Applied Ergonomics 49 (2015) 55e62
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Effect of gait on formation of thermal environment inside footwear Yasuhiro Shimazaki*, Masaaki Murata Department of Systems Engineering for Sports, Okayama Prefectural University, 111 Kuboki, Soja, Okayama 719-1197, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Accepted 28 January 2015 Available online
In this study, the relationship between the gait condition and foot temperature distributions inside footwear was investigated using subject experiments. Mechanical, physical, and physiological variables such as the foot contact force, landing speed, and metabolic heat generation were also measured. Gait motion measurements showed that a large contact force was concentrated in the small area of the heel at the initial contact and later at the forefoot. A faster gait produced a larger contact force, higher landing velocity, higher skin temperature, and larger metabolism during gait. The temperature at the bottom of the foot increased, and the temperature on the upper side decreased. The metabolic heat generation had a basic impact on the temperature profile, and skin temperatures tended to increase gradually. In addition, high-temperature-elevation regions such as the big toe and heel coincided with regions with high-contact loads, which suggested a relationship between the temperature elevation and contact load. © 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Skin temperature Foot contact Heat transfer
1. Introduction People normally wear shoes in daily life, and research on footwear is of interest to many people. The foot has an important thermal radiator function for human thermoregulation (Day, 1969). Therefore, when feet are contained within footwear for long periods, the temperature and humidity inside the footwear rise. In one study, the midsole temperature became greater than 50 C while running during daytime in summer (Kinoshita and Bates, 1996). This can cause discomfort, especially during summer or in hot and humid regions. Skin temperature contributes to the thermal comfort of the entire body and the autonomic thermoregulatory response (Frank et al., 1999). A significant weighting value of 0.07 is used for the foot temperature in a common expression of whole body skin temperature (Hardy and DuBois, 1938), and a whole-body thermal sensation evaluation can be performed using the skin temperature (Takada et al., 2013). The thermal environment such as the temperature inside footwear becomes important in this respect. Moreover, the temperature and humidity elevations inside footwear are recognized not only as a source of discomfort but also as a severe problem that can cause injury or bacterial infection in some cases.
* Corresponding author. Tel.: þ81 866 94 2123. E-mail addresses: [email protected] (Y. Shimazaki), [email protected] (M. Murata). http://dx.doi.org/10.1016/j.apergo.2015.01.007 0003-6870/© 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.
In the context of thermal comfort, many measurements and assessments addressing footwear have been carried out. The footskin temperature, air temperature inside footwear, and humidity have commonly been measured, and elevations in these values have been recognized (Kawabata and Tokura, 1993). An in-shoe portable device for plantar pressure, temperature, and humidity measurement was invented (Maluf et al., 2001), and shoe comfort assessments and product evaluations have frequently been conducted (Schols et al., 2004). However, no studies have considered the basics of the heat transfer and temperature distribution inside footwear. Therefore, the authors were interested in the formation of the thermal environment inside footwear. The heat transfer inside footwear is mainly divided into two mechanisms: (1) expelled heat generated inside the body and (2) heat transfer to/from the footwear surface or through the surface. Thus, a hot feeling may be the result of inner heat generation due to metabolism. In general, heat transfers from a higher to lower temperature region. Therefore, another possibility when feeling uncomfortably warm or hot is that the person's surroundings have a higher temperature than their skin. When considering the conservation of energy, one mechanism that could potentially cause heating inside footwear is the transformation of the mechanical energy of the gait motion into thermal energy. In fact, the maximum contact force at landing was reported to be more than the body weight during running at 3.3 m/s (Chuckpaiwong et al., 2008). This contact force seems to have enough potential energy to heat the inside of footwear. Normally, the walking speed strongly
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affects the peak plantar pressure (Pataky et al., 2008), and this may cause the formation of a different thermal environment in footwear. The purpose of this study was to identify the influence of the heat generation and transfer changes associated with the gait speed on the formation of the thermal environment inside footwear. A second purpose of the study was to provide a database for walking or even running to be used for thermal comfort optimisation in footwear design. 2. Procedure Experiments with subjects were conducted to simultaneously evaluate the contact force and skin temperature changes inside footwear. Four gait speeds were selected for the experiment: 3.0, 6.0, 9.0, and 12.0 km/h. The 9.0 and 12.0 km/h gaits were faster than walking. Thus, the gait transitioned from walking to running at 9.0 km/h. In this sense, our gait speeds covered a wide range from walking to running. The foot-contact force distribution was measured using pressure sensors (Tekscan F-Scan system) at 0.01 s intervals (100 Hz) at a given gait on a treadmill (Ahroni et al., 1998). First, a comparison was made of the force value by a calibration (Hamzah et al., 2008) using piezoelectric force platforms (Kistler model9287) during simple vertical stepping to ensure the accuracy and timing of the contact force. The time-averaged value was 6.4% lower in the F-scan system. However, the overall output shapes looked similar, and the measurements were repeatable. In fact, some studies have addressed the accuracy of the system, and the present gait condition was similar to that used in previous studies (e.g., Verdejo and Mills, 2004), allowing an analysis of this kind of gait speed. For the purpose of analysing the gait motion, a 9-axis motion sensor (ZMP IMU-Z2) was placed on the very rear surface of the footwear, and measurements were acquired at 0.01 s intervals (100 Hz). The temperatures were recorded (using J thermocouples) at eight different points at 1 min intervals. The measuring points were determined by referring to a previous planter pressure analysis (Burnfield et al., 2004), and included the little toe, big toe, arch, heel, instep, medial malleolus, lateral malleolus, and sublingual to represent the entire foot, as shown in Fig. 1. Since the temperature measurement used several thermocouples simultaneously, the sensors were compared using a calibration bath before the experiment. An exhaled gas analyser
Fig. 1. Temperature measurement points on foot.
(S&Me VO2000) was also used to evaluate the metabolism. Prior to the experiment, an oxygen flow sensor was installed and calibrated, and automatic calibration was performed before every measurement. All of this equipment was synchronized. The body mass was measured before and after the experiment to observe the weight change from sweating. The surrounding weather factors were also measured at 1 min intervals. The global solar radiation, solar radiation reflected from the ground, infrared radiation from the atmosphere and ground (EKO MR-60), air temperature (Pt-100 resistance), wind speed (ultrasonic anemometer), and humidity (capacitance hygrometer) were measured. For reference, the average air temperature, humidity, and wind speed were approximately 28.6 C (SD 0.2 C), 72.0% RH (SD 2.0% RH), and calm wind, respectively. All the measurements were taken once a day during a single session, thus preventing the order effect. The participant changed into specified clothing and was weighed to calibrate the pressure measurement prior to the experiment. Sensors were installed 15 min before the measurements began, by which point the participants were acclimated to the initial environmental state in a chamber. The measurements were taken for 50 min in an indoor chamber. After 10 min of rest, a subject performed a gait exercise for 30 min. After this exercise, the subject recovered for 10 min. The subjects wore shoes on their bare feet. The shoes used in the study were designed as running shoes and had a length of 27.0 cm and a weight of 0.18 kg. The subjects were selected to fit the size. The shoes had a slightly raised heel with a maximum height of 2.0 cm, with rubber cushioning. The upper part of the shoes was made of a porous material. A total of 17 healthy males, between the ages of 19 and 23, participated in the study. The participants had a mean age of 21.1 years (SD 1.6 year), mean height of 170.3 cm (SD 4.3 cm), and mean weight of 60.1 kg (SD 6.3 kg). All the subjects agreed to be part of the study, and the research was conducted with the approval of the Research Ethics Committee of Okayama Prefectural University. The data are normally presented as the mean response. In particular, the analysis results for the temperature changes are presented as temperature elevations to remove individual temperature level differences. 3. Results 3.1. Gait motion An example of the time-dependent gait motion is analysed. The contact force, contact area, and contact force distribution are shown in Figs. 2 and 3. Fig. 2(a) shows the values for a gait of 3.0 km/h and (b) shows them for a gait of 12.0 km/h. In the figures, the contact force is normalized for the body weight. A gait event includes the initial contact, loading response, mid-stance, terminal stance, pre-swing, initial swing, mid-swing, and terminal swing in time order (Uustal and Baerga, 2004). The time is defined as zero at the initial contact. In Fig. 2(a), the contact force and contact area rapidly increase after the heel strike (initial contact). Then, these values increase with time. After approximately 0.50 s, they reach the maximum. Then, the contact force and contact area decrease. After 0.95 s, toe off occurs, and the values become nearly zero again. The time-dependent contact force distributions are also shown in Fig. 3. After the foot contact, when all the force is concentrated on the small area of the heel in Fig. 3(a), the contact force and contact area gradually increase, as seen in Fig. 3(b). Then, the contact force moves from the heel to the forefoot, as seen in Fig. 3(c). Finally, at the time when the heel rises, the contact force is concentrated at the metatarsal and toe neck, and the foot contact terminates. After 1.5 s, another landing occurs, and a new cycle of gait motion starts.
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3.2. Influence of gait speed
Fig. 2. Time-dependent contact force and contact changes.
Because the bottom of the foot has contact with the insole even during the swing period, the contact force and contact area cannot be zero. The movement series are similar for the other gait speeds, as shown in Fig. 2(b).
The influence of the gait speed on the maximum contact force is analysed in Fig. 4(a). Faster walking results in higher values for the maximum contact force. There is a large gap between the 6.0 and 9.0 km/h gaits. This gap can be explained by alternating walking with running. The maximum contact force at the 3.0 and 6.0 km/h gaits is approximately 100% BW, and this value increases with the increasing gait speed at 9.0 km/h or higher. The maximum contact force is approximately 170%BW at the highest gait of 12.0 km/h. Thus, it almost doubles between 3.0 and 12.0 km/h. The mean contact force during the contact period is shown in Fig. 4(b). Faster walking also results in higher values for the mean contact force, and there is a large gap between the 6.0 and 9.0 km/h gaits. This gap can be explained by alternating walking with running. The mean contact force is approximately 60%BW at the 3.0 and 6.0 km/h gaits and 80e90% BW at the 9.0 and 12.0 km/h gaits. The influence of the gait speed on the maximum contact area is analysed in Fig. 5(a). The maximum contact areas differ between the low and fast gaits. The maximum contact area for the 3.0 and 6.0 km/h gaits is approximately 0.009 m2, and the maximum contact area for gaits of 9.0 km/h or higher is approximately 0.013 m2. The maximum contact area is shown at the early stage for the flat foot. Faster walking results in slightly higher values for the maximum contact area. The mean contact area during the contact period is shown in Fig. 5(b). The mean contact area is almost constant at 0.006 m2 for all the gaits. The instant foot velocity is obtained by integrating the instant acceleration at landing. The average foot velocity during the deceleration period of the swing is shown in Fig. 6. Faster walking results in higher values for the landing velocity. The lowest landing velocity is 1.14 m/s at the 3.0 km/h gait, and the highest is 3.45 m/s at the 12.0 km/h gait. There is approximately a 3-fold increase from the 3.0 km/h gait to the 12.0 km/h gait. The influence of the gait speed on the stance time and swing time is shown in Fig. 7. The longest stance time is 0.97 s for the 3.0 km/h gait. The stance times for 9.0 and 12.0 km/h are almost the same (0.30 s). Faster walking results in a shorter stance time. On the other hand, the swing time stays constant at approximately 0.50 s. A gaitemotion cycle (¼stance þ swing) became shorter with increasing gait speed.
Fig. 3. Contact force distribution during cycle of gait motion.
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Fig. 6. Influence of gait speed on foot landing speed.
When taken together, a higher gait caused a larger contact force to be concentrated in a relatively smaller area in a short period of time compared with a lower gait. 3.3. Body temperatures
Fig. 4. Influence of gait speed on contact force.
Fig. 5. Influence of gait speed on contact area.
Examples of the time-dependent skin temperatures and sublingual temperatures at the 6.0 km/h gait are shown in Fig. 8. At the beginning, the temperatures are stable but not equal at all the measuring points. The temperatures on the upper side of the foot, such as the instep, medial malleolus, and lateral malleolus, are relatively low. After 10 min, the temperatures on the upper side of the foot drop, and those at the other measuring points rise. In particular, the temperatures for the big toe and heel rise sharply. During gait, all the foot temperatures tend to gradually increase. After 30 min of gait, the temperatures at the upper side of the foot increase sharply, and the temperatures at the big toe and heel start to decrease. With the exception of the heel, the temperatures settle at a certain level. The heel temperature continues to decrease. At the end of the 50 min experiment, all of the temperatures are higher than they were before the experiment. The heel temperature shows the highest amount of elevation at 3.0 C. The instep temperature shows the lowest amount of elevation; however, it still rises 1.5 C. The temperatures show similar tendencies at the other gait speeds. The influence of the gait speed on each body temperature elevation is analysed in Fig. 9(a)e(g). The skin temperature tendencies are divided into three groups. The big toe and heel belong to the first group. The temperatures rise sharply during gait and drop after walking in this group. The highest temperature elevations are 8.2 C for the heel and 4.8 C for the big toe at the 12.0 km/ h gait. Even at the lowest gait of 3.0 km/h, the temperature rises 3.5 C for the heel and 1.6 C for the big toe. The greatest characteristic of this group is the rate of temperature increase at the beginning of gait. For instance, the rate of heel temperature increase is 0.4e0.6 C/min, and that for the big toe is 0.2e0.5 C/min for the first 5 min of gait. The little toe and arch belong to another group. The temperatures of this group tend to increase gradually, and the amount of temperature elevation is small compared with the first group. Actually, the maximum temperature elevations are 2.0 C for the little toe and 3.7 C for the arch during gait. The rates of temperature increase are 0e0.1 C/min for the little toe and 0.1e0.3 C/min for the arch during the first 5 min of gait. The temperatures of the last group drop sharply after walking begins and rise sharply after walking ends. The instep, medial malleolus, and lateral malleolus belong to this group. The overall temperature trend of this group is a slight increase with time. The temperature
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whole body were estimated by using Kurazumi's method for Japanese people (Kurazumi et al., 1994). The influence of the gait speed on the time average latent heat loss is also shown in Fig. 11. The amount of evaporative heat loss increases with the gait speed. The latent heat loss is slightly smaller than the metabolism at a slower gait of 3.0 km/h. The difference between them increases with the gait speed. There is a cooling capacity limit for sweating, and the maximum latent heat flux was 208 W/m2 at the 12.0 km/h gait. Therefore, the effect of the latent heat on the thermal environment formation inside footwear varied with the gait speed. 4. Discussion
Fig. 7. Influence of gait speed on period of gait motion.
elevation was approximately 2.0 C after the 50 min experiment. However, the temperatures clearly decreased during gait for a higher gait. The temperatures at the 12.0 km/h gait dropped by approximately 2.0 C just after starting the gait. Interestingly, the temperatures at the bottom side increase and those at the upper side decrease during gait. In general, the temperature rise or drop tends to increase with increasing gait speed. The sublingual temperature remained almost constant at all gaits.
3.4. Other human factors The time dependent metabolism at each gait is shown in Fig. 10. The metabolism is Weir's proposed variable (Weir, 1949). At the beginning, the metabolism stays around 60 W/m2 and is almost constant at all gait conditions. Immediately after walking begins, the metabolism rises sharply and then stays almost constant. Faster walking results in a higher metabolism. The highest metabolism is approximately 480 W/m2 at the 12.0 km/h gait, and the lowest metabolism is approximately 110 W/m2 at the 3.0 km/h gait. Immediately after 30 min of walking, the metabolism drops sharply and returns to the same level as that of the previous standing-up period. Some delayed response is observed for recovery at a higher gait speed. The amount of heat generation at the feet was estimated by assuming that heat generation occurs uniformly over the entire body, as shown in Fig. 11. Sweat is produced to cool the body by evaporation. The amount of evaporative heat loss for the whole body was predicted by multiplying the body mass change during the experiment and the evaporative latent heat. The efficiency of sweating, which is defined as the ratio of secreted and evaporated sweat, was 0.5, based on €m and Holme r (1985). The latent heat on the foot AlbereWallerstro was assigned on the basis of an area factor. The areas of the foot and
Fig. 8. Time-dependent temperatures changes at 6.0 km/h gait.
Some differences in the temperature distributions were observed in the experiment. Faster walking resulted in a higher temperature elevation during gait motion. The temperature on the bottom side of the foot increased, and the temperature on the upper side decreased during gait. In particular, the temperatures on the big toe and heel clearly increased. Some possible factors for the temperature elevation on a shoed foot during gait are the foot contact, slipping friction between the foot and the shoe sole, and environmental variables such as air temperature and solar radiation. In the experiment, the environmental factors were controlled and had insignificant effects. The friction force is normally calculated by multiplying a mass by a friction coefficient, which is normally a decimal value. Considering the fact that the contact force was greater than the body weight, the slipping friction may have had a lower impact than the foot contact. Actually, faster walking resulted in a larger contact force and higher landing velocity in the study. The variation in the temperature increase during gait increased with the increasing contact force from a different perspective. Because the contact load can be defined by the mass, velocity magnitude, and time of loading, an increase in the vertical ground reaction force with a faster walking velocity was previously reported for young adults (Nilsson and Thorstensson, 1989). An elevated heel contact pressure was also reported to be associated with a higher approach velocity (Morag and Cavanagh, 1999). Moreover, the frequency of ground contact increases with increasing gait speed. Of course, the inner heat production increases with increasing gait speed. The metabolism changes sharply at the times of starting and stopping the gait, and then becomes almost constant shortly thereafter. A temperature elevation caused by the metabolism may have occurred with relative uniformity over the foot skin, because the heat was transferred from the same blood flow. The progressive temperature elevation is explainable on this basis and also as a result of the shoe insulation, which is the reason for the temperate rise after stopping the gait. The meaning of insulation here is the state where the generated quantity of heat is larger than the amount of heat dissipated from the skin surface. The skin temperature elevations on the big toe and heel were sharp. During walking, the body weight is solely on the heel at the initial contact, and a strong contact force is observed in this period (Rosenbaum et al., 1994). A strong contact force is observed again in the late stance period, because all the contact force is concentrated over a small region of the forefoot (Eilis et al., 2002). In fact, the higher temperature elevation region coincides with the area of the higher contact load. It is known that a repeated compression load produces heat energy converted from mechanical energy. The temperature elevation in these regions is assumed to be due to the absorption of the contact force on the skin itself, as conducted through the shoe cushioning. Regions with less contact, such as the little toe, arch, and upper side, tended to show relatively lower temperature elevations. The foot temperatures in these regions were mainly affected by metabolic heat generation and thermal
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Fig. 9. Time-dependent temperature changes at each body part.
insulation, which retained the inner heat inside the footwear. This is why the temperatures gradually increased. The big toe and heel temperatures may also have been affected by the contact force. On the other hand, the upper-side temperatures dropped. Air restriction inside footwear causes high sweat production (Purvis and Tunstall, 2004), and local heating, such as of the foot in this study, increases the local sweating rate (Shibasaki et al., 2006), Thus, sweating was a contributing factor in this upper-side temperature drop. A foot produces large quantities of sweat. It contains
a dense region of sweat glands (Park and Tamura, 1992), which consist of two types. Eccrine sweat glands are distributed over the body, and the secretion from eccrine glands is commonly called sweat and contributes to evaporative heat loss (Sato et al., 1989). Apocrine sweat glands are distributed in specific regions such as the sole of the foot, and the secretion from these glands is caused mainly by emotional strain (Ebling, 1989). In this regard, evaporation was considered to occur on the upper side of the foot. This may be a reason that the instep temperature dropped during the gait.
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However, because the micro-climate created under clothing, including sock, and footwear, has important implications in thermoregulation (Shigaki et al., 1993), human perceptions of foot temperature are strongly influenced by sock characteristics (Barkley et al., 2011). Future experiments could investigate the role of footwear with socks in the perception of comfort. Because other factors such as individual differences in fitness, foot shape, and action would also affect the results, it is expected that the roles of such complex factors will be identified in future work.
5. Conclusions
Fig. 10. Time-dependent metabolism change.
However, the metabolism always has an advantage compared with latent heat loss. Therefore, sweating in itself was insufficient to explain these temperature drops. The foot was constantly moving during the gait, and this movement induced a strong convective heat transfer. In general, convective heat transfer is dominated by the temperature gradient between the skin and the ambient fluid, along with the relative movement against the ambient fluid. Because faster walking resulted in greater foot movement, this led to a larger convective heat loss from the skin surface. The porous material also affected the temperature drop by allowing air ventilation. In addition, the bellows action also has a temperature reduction potential during gait motion. A greater amount of ventilation assumed to be due to the bellows action has been reported at an open space like beneath the arch in a closed shoe (Satsumoto et al., 2011). In fact, a relatively lower temperature elevation on the arch was observed. The relation between the gait conditions and foot temperature distributions inside the footwear became apparent. In particular, a larger contact force generated a higher temperature increase on the bottom part of the foot such as the big toe and heel. Since faster walking is accompanied by a larger contact force, reducing the contact force would be difficult. Many researchers have focused on developing shoe cushioning to reduce the contact force and improve the safety. Therefore, an optimum shoe cushioning design might be a solution to both provide comfort and prevent injury. In future research, key aspects of thermally comfortable design concepts and applications will involve predictions of the heat generation mechanism inside the insole and the heat release mechanism-associated airflows inside footwear. One of the limitations of the present study was that the subjects wore shoes on their bare feet throughout the experiment to ensure that the same conditions were maintained, and socks effects were prevented.
Fig. 11. Equivalent metabolism and equivalent latent heat.
In order to identify the factors influencing the formation of the thermal environment inside footwear, experiments with subjects were conducted at four different gait speeds to evaluate the skin temperature changes and mechanical gait parameters. Basically, metabolic heat generation had an impact on the temperature profile. Therefore, the skin temperatures tended to gradually increase. In addition, high temperature elevation regions such as the big toe and heel coincided with the regions with high contact loads. Faster walking resulted in a larger contact force, higher landing velocity, higher skin temperature elevation, and larger metabolism during gait. Considering the time-dependent temperature changes, the results suggested a relationship between the temperature elevations on the big toe and heel and the contact force. The foot temperatures in other regions were mainly affected by the thermal insulation, which retained the inner heat inside the footwear. The upper-side temperature dropped as a result of the convective heat loss and ventilation due to foot movement. Sweat evaporation had a beneficial effect on the instep temperature, but a limited effect on the entire thermal environment formation. Future experiments could investigate factors such as socks and individual differences to design comfortable footwear.
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Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo
Effect of gait on formation of thermal environment inside footwear Yasuhiro Shimazaki*, Masaaki Murata Department of Systems Engineering for Sports, Okayama Prefectural University, 111 Kuboki, Soja, Okayama 719-1197, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 April 2014 Accepted 28 January 2015 Available online
In this study, the relationship between the gait condition and foot temperature distributions inside footwear was investigated using subject experiments. Mechanical, physical, and physiological variables such as the foot contact force, landing speed, and metabolic heat generation were also measured. Gait motion measurements showed that a large contact force was concentrated in the small area of the heel at the initial contact and later at the forefoot. A faster gait produced a larger contact force, higher landing velocity, higher skin temperature, and larger metabolism during gait. The temperature at the bottom of the foot increased, and the temperature on the upper side decreased. The metabolic heat generation had a basic impact on the temperature profile, and skin temperatures tended to increase gradually. In addition, high-temperature-elevation regions such as the big toe and heel coincided with regions with high-contact loads, which suggested a relationship between the temperature elevation and contact load. © 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.
Keywords: Skin temperature Foot contact Heat transfer
1. Introduction People normally wear shoes in daily life, and research on footwear is of interest to many people. The foot has an important thermal radiator function for human thermoregulation (Day, 1969). Therefore, when feet are contained within footwear for long periods, the temperature and humidity inside the footwear rise. In one study, the midsole temperature became greater than 50 C while running during daytime in summer (Kinoshita and Bates, 1996). This can cause discomfort, especially during summer or in hot and humid regions. Skin temperature contributes to the thermal comfort of the entire body and the autonomic thermoregulatory response (Frank et al., 1999). A significant weighting value of 0.07 is used for the foot temperature in a common expression of whole body skin temperature (Hardy and DuBois, 1938), and a whole-body thermal sensation evaluation can be performed using the skin temperature (Takada et al., 2013). The thermal environment such as the temperature inside footwear becomes important in this respect. Moreover, the temperature and humidity elevations inside footwear are recognized not only as a source of discomfort but also as a severe problem that can cause injury or bacterial infection in some cases.
* Corresponding author. Tel.: þ81 866 94 2123. E-mail addresses: [email protected] (Y. Shimazaki), [email protected] (M. Murata). http://dx.doi.org/10.1016/j.apergo.2015.01.007 0003-6870/© 2015 Elsevier Ltd and The Ergonomics Society. All rights reserved.
In the context of thermal comfort, many measurements and assessments addressing footwear have been carried out. The footskin temperature, air temperature inside footwear, and humidity have commonly been measured, and elevations in these values have been recognized (Kawabata and Tokura, 1993). An in-shoe portable device for plantar pressure, temperature, and humidity measurement was invented (Maluf et al., 2001), and shoe comfort assessments and product evaluations have frequently been conducted (Schols et al., 2004). However, no studies have considered the basics of the heat transfer and temperature distribution inside footwear. Therefore, the authors were interested in the formation of the thermal environment inside footwear. The heat transfer inside footwear is mainly divided into two mechanisms: (1) expelled heat generated inside the body and (2) heat transfer to/from the footwear surface or through the surface. Thus, a hot feeling may be the result of inner heat generation due to metabolism. In general, heat transfers from a higher to lower temperature region. Therefore, another possibility when feeling uncomfortably warm or hot is that the person's surroundings have a higher temperature than their skin. When considering the conservation of energy, one mechanism that could potentially cause heating inside footwear is the transformation of the mechanical energy of the gait motion into thermal energy. In fact, the maximum contact force at landing was reported to be more than the body weight during running at 3.3 m/s (Chuckpaiwong et al., 2008). This contact force seems to have enough potential energy to heat the inside of footwear. Normally, the walking speed strongly
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affects the peak plantar pressure (Pataky et al., 2008), and this may cause the formation of a different thermal environment in footwear. The purpose of this study was to identify the influence of the heat generation and transfer changes associated with the gait speed on the formation of the thermal environment inside footwear. A second purpose of the study was to provide a database for walking or even running to be used for thermal comfort optimisation in footwear design. 2. Procedure Experiments with subjects were conducted to simultaneously evaluate the contact force and skin temperature changes inside footwear. Four gait speeds were selected for the experiment: 3.0, 6.0, 9.0, and 12.0 km/h. The 9.0 and 12.0 km/h gaits were faster than walking. Thus, the gait transitioned from walking to running at 9.0 km/h. In this sense, our gait speeds covered a wide range from walking to running. The foot-contact force distribution was measured using pressure sensors (Tekscan F-Scan system) at 0.01 s intervals (100 Hz) at a given gait on a treadmill (Ahroni et al., 1998). First, a comparison was made of the force value by a calibration (Hamzah et al., 2008) using piezoelectric force platforms (Kistler model9287) during simple vertical stepping to ensure the accuracy and timing of the contact force. The time-averaged value was 6.4% lower in the F-scan system. However, the overall output shapes looked similar, and the measurements were repeatable. In fact, some studies have addressed the accuracy of the system, and the present gait condition was similar to that used in previous studies (e.g., Verdejo and Mills, 2004), allowing an analysis of this kind of gait speed. For the purpose of analysing the gait motion, a 9-axis motion sensor (ZMP IMU-Z2) was placed on the very rear surface of the footwear, and measurements were acquired at 0.01 s intervals (100 Hz). The temperatures were recorded (using J thermocouples) at eight different points at 1 min intervals. The measuring points were determined by referring to a previous planter pressure analysis (Burnfield et al., 2004), and included the little toe, big toe, arch, heel, instep, medial malleolus, lateral malleolus, and sublingual to represent the entire foot, as shown in Fig. 1. Since the temperature measurement used several thermocouples simultaneously, the sensors were compared using a calibration bath before the experiment. An exhaled gas analyser
Fig. 1. Temperature measurement points on foot.
(S&Me VO2000) was also used to evaluate the metabolism. Prior to the experiment, an oxygen flow sensor was installed and calibrated, and automatic calibration was performed before every measurement. All of this equipment was synchronized. The body mass was measured before and after the experiment to observe the weight change from sweating. The surrounding weather factors were also measured at 1 min intervals. The global solar radiation, solar radiation reflected from the ground, infrared radiation from the atmosphere and ground (EKO MR-60), air temperature (Pt-100 resistance), wind speed (ultrasonic anemometer), and humidity (capacitance hygrometer) were measured. For reference, the average air temperature, humidity, and wind speed were approximately 28.6 C (SD 0.2 C), 72.0% RH (SD 2.0% RH), and calm wind, respectively. All the measurements were taken once a day during a single session, thus preventing the order effect. The participant changed into specified clothing and was weighed to calibrate the pressure measurement prior to the experiment. Sensors were installed 15 min before the measurements began, by which point the participants were acclimated to the initial environmental state in a chamber. The measurements were taken for 50 min in an indoor chamber. After 10 min of rest, a subject performed a gait exercise for 30 min. After this exercise, the subject recovered for 10 min. The subjects wore shoes on their bare feet. The shoes used in the study were designed as running shoes and had a length of 27.0 cm and a weight of 0.18 kg. The subjects were selected to fit the size. The shoes had a slightly raised heel with a maximum height of 2.0 cm, with rubber cushioning. The upper part of the shoes was made of a porous material. A total of 17 healthy males, between the ages of 19 and 23, participated in the study. The participants had a mean age of 21.1 years (SD 1.6 year), mean height of 170.3 cm (SD 4.3 cm), and mean weight of 60.1 kg (SD 6.3 kg). All the subjects agreed to be part of the study, and the research was conducted with the approval of the Research Ethics Committee of Okayama Prefectural University. The data are normally presented as the mean response. In particular, the analysis results for the temperature changes are presented as temperature elevations to remove individual temperature level differences. 3. Results 3.1. Gait motion An example of the time-dependent gait motion is analysed. The contact force, contact area, and contact force distribution are shown in Figs. 2 and 3. Fig. 2(a) shows the values for a gait of 3.0 km/h and (b) shows them for a gait of 12.0 km/h. In the figures, the contact force is normalized for the body weight. A gait event includes the initial contact, loading response, mid-stance, terminal stance, pre-swing, initial swing, mid-swing, and terminal swing in time order (Uustal and Baerga, 2004). The time is defined as zero at the initial contact. In Fig. 2(a), the contact force and contact area rapidly increase after the heel strike (initial contact). Then, these values increase with time. After approximately 0.50 s, they reach the maximum. Then, the contact force and contact area decrease. After 0.95 s, toe off occurs, and the values become nearly zero again. The time-dependent contact force distributions are also shown in Fig. 3. After the foot contact, when all the force is concentrated on the small area of the heel in Fig. 3(a), the contact force and contact area gradually increase, as seen in Fig. 3(b). Then, the contact force moves from the heel to the forefoot, as seen in Fig. 3(c). Finally, at the time when the heel rises, the contact force is concentrated at the metatarsal and toe neck, and the foot contact terminates. After 1.5 s, another landing occurs, and a new cycle of gait motion starts.
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3.2. Influence of gait speed
Fig. 2. Time-dependent contact force and contact changes.
Because the bottom of the foot has contact with the insole even during the swing period, the contact force and contact area cannot be zero. The movement series are similar for the other gait speeds, as shown in Fig. 2(b).
The influence of the gait speed on the maximum contact force is analysed in Fig. 4(a). Faster walking results in higher values for the maximum contact force. There is a large gap between the 6.0 and 9.0 km/h gaits. This gap can be explained by alternating walking with running. The maximum contact force at the 3.0 and 6.0 km/h gaits is approximately 100% BW, and this value increases with the increasing gait speed at 9.0 km/h or higher. The maximum contact force is approximately 170%BW at the highest gait of 12.0 km/h. Thus, it almost doubles between 3.0 and 12.0 km/h. The mean contact force during the contact period is shown in Fig. 4(b). Faster walking also results in higher values for the mean contact force, and there is a large gap between the 6.0 and 9.0 km/h gaits. This gap can be explained by alternating walking with running. The mean contact force is approximately 60%BW at the 3.0 and 6.0 km/h gaits and 80e90% BW at the 9.0 and 12.0 km/h gaits. The influence of the gait speed on the maximum contact area is analysed in Fig. 5(a). The maximum contact areas differ between the low and fast gaits. The maximum contact area for the 3.0 and 6.0 km/h gaits is approximately 0.009 m2, and the maximum contact area for gaits of 9.0 km/h or higher is approximately 0.013 m2. The maximum contact area is shown at the early stage for the flat foot. Faster walking results in slightly higher values for the maximum contact area. The mean contact area during the contact period is shown in Fig. 5(b). The mean contact area is almost constant at 0.006 m2 for all the gaits. The instant foot velocity is obtained by integrating the instant acceleration at landing. The average foot velocity during the deceleration period of the swing is shown in Fig. 6. Faster walking results in higher values for the landing velocity. The lowest landing velocity is 1.14 m/s at the 3.0 km/h gait, and the highest is 3.45 m/s at the 12.0 km/h gait. There is approximately a 3-fold increase from the 3.0 km/h gait to the 12.0 km/h gait. The influence of the gait speed on the stance time and swing time is shown in Fig. 7. The longest stance time is 0.97 s for the 3.0 km/h gait. The stance times for 9.0 and 12.0 km/h are almost the same (0.30 s). Faster walking results in a shorter stance time. On the other hand, the swing time stays constant at approximately 0.50 s. A gaitemotion cycle (¼stance þ swing) became shorter with increasing gait speed.
Fig. 3. Contact force distribution during cycle of gait motion.
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Fig. 6. Influence of gait speed on foot landing speed.
When taken together, a higher gait caused a larger contact force to be concentrated in a relatively smaller area in a short period of time compared with a lower gait. 3.3. Body temperatures
Fig. 4. Influence of gait speed on contact force.
Fig. 5. Influence of gait speed on contact area.
Examples of the time-dependent skin temperatures and sublingual temperatures at the 6.0 km/h gait are shown in Fig. 8. At the beginning, the temperatures are stable but not equal at all the measuring points. The temperatures on the upper side of the foot, such as the instep, medial malleolus, and lateral malleolus, are relatively low. After 10 min, the temperatures on the upper side of the foot drop, and those at the other measuring points rise. In particular, the temperatures for the big toe and heel rise sharply. During gait, all the foot temperatures tend to gradually increase. After 30 min of gait, the temperatures at the upper side of the foot increase sharply, and the temperatures at the big toe and heel start to decrease. With the exception of the heel, the temperatures settle at a certain level. The heel temperature continues to decrease. At the end of the 50 min experiment, all of the temperatures are higher than they were before the experiment. The heel temperature shows the highest amount of elevation at 3.0 C. The instep temperature shows the lowest amount of elevation; however, it still rises 1.5 C. The temperatures show similar tendencies at the other gait speeds. The influence of the gait speed on each body temperature elevation is analysed in Fig. 9(a)e(g). The skin temperature tendencies are divided into three groups. The big toe and heel belong to the first group. The temperatures rise sharply during gait and drop after walking in this group. The highest temperature elevations are 8.2 C for the heel and 4.8 C for the big toe at the 12.0 km/ h gait. Even at the lowest gait of 3.0 km/h, the temperature rises 3.5 C for the heel and 1.6 C for the big toe. The greatest characteristic of this group is the rate of temperature increase at the beginning of gait. For instance, the rate of heel temperature increase is 0.4e0.6 C/min, and that for the big toe is 0.2e0.5 C/min for the first 5 min of gait. The little toe and arch belong to another group. The temperatures of this group tend to increase gradually, and the amount of temperature elevation is small compared with the first group. Actually, the maximum temperature elevations are 2.0 C for the little toe and 3.7 C for the arch during gait. The rates of temperature increase are 0e0.1 C/min for the little toe and 0.1e0.3 C/min for the arch during the first 5 min of gait. The temperatures of the last group drop sharply after walking begins and rise sharply after walking ends. The instep, medial malleolus, and lateral malleolus belong to this group. The overall temperature trend of this group is a slight increase with time. The temperature
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whole body were estimated by using Kurazumi's method for Japanese people (Kurazumi et al., 1994). The influence of the gait speed on the time average latent heat loss is also shown in Fig. 11. The amount of evaporative heat loss increases with the gait speed. The latent heat loss is slightly smaller than the metabolism at a slower gait of 3.0 km/h. The difference between them increases with the gait speed. There is a cooling capacity limit for sweating, and the maximum latent heat flux was 208 W/m2 at the 12.0 km/h gait. Therefore, the effect of the latent heat on the thermal environment formation inside footwear varied with the gait speed. 4. Discussion
Fig. 7. Influence of gait speed on period of gait motion.
elevation was approximately 2.0 C after the 50 min experiment. However, the temperatures clearly decreased during gait for a higher gait. The temperatures at the 12.0 km/h gait dropped by approximately 2.0 C just after starting the gait. Interestingly, the temperatures at the bottom side increase and those at the upper side decrease during gait. In general, the temperature rise or drop tends to increase with increasing gait speed. The sublingual temperature remained almost constant at all gaits.
3.4. Other human factors The time dependent metabolism at each gait is shown in Fig. 10. The metabolism is Weir's proposed variable (Weir, 1949). At the beginning, the metabolism stays around 60 W/m2 and is almost constant at all gait conditions. Immediately after walking begins, the metabolism rises sharply and then stays almost constant. Faster walking results in a higher metabolism. The highest metabolism is approximately 480 W/m2 at the 12.0 km/h gait, and the lowest metabolism is approximately 110 W/m2 at the 3.0 km/h gait. Immediately after 30 min of walking, the metabolism drops sharply and returns to the same level as that of the previous standing-up period. Some delayed response is observed for recovery at a higher gait speed. The amount of heat generation at the feet was estimated by assuming that heat generation occurs uniformly over the entire body, as shown in Fig. 11. Sweat is produced to cool the body by evaporation. The amount of evaporative heat loss for the whole body was predicted by multiplying the body mass change during the experiment and the evaporative latent heat. The efficiency of sweating, which is defined as the ratio of secreted and evaporated sweat, was 0.5, based on €m and Holme r (1985). The latent heat on the foot AlbereWallerstro was assigned on the basis of an area factor. The areas of the foot and
Fig. 8. Time-dependent temperatures changes at 6.0 km/h gait.
Some differences in the temperature distributions were observed in the experiment. Faster walking resulted in a higher temperature elevation during gait motion. The temperature on the bottom side of the foot increased, and the temperature on the upper side decreased during gait. In particular, the temperatures on the big toe and heel clearly increased. Some possible factors for the temperature elevation on a shoed foot during gait are the foot contact, slipping friction between the foot and the shoe sole, and environmental variables such as air temperature and solar radiation. In the experiment, the environmental factors were controlled and had insignificant effects. The friction force is normally calculated by multiplying a mass by a friction coefficient, which is normally a decimal value. Considering the fact that the contact force was greater than the body weight, the slipping friction may have had a lower impact than the foot contact. Actually, faster walking resulted in a larger contact force and higher landing velocity in the study. The variation in the temperature increase during gait increased with the increasing contact force from a different perspective. Because the contact load can be defined by the mass, velocity magnitude, and time of loading, an increase in the vertical ground reaction force with a faster walking velocity was previously reported for young adults (Nilsson and Thorstensson, 1989). An elevated heel contact pressure was also reported to be associated with a higher approach velocity (Morag and Cavanagh, 1999). Moreover, the frequency of ground contact increases with increasing gait speed. Of course, the inner heat production increases with increasing gait speed. The metabolism changes sharply at the times of starting and stopping the gait, and then becomes almost constant shortly thereafter. A temperature elevation caused by the metabolism may have occurred with relative uniformity over the foot skin, because the heat was transferred from the same blood flow. The progressive temperature elevation is explainable on this basis and also as a result of the shoe insulation, which is the reason for the temperate rise after stopping the gait. The meaning of insulation here is the state where the generated quantity of heat is larger than the amount of heat dissipated from the skin surface. The skin temperature elevations on the big toe and heel were sharp. During walking, the body weight is solely on the heel at the initial contact, and a strong contact force is observed in this period (Rosenbaum et al., 1994). A strong contact force is observed again in the late stance period, because all the contact force is concentrated over a small region of the forefoot (Eilis et al., 2002). In fact, the higher temperature elevation region coincides with the area of the higher contact load. It is known that a repeated compression load produces heat energy converted from mechanical energy. The temperature elevation in these regions is assumed to be due to the absorption of the contact force on the skin itself, as conducted through the shoe cushioning. Regions with less contact, such as the little toe, arch, and upper side, tended to show relatively lower temperature elevations. The foot temperatures in these regions were mainly affected by metabolic heat generation and thermal
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Fig. 9. Time-dependent temperature changes at each body part.
insulation, which retained the inner heat inside the footwear. This is why the temperatures gradually increased. The big toe and heel temperatures may also have been affected by the contact force. On the other hand, the upper-side temperatures dropped. Air restriction inside footwear causes high sweat production (Purvis and Tunstall, 2004), and local heating, such as of the foot in this study, increases the local sweating rate (Shibasaki et al., 2006), Thus, sweating was a contributing factor in this upper-side temperature drop. A foot produces large quantities of sweat. It contains
a dense region of sweat glands (Park and Tamura, 1992), which consist of two types. Eccrine sweat glands are distributed over the body, and the secretion from eccrine glands is commonly called sweat and contributes to evaporative heat loss (Sato et al., 1989). Apocrine sweat glands are distributed in specific regions such as the sole of the foot, and the secretion from these glands is caused mainly by emotional strain (Ebling, 1989). In this regard, evaporation was considered to occur on the upper side of the foot. This may be a reason that the instep temperature dropped during the gait.
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However, because the micro-climate created under clothing, including sock, and footwear, has important implications in thermoregulation (Shigaki et al., 1993), human perceptions of foot temperature are strongly influenced by sock characteristics (Barkley et al., 2011). Future experiments could investigate the role of footwear with socks in the perception of comfort. Because other factors such as individual differences in fitness, foot shape, and action would also affect the results, it is expected that the roles of such complex factors will be identified in future work.
5. Conclusions
Fig. 10. Time-dependent metabolism change.
However, the metabolism always has an advantage compared with latent heat loss. Therefore, sweating in itself was insufficient to explain these temperature drops. The foot was constantly moving during the gait, and this movement induced a strong convective heat transfer. In general, convective heat transfer is dominated by the temperature gradient between the skin and the ambient fluid, along with the relative movement against the ambient fluid. Because faster walking resulted in greater foot movement, this led to a larger convective heat loss from the skin surface. The porous material also affected the temperature drop by allowing air ventilation. In addition, the bellows action also has a temperature reduction potential during gait motion. A greater amount of ventilation assumed to be due to the bellows action has been reported at an open space like beneath the arch in a closed shoe (Satsumoto et al., 2011). In fact, a relatively lower temperature elevation on the arch was observed. The relation between the gait conditions and foot temperature distributions inside the footwear became apparent. In particular, a larger contact force generated a higher temperature increase on the bottom part of the foot such as the big toe and heel. Since faster walking is accompanied by a larger contact force, reducing the contact force would be difficult. Many researchers have focused on developing shoe cushioning to reduce the contact force and improve the safety. Therefore, an optimum shoe cushioning design might be a solution to both provide comfort and prevent injury. In future research, key aspects of thermally comfortable design concepts and applications will involve predictions of the heat generation mechanism inside the insole and the heat release mechanism-associated airflows inside footwear. One of the limitations of the present study was that the subjects wore shoes on their bare feet throughout the experiment to ensure that the same conditions were maintained, and socks effects were prevented.
Fig. 11. Equivalent metabolism and equivalent latent heat.
In order to identify the factors influencing the formation of the thermal environment inside footwear, experiments with subjects were conducted at four different gait speeds to evaluate the skin temperature changes and mechanical gait parameters. Basically, metabolic heat generation had an impact on the temperature profile. Therefore, the skin temperatures tended to gradually increase. In addition, high temperature elevation regions such as the big toe and heel coincided with the regions with high contact loads. Faster walking resulted in a larger contact force, higher landing velocity, higher skin temperature elevation, and larger metabolism during gait. Considering the time-dependent temperature changes, the results suggested a relationship between the temperature elevations on the big toe and heel and the contact force. The foot temperatures in other regions were mainly affected by the thermal insulation, which retained the inner heat inside the footwear. The upper-side temperature dropped as a result of the convective heat loss and ventilation due to foot movement. Sweat evaporation had a beneficial effect on the instep temperature, but a limited effect on the entire thermal environment formation. Future experiments could investigate factors such as socks and individual differences to design comfortable footwear.
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