle was reduced by 49% and 41%, respectively [31
]. However, contrary to our current knowledge, the subjective feeling of thermal discomfort is strong when the exoskeleton reduces muscle activity.
Cold exposure would cause impairments in human physiological functions, especially muscular functioning. The accuracy of the worker’s operation decreases under the cold operation, largely due to the decrease of the hand temperature [32
]. In general, people in cold environments need protective measures to improve thermal comfort [34
]. In standing and assembly tasks, wearable personal heating devices for limbs and torso have been proven to significantly improve the thermal comfort of workers [35
]. The required clothing insulation during the light physical activity under 4 to 10 °C is 1.6 to 1.9 CLO [36
]. However, with the increase in activity level, the body’s thermal sensation can be significantly improved. When the activity level is high, it may cause increased sweating, and people will take the initiative to reduce clothing for heat dissipation. In a cold environment, the risk of hyperthermia is possible when working with protective clothing [37
]. Although changes in activity status make the thermal response show different results in a cold environment, the effect of the exoskeleton’s activity intervention on the thermal response is still unclear.
A key feature of the exoskeleton is wearability, which forms a dynamic structure with the human body’s movement joints. The passive upper or lower limbs exoskeleton mainly forms contact with the chest, trunk, thighs, shoulders and upper arms to support and reduce joint torque under static or dynamic operations. The material used in the exoskeleton is different from that of common clothing. The outer layer of the exoskeleton can be made of ABS plastic, carbon fiber or alloy materials [38
]. The inner side in contact with the human body uses memory foam, interlining, and mesh to prevent the high-strength surface material from harming the workers [41
]. Organic polymer materials are widely used in the clothing industry, and the combination of inorganic polymer materials and fabrics is reported to have better thermal insulation properties [42
]. Cold protective clothing with the metal coating is reported to increase the thermal resistance of fabrics by 30 to 75% [44
]. The mixed structure of multiple materials used in the exoskeleton may potentially increase the clothing insulation, which may also affect the thermal response of workers.
Unlike other wearable devices, the exoskeleton can cause changes in muscle activity, but its influence on the thermal response of the human body is unknown. This paper aims to study the effects of a passive exoskeleton on the thermal response of the human body. The exoskeleton is mainly used to reduce the lower back load of the human body during repeated operations. Although previous studies have shown that exoskeletons can provide good assistance, they did not explain the impact of assistance on human thermal response and report the temperature under the experiment. Metabolic heat production is closely related to the activity level, and it is also meaningful to compare the thermoregulatory activity of workers using it under different temperature. This study analyzes other thermal responses, including the mean skin temperature, thermal sensation, and thermal comfort, which helps to evaluate wearability under environmental variables. To the best of our knowledge, this is the first study to investigate the thermal response when wearing the exoskeleton under two temperatures. We hope to explore whether the exoskeleton’s effect on human thermal comfort will become a potential problem in its promotion and application through this research.