The protective equipment required in the sport of football covers much of the body surface and contains rigid materials and thick padding. Thus, football uniforms represent a precarious balance between musculoskeletal protection related to the rigors of the sport and potential harm from the thermal environment as both dry and evaporative heat loss is impeded. The National Collegiate Athletic Association® (NCAA) provides rules regarding the required game uniform for football; however, enough flexibility exists to allow various configurations of game uniforms to be worn depending on weather conditions. Moreover, no regulations cover specific configurations of uniforms that are worn during football practice sessions, and shorts or “girdles” containing hip, thigh, and tailbone pads are commonly substituted for full football pants. The primary difference between NCAA rules and National Football League® (NFL) rules for protective clothing concerns the socks. Specifically, NFL players are required to wear long socks, whereas collegiate players may wear long or short socks, as long as the entire team wears the same type of socks.
The biophysical heat exchange between an exercising human and the environment is predictably modified by clothing systems according to the proportion of the body surface area covered, characteristics of the fabrics and protective materials, and the air trapped within and between material layers. Although entire databases exist for quantifying the effects of indoor and outdoor “everyday” clothing and industrial clothing on heat exchange (8,9), there are no published data for athletic uniforms. If such data were available, they could be incorporated into so-called rational approaches to environmental safety, i.e., solving heat balance equations to predict heat storage and thermal strain (6).
When the exercising body is in thermal equilibrium with the environment, heat gain equals heat loss, that is,
where Mnet is the net metabolic heat production (metabolic heat produced minus any external work performed), (R + C) is the combined dry heat exchange via radiation and convection, and Esk is evaporative heat loss from the skin. Heat is neither significantly gained nor lost and body temperature remains constant. The combined factor (R + C) is determined by the thermal gradient between the mean skin temperature (T sk) and the environmental dry-bulb temperature (Tdb) as modified by the total resistance provided by the clothing worn (Rt) according to the relationship (R + C) = (T sk - Tdb)/Rt. Similarly, Esk = w·(Ps,sk - Pa)/Re,t, where w is the skin wettedness factor, (Ps,sk - Pa) is the vapor pressure gradient from fully saturated skin and the air, and Re,t is the total resistance to evaporation of sweat. Because Mnet, T sk, Tdb, Ps,sk, and Pa can be measured or estimated, knowledge of the parameters Rt and Re,t would allow one to model heat exchange between the exercising human and the environment, providing useful information about the rate of rise of body temperature and safe time limits for prolonged exercise-heat exposures.
In the present study, five configurations of NCAA regulation football uniforms and practice ensembles were selected for evaluation after consultation with football coaches, equipment managers, and athletic trainers at several universities. The insulation value, Rt, was directly measured using the constant temperature method specified in ASTM F 1291, Standard Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin (1), which involved measuring the thermal resistance of each clothing system using an electrically heated manikin in thermal equilibrium with its surroundings. To determine evaporative resistance, Re,t, the manikin was covered with a cotton knit “skin” and sprayed with distilled water to simulate skin saturated with sweat, following procedures published by McCullough et al. (9).
METHODS AND PROCEDURES
The thermal resistance values for the football uniform configurations were measured using an electrically heated manikin in an environmental chamber. The manikin used in the present study consisted of a black anodized copper skin formed to simulate the physical shape and size of a reference man (i.e., 1.8 m2 surface area, 176 cm height). Heating wires were bonded to the inside surface of the copper skin to provide internal heating distributed so as to simulate the skin temperature distribution of a human. The skin temperature of the manikin was measured using 16 thermistors located on different parts of the body. The power cables and thermistor wires pass from the body cavity through the left side of the neck to the control and measurement equipment. The entire system was computer operated.
One garment of each of the football ensembles was provided by the athletic department of an NCAA Division I university. The ensembles are described in Table 1. Three ensembles were full game uniforms adhering to NCAA regulations, while the remaining two were practice configurations in which football pants were replaced by a hip girdle with pads or mesh shorts, respectively, and cut-off T-shirts were worn. In addition, a reference ensemble consisting of a T-shirt, briefs, shorts, ankle-length socks, and athletic shoes was included for comparison.
Experimental Procedures and Calculations
Dry manikin tests.
The thermal resistance (also presented as the insulation value in clo units) was measured according to ASTM F 1291, Standard Test Method for Measuring the Thermal Insulation of Clothing Using a Heated Manikin, Option #L (1). The garments were initially hung on a clothing rack in an environmental chamber. The environmental conditions for the dry insulation tests were controlled as follows: ambient air temperature = 20°C, air velocity = 0.2 m·s-1, dew point temperature = 6°C, relative humidity = 40%, and manikin surface temperature = 33.3°C.
The air temperature was measured continuously with a matrix of four thermistors mounted on a vertical wooden stand located between the wall air supply and the manikin. The dew point temperature was measured continuously with a sensor in the same location, and air velocity was measured periodically using an anemometer.
To conduct each test, the manikin was dressed in an ensemble, and all closures were secured. The manikin hung from a metal stand by a hook in the head. The feet touched the floor, and arms hung at the sides. Equilibrium was maintained for at least 1.5 h before testing and was indicated by a steady-state power reading that had not changed more than 2%. Data were collected by computer every 30 s over a 30-min period.
The total thermal resistance (Rt) is the total resistance to dry heat loss from the body surface, which includes the resistance provided by the clothing and the air layer around the clothed body. Rt was directly measured with the manikin and is calculated as:
where Rt = total thermal insulation of the clothing plus the boundary air layer (°C·m-2·W-1), T s = mean surface temperature (°C), Ta = ambient air temperature (°C), As = manikin surface area (m2), and H = power input (W).
The thermal resistance of clothing is more commonly expressed in clo units where 1 clo of insulation is equal to 0.155 m2·°C·W-1 (2). Gagge et al. (3) first defined the thermal resistance associated with 1 clo by first considering that the resting metabolic heat production of an average man is about 58 W·m-2. Approximately 25% of this heat is lost via the respiratory system and by diffusion of moisture through the skin. Therefore, 44 W·m-2 remains to be lost through the clothing via radiation, conduction, and convection (3). The temperature difference across the clothing is equal to the difference between the mean skin temperature (T sk) and the ambient air temperature (Tdb), assuming the mean radiant temperature of the surroundings is equal to Tdb. Consequently, a clothed person with a comfortable skin temperature of 33°C in a comfortable environment at 21°C has a 12°C temperature gradient across which 44 W·m-2 is transferred. A heat transfer coefficient of 0.275 m2·°C·W-1 is calculated by dividing the temperature difference by the heat flow (5). About 0.12 of the 0.275 m2·°C·W-1 total is contributed by the surrounding air layer, so 0.155 m2·°C·W-1 is contributed by the clothing alone. Thus, 1 clo of insulation is equal to 0.155 m2·°C·W-1, and
In the present study, the total insulation value for each ensemble was reported as the average of three independent replications made on each sample set of clothing. All replications were within ± 0.001°C·m-2·W-1 of the mean. Because It includes the air layer resistance at the surface of the clothed body, it is influenced by air velocity and temperature level (as it relates to incident radiation). These factors can be easily dealt with in biophysiological models that predict heat exchange under a specific set of conditions. However, in some models and applications, it may be preferable to separate the resistance of the clothing from the resistance of the air layer.
Intrinsic clothing insulation (Icl) indicates the insulation provided by the clothing only and does not include the insulation provided by the surface air layer. Icl is defined by
where Icl = intrinsic clothing insulation (clo), It = total thermal insulation of the clothing plus the boundary air layer (clo), Ia = resistance of the boundary air layer around the nude manikin (clo), and fcl = clothing area factor (unitless).
The insulation provided by the external air layer (Ia) was determined by testing the manikin nude. The fcl is the ratio of the clothed body surface area to the nude body surface area. The term (Ia/fcl) is the resistance provided by the air layer around the clothed body. It is smaller than the air layer resistance for the nude body (Ia) because the clothing increases the surface area and thus provides a greater area for heat transfer. The fcl varies from 1.00 (nude) to a maximum of about 1.50. It can be measured with a photographic method (8) or estimated from ISO Standard 9920 (7). In this study, fcl values were determined by taking photographs of the manikin from the front and side angles, and comparing the projected area of the manikin clothed with the projected area of the manikin nude (8).
Manikin procedures for evaporative resistance tests.
The environmental conditions for the sweating manikin tests were controlled as follows: ambient air temperature = 26.7°C, air velocity = 0.15 m·s-1, dew point temperature = 12°C, relative humidity = 40%, and manikin surface temperature = 32.8°C. For the evaporative tests, the manikin was covered with a cotton knit “skin” and sprayed with distilled water to simulate skin saturated with sweat (i.e., 100% skin wettedness). The procedures were similar to those used for the dry tests; however, the environmental conditions were warmer. The basic equation for calculating the total resistance to evaporative heat transfer provided by the clothing is:
where Re,t = resistance to evaporative heat transfer provided by the clothing and the boundary air layer (m2·kPa·W-1), Ps,s = saturated water vapor pressure at the manikin surface (kPa), and Pa = the water vapor pressure in the air (kPa).
Ideally, the air temperature and the manikin temperature would be exactly the same, and all of the heat transferred would be due to moisture evaporation. However, the Tdb was ˜5°C lower than the T s in the sweating tests, and consequently, some dry heat transfer occurred. It was therefore necessary to correct H for this heat transfer by the relationship
Therefore, the mean Rt value from the dry tests on the ensemble was needed to calculate Re,t. The actual calculation for Re,t is then
Data from three replications of the wet tests were averaged to determine the mean Re,t for each ensemble (including the air layer) and for the air layer alone (tested using the wet cotton “skin” alone). All replications were within ± 0.001 m2·kPa·W-1 of the mean.
The equation for calculating the intrinsic evaporative resistance provided by the clothing alone (Re,cl) is analogous to that for dry resistance:
where Re,a = the resistance to evaporative heat transfer for a still air layer (m2·kPa·W-1).
Finally, the moisture permeability index (im) indicates the maximum evaporative heat transfer permitted by a clothing system as compared to ideal maximum from an uncovered surface (i.e., a sling psychrometer). It was defined by Woodcock (11) as
The value of the ratio Ra/Re,a varies somewhat with temperature but is typically taken to have the value of 16.65°C·kPa-1 (i.e., the Lewis relation). The im value for a clothing system (including the air layer resistance) can thus be calculated by the simplified relationship
Although theoretically the im values could range from 0 for a system with no evaporative heat loss to 1 for a system that had little or no resistance to evaporative heat transfer, values greater than 0.50 are seldom obtainable for a motionless nude manikin in relatively still air (4).
The evaporative heat transfer characteristics of different clothing systems can be directly compared using the im values if their insulation values are the same. If the insulation values are different, the relative evaporative transmissibility of the ensembles can be determined by dividing each im value by its corresponding total insulation value. The resulting parameter (im/It) is a coefficient of evaporative transmissibility that is a function of air velocity (4). It is a good indicator of the percent of evaporative cooling permitted by the clothing ensemble.
The insulation values for the football uniform ensembles are presented in Table 2. The thermal resistance (insulation) associated with the still air layer around the nude manikin was 0.094 m2·°C·W-1 or 0.61 clo. For comparative purposes, the insulation values for a T-shirt/shorts ensemble are given also. For minimal clothing like the T-shirt/shorts ensemble, the Ia is greater than the Icl; that is, the boundary air layer provides more resistance to dry heat exchange than the clothing itself under still air conditions. A practice uniform (P1) with a helmet, padding under the shorts, and shoulder pads under the jersey provided twice the clothing insulation as the reference T-shirt/shorts ensemble. The cold-weather game uniform (G3) provided almost three times more insulation than the T-shirt/shorts ensemble. Because of the simple algebraic relation between (R + C) and Rt, wearing a warm weather (minimal allowable) game uniform would decrease dry heat loss by 42% compared with the T-shirt/shorts reference ensemble.
The evaporative resistance values, permeability index values, and evaporative transmissibility are listed in Table 3. The Re,a value for the still air layer was 0.012 m2·kPa·W-1. The intrinsic evaporative resistance values for the clothing ranged from 0.017 to 0.029 m2·kPa·W-1, with the regular game uniform (G2) and the cold weather game uniform (G3) having three times the clothing evaporative resistance of the T-shirt/shorts ensemble.
This paper presents, for the first time, quantitative data that govern heat exchange through football uniforms. These data can be used to more accurately model the heat balance and heat storage of players during football practices and games. These data were collected under still air conditions with a stationary manikin. During a football practice or game, higher levels of air velocity and body motion will increase the convective and evaporative heat loss from the player’s body. Rational models can incorporate such situation-specific variables.
A high insulation value may or may not be desirable in a football uniform, depending upon the environmental conditions in which the game is played. Football uniforms provide uneven coverage of the body. Some body parts like the head, shoulders, hips, and thighs have thick, protective padding. Other parts like the arms, calves, and hands are often not covered with clothing at all. As expected, the cold weather game uniform G3, which included thermal leg liners, had the highest insulation value, followed by the temperate weather uniform G2 without leg liners. Adding the leg liners made little difference in the clothing insulation, as the lower leg was already covered by long socks. Both of these ensembles covered the entire body surface except for the face and neck and had an intrinsic clothing insulation value of about 1 clo—the insulation associated with a heavy, three-piece men’s business suit. For comparison, a long-sleeved sweat shirt and sweat pants worn with socks, shoes, and briefs has an Icl value of 0.74 clo (9). Clothing insulation increases as the percentage of the body surface area covered with garments increases and as the thickness of the garment layers increases (8).
The higher the evaporative resistance, the less permeable the clothing is to moisture transfer. Low evaporative resistance values are usually associated with clothing comfort in both hot and cold environments. The average Re,cl for a wide range of indoor clothing ensembles has been reported as 0.019 m2·kPa·W-1, and the average Re,t value as 0.031 m2·kPa·W-1 (9). In this study, the warm weather football uniform (G1) had an evaporative resistance value that was higher than that of a sweat suit ensemble or jeans and a heavy long-sleeved, rugby-style shirt (Re,cl = 0.017 m2·kPa·W-1 and 0.020 m2·kPa·W-1, respectively) (9). The practice uniform without hip pads (P2) had the lowest intrinsic evaporative resistance of all of the football ensembles, but it was still double that of the T-shirt/shorts reference ensemble due to the thick padding and rigid material used in the helmet and shoulder pads.
The evaporative resistance of an ensemble depends upon the moisture permeability characteristics and wicking properties of the component materials used in an ensemble and the amount of skin surface covered by those materials. Fiber content has little to do with the moisture permeability of textiles. The openness of the fabric structure and the type of surface treatments have more of an impact. Hard solid materials (used in helmets and shoulder pads) and foam padding, with or without a vinyl covering, particularly impede the evaporation of sweat. In addition, the padding in the hips and legs is tight fitting, minimizing air circulation between the skin and the clothing.
The higher the permeability index or im value, the greater the moisture permeability of the clothing relative to its level of insulation. Consequently, high permeability index values are desirable for effective evaporative heat transfer and thermal comfort. Manikin-based im values typically range from 0 (no permeability) to about 0.5 (nude). Values for most indoor clothing range from approximately 0.34 to 0.41 (9), and the football ensembles had im values within this range. The football uniforms with the most body surface coverage, G2 and G3, had the lowest permeability index values, whereas the practice uniforms (and T-shirt/shorts ensemble) provided the highest permeability index values, primarily because they had the highest exposed body surface area for unimpeded evaporation of sweat. When the im values were divided by the insulation values (im/It), the differences between the evaporative cooling potential of the ensembles became larger.
To illustrate the impact of the present data, consider the example of a 6-foot 3-inch football player weighing 240 lbs (109.1 kg) with a surface area of 2.36 m2. If he runs on a level surface at an average oxygen uptake of 2.0 L·min-1, he will generate an Mnet of approximately 297 W·m-2. On an 86°F (30°C) day with 70% relative humidity, the Pa would be 2.96 kPa. Assuming a fully wetted skin at a T sk of 36°C, Ps,sk = 5.89 kPa. Thus, the dry and evaporative gradients from skin to air are (T sk - Tdb) = 6°C and (Ps,sk - Pa) = 2.93 kPa, respectively. Wearing just shorts and a T-shirt, (R + C) = 6°C/0.140 m-2·°C·W-1 = 43 W·m-2 and Esk = 2.93 kPa/0.020 m-2·kPa·W-1 = 147 W·m-2. The relation between heat storage and the change in mean body temperature ([DELTA]T b) is given as:
where [DELTA]t is the elapsed time in h, 0.97 W·h/(kg·°C) is the specific heat of the body, mb is body mass, and As is surface area. In this scenario, if he runs for 30 min, total heat storage, S = Mnet - (R + C) - Esk = 107 W·m-2. Given that mean skin temperature (T sk) rarely exceeds 36°C during exercise-heat stress, and (10)
core temperature (Tc) would be predicted to increase approximately 1°C. Wearing the warm weather football uniform G1, (R + C) = 30 W·m-2 and Esk = 92 W·m-2. The new S = 175 W·m-2 and core temperature would be predicted to increase approximately 2°C.
Although heat-related illness in football is more prevalent than hypothermia, a similar heat balance approach can be used for cold weather application of the present insulation values. For example, consider the above described player standing on the sidelines on a 0°C day in Green Bay, WI. Esk has minimal impact in this instance, and
Assuming Mnet = 44 W·m-2 and T sk = 32°C, and weighting [DELTA]T b for vasoconstricted skin (2), i.e.,
S in uniform G1 would be -117 W·m-2, leading to a predicted fall in Tc of approximately 1.5°C over a 30-min period (assuming a drop in T sk from 34°C to 32°C). Switching to the cold weather uniform, G3, reduces S to -93 W·m-2, attenuating the decline in Tc by 33% over the 30-min period.
There is a need for quantitative information that exercise scientists, coaches, team physicians, and athletic trainers can use to prevent heat related injuries and deaths associated with the sport of football. Mathematical models for estimating the heat exchange between the body and the environment are widely available, and user-friendly computer programs could be developed. However, quantifying some of the key input variables renders such an approach difficult. The environmental conditions during a game or practice can be measured directly or downloaded from local weather web sites. The database of thermal and evaporative resistance values for the football uniforms from this article can be incorporated into a selection menu in the program. At this time, estimating the metabolic heat production of individual players remains the greatest challenge facing physiologists.
The authors gratefully acknowledge the financial support of the Gatorade Sports Science Institute.