Talk to the Hand: U.S. Army Biophysical Testing

MILITARY MEDICINE, 182, 7/8:e1702, 2017 Talk to the Hand: U.S. Army Biophysical Testing William R. Santee, PhD; Adam W. Potter, MS, MBA; COL Karl E. Friedl, MS USA (Ret.) The human hand is uniquely dexterous among primates, and hands play a critical role in the performance of the soldier system. Impairment of manual hand performance during cold exposure is related to the skin temperature.1,2 Soldier hands provide fine motor dexterity in tactical functions, ranging from pulling a trigger to pulling a parachute ripcord. Hands have a significant role in thermoregulatory control because of the large changes in blood flow that can be achieved, combined with a surface area to mass ratio which is 4 to 5 times larger than for the whole body.3 If properly designed, handwear can protect and augment human performance capabilities, whereas improper handwear can lead to critical Biophysics and Biomedical Modeling Division, U.S. Army Research Institute of Environmental Medicine, 10 General Greene Avenue, Building 42, Natick, MA 01760-5007. The opinions or assertions contained herein are the private views of the authors and should not be construed as official or reflecting the views of the Army or the Department of Defense. Any citations of commercial organizations and trade names in this report do not constitute an official Department of the Army endorsement of approval of the products or services of these organizations. This article is approved for public release; distribution is unlimited. The authors have no conflicts of interest to report. doi: 10.7205/MILMED-D-16-00156 e1702 mission failures or injuries. These injuries can lead to lifelong sensitivities to cold or more severe tissue loss and amputation. The U.S. Army has been on the forefront of the biophysical analysis of clothing systems and subsystems (e.g., gloves, boots, headwear) since initial environmental research established at the Armored Medical Research Laboratory (Fort Knox, Tennessee) and Climatic Research Laboratory (Lawrence, Massachusetts) during World War II (Fig. 1). Concerns about protection in the cold increased with the large number of nonbattle cold injuries in northern Europe, in the Aleutian Islands, and later in Korea.4 In the Winter of 1950–1951, an estimated 5,600 soldiers were evacuated from Korea for cold injuries, primarily freezing cold injury to hands and feet.5 Even in training, extremity cold injuries continue to be a problem for the Army.6 Thermal manikin testing methodologies were developed to provide an efficient and consistent analytical tool for the rapid evaluation of new clothing concepts. These methods have been upgraded since the original World War II and Korean War eras to include articulation and sweating capabilities. The mathematical modeling that makes these measurements relevant to human physiology has also dramatically evolved. The important role of test manikins in protecting soldier readiness should MILITARY MEDICINE, Vol. 182, July/August 2017 Downloaded from https://academic.oup.com/milmed/article-abstract/182/7/e1702/4158536 by guest on 01 June 2020 ABSTRACT Background: Many people are unaware of the science underlying the biophysical properties of Soldier clothing and personal protective equipment, yet there is a well-refined biomedical methodology initiated by Army physiologists in World War II. This involves a methodical progression of systematic material testing technologies, computer modeling, and human testing that enables more efficient development and rapid evaluation of new concepts for Soldier health and performance. Sophisticated manikins that sweat and move are a central part of this testing continuum. This report briefly summarizes the evolution and use of one specialized form of the manikin technologies, the thermal hand model, and its use in research on Soldier hand-wear items that sustain dexterity and protect the hand in extreme environments. Methods: Thermal manikin testing methodologies were developed to provide an efficient and consistent analytical tool for the rapid evaluation of new clothing concepts. These methods have been upgraded since the original World War II and Korean War eras to include articulation and sweating capabilities, as characterized and illustrated in this article. The earlier “retired” versions of thermal hand models have now been transferred to the National Museum of Health and Science. Findings: The biophysical values from manikin testing are critical inputs to the U.S. Army Research Institute of Environmental Medicine mathematical models that provide predictions of soldier comfort, duration of exposure before loss of manual dexterity, and time to significant risk of freezing (skin temperature <−1°C) and nonfreezing cold injuries (skin temperature <5°C). The greater thickness of better insulated handwear reduces dexterity and also increases surface area which makes added insulation increasingly less effective in retaining heat. Measurements of both thermal resistance (insulation) and evaporative resistance (permeability) collectively characterize the biophysical properties and enable mathematical modeling of the human thermophysiological responses. This information can help guide the hand-wear development and selection process which often requires trade-offs between factors such as material, cost, and sizing. Impact: Soldier hands provide fine motor dexterity in tactical functions, ranging from pulling a trigger to pulling a parachute ripcord; thus, protecting hand function is critical to soldier readiness. Also, the importance of protection against nonbattle cold injuries was highlighted during World War II in northern Europe, in the Aleutian Islands, and later in Korea. The U.S. Army has been on the forefront of the biophysical analysis of clothing including gloves since environmental research was established at the Armored Medical Research Laboratory and Climatic Research Laboratory during World War II. U.S. Army Research Institute of Environmental Medicine does not make the equipment but works with their Natick Soldier Research, Development, and Engineering Center partners to make the equipment better. Talk to the Hand be noted as earlier generations of these hand models pass into history (Fig. 2A–2D). The first thermal manikins, including the early rigid thermal hand models (Fig. 2A) were built by General Electric Company (Bridgeport, Connecticut) for the U.S. Army and Air Force; these and subsequent models have a flat black surface to match emissivity of human skin (emission of radiative heat from the surface). This was replaced by a more elaborate 22-zone articulated copper hand (Fig. 2B). The addition of the articulation feature was an important improvement as it allowed mounting handwear without the need for cutting or modification of the material. This model FIGURE 2. The progression of thermal hand manikins used for soldier glove testing including: (A) original rigid hand built by General Electric Company; (B) sectionalized hand calorimeter built by the Dunn Engineering Corp (Cambridge, Massachusetts); (C) Thermal Hand Test System (THTS-1) cast from a real hand, developed for USARIEM by Measurement Technology Northwest; and the (D) 8 Zone Sweating Hand (506-18), by Measurement Technology Northwest. MILITARY MEDICINE, Vol. 182, July/August 2017 e1703 Downloaded from https://academic.oup.com/milmed/article-abstract/182/7/e1702/4158536 by guest on 01 June 2020 FIGURE 1. Army scientists and personnel conducting evaluations on improved textiles and insulating materials at the Climatic Research Laboratory, Lawrence, Massachusetts, ca. 1945. Included in this image is a thermal hand manikin being prepared for data collection, testing of a textile sample with a thermal flat plate, and a small wind tunnel environmental chamber. Painted by Technician fourth Grade Moore, Climatic Research Laboratory Enlisted Soldier. Courtesy copy of the original on display at U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts. was supplemented, and eventually replaced by a simpler, field portable seven-zone aluminum model (Thermal Hand Test System, Thermetrics/Measurement Technology Northwest, Seattle, Washington; Fig. 2C). In this simplified version, temperature is measured at the surface of the hand, instead of internal temperature measurements of the previous models. Ultimately, sweating was added to the hand manikin (Thermetrics/Measurement Technology Northwest; Fig 2D), with an added layer of material to represent skin and retain moisture. This enabled the measurement of evaporative resistance, an indication of water vapor permeability. The basic operating principle for all anthropometric thermal models, require accurate measurements of the power required to maintain a constant surface (“skin”) temperature. Temperature of the surrounding environment (within an environmental chamber) is also held constant, thus creating a constant thermal gradient. At steady-state conditions the power supplied to maintain the surface temperature of the model is equal to heat loss to the environment, allowing for calculations of thermal and evaporative resistances.7 These data values of handwear can be used to determine suitability for protection within different environmental conditions. An example test would demonstrate a range of values between insulation of the bare hand (0.04 m2·K·W−1) to light duty glove (0.12), trigger finger mitten (0.21), and the arctic mitten set (0.35). This method can also be applied to wet handwear, plotting the decrease in power demand as the handwear dries, as well as to wind effects.8 Measurements of both thermal resistance (insulation) and evaporative resistance (permeability) collectively characterize the biophysical properties and enable mathematical modeling of the human thermophysiological responses. This information can help guide the handwear development and selection process which often requires a trade-off between material, cost, sizing, and so on. The greater thickness of better insulated handwear reduces dexterity and increases surface area Talk to the Hand which makes added insulation increasingly less effective in retaining heat (Fig. 3).9 There are also large differences between overall and local insulation; typically, insulation around the fifth fingertip is used as a critical indicator of the overall effectiveness of a glove. In mittens, the fifth digit is less isolated; however, it still likely to be cooler than the other fingers. The biophysical values from manikin testing are critical inputs to the U.S. Army Research Institute of Environmental Medicine (USARIEM) mathematical models that provide predictions of soldier comfort, duration of exposure before loss of manual dexterity, and time to significant risk of freezing (skin temperature <−1 C) and nonfreezing cold injuries (skin temperature <5 C).1,10 Physiological factors also play a part in hand protection, including for example the effect of warming the torso on increased hand blood flow and skin temperature. As the body cools, blood flow is reduced to the extremities, increasing the likelihood of cold related injuries (e.g., frostbite). Typically, when the skin temperature of the hand reaches approximately 5°C nonfreezing injuries occur; whereas freezing injuries occur around −1°C when the tissue begins to freeze.11 Values from whole-body and hand-specific thermal models coupled with thermophysiological modeling allow for predictions of the amount of thermal insulation needed to protect an individual from cold injuries on the basis of their environment and activities. A combination of thermal testing and physiological modeling provides a quantitative and systematic means of comparing both the biophysical properties and predicted physiological responses of wearers, allowing for comparison of different handwear. This systematic process provides a basis for selection of suitable handwear. Acceptability for military procurement is also dependent on other proper- e1704 ties such as durability and performance impacts from the handwear. Mittens generally provide more warmth than gloves as there is no barrier to heat exchange between the hand regions and surface area is decreased, but dexterity is generally better with gloves. Military handwear often consists of an outer shell and an inner liner. Under actual use, it may be necessary to remove the outer shell and work either bare handed or with just the thin inner liner. Thus both the thermal resistance and other properties of the inner glove are important and need to be tested separately as well as in a complete ensemble of inner and outer elements. Manikin testing of the thermal properties of the handwear is often followed by human testing. In some studies, the hand is inserted into a small cold chamber or ice bucket for a cold pressor test,12 but an ideal test consists of individuals wearing a complete cold weather uniform inside a large environmental chamber. As the overall thermal state and heat production for the body determine factors such as blood flow to the extremities, controlling for activity level, and positioning of the hand and movement are important. Dexterity tests may also be incorporated into a study, but the test design must take into account the effect of movement or posture on circulation and heat production. For human testing of handwear, the test volunteers are often required to be sedentary, and some test designs are very specific regarding the positioning of the hands.13 Testing at cold temperatures may also impact test instrumentation. Because of the cost, in terms of both monetary and human resources, well-controlled handwear studies are often not included in the development program, and testing may go directly to a limited issue field trial of issues of durability, manual dexterity, and fit. Hand, foot, and head manikins complement the evaluations that are done with moving and sweating whole body MILITARY MEDICINE, Vol. 182, July/August 2017 Downloaded from https://academic.oup.com/milmed/article-abstract/182/7/e1702/4158536 by guest on 01 June 2020 FIGURE 3. Relative mitten size necessary for insulation to protect at various exposure times in −20°C conditions (recreation developed on the basis of Goldman, 1964).9 Talk to the Hand 3. 4. 5. 6. 7. 8. 9. 10. ACKNOWLEDGMENTS This work was supported in part by appointments for both Drs Santee and Friedl to the Knowledge Preservation Program at the U.S. Army Research Institute of Environmental Medicine (USARIEM) administered by the Oak Ridge Institute for Science and Education (ORISE) program under the U.S. Department of Energy. 11. 12. 13. REFERENCES 1. Xu X, Tikuisis P: Thermoregulatory modeling for cold stress. Compr Physiol 2014; 4 (3): 1057–81. 2. 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Santee WR, Blanchard LA, Chang SKW, Gonzalez RR: Biophysical Model for Handwear Insulation Testing. Technical Report T7/93, Natick, MA, US Army Research Institute of Environmental Medicine, 1993. NTIS accession number ADA262926. Available at http://oai.dtic .mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA262926; accessed December 16, 2016. Goldman RF: The Arctic soldier: Possible research solutions for his protection. US Army Research Institute of Environmental Medicine, Natick, MA. 1964. NTIS accession number ADA613189. Available at: http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html& identifier=AD0613189; accessed December 16, 2016. Xu X, Tikuisis P, Gonzalez R, Giesbrecht G: Thermoregulatory model for prediction of long-term cold exposure. Comput Biol Med 2005; 35: 287–98. Castellani JW, Young AJ, Ducharme MB, Giesbrecht GG, Glickman E, Sallis RE: Prevention of cold injuries during exercise. Med Sci Sports Exerc 2006; 38: 2012–29. O’Brien C: Reproducibility of the cold-induced vasodilation response in the human finger. J Appl Physiol 2005; 98(4): 1334–40. Santee WR, Mullen SP, Laprise BS, Blanchard LA: An Evaluation of Prototype Electrically Heated Handwear. Technical report, Natick, MA, US Army Research Institute of Environmental Medicine, 1999. NTIS accession number ADA363276. Available at: http://oai.dtic.mil/oai/oai? verb=getRecord&metadataPrefix=html&identifier=ADA363276; accessed December 16, 2016. Chauncy T: The copper thermal manikin. Mil Med 2015; 180: 718–9. e1705 Downloaded from https://academic.oup.com/milmed/article-abstract/182/7/e1702/4158536 by guest on 01 June 2020 manikins at USARIEM (“Chauncy,” one such whole body manikin, is currently on view as part of the collection at the National Museum of Health and Science).14 Collectively, these have been developed as part of the Army Medical Department’s historic mission of thermal physiology research with biophysics and biomedical modeling. USARIEM does not make the clothing but works with their Natick Soldier Research, Development, and Engineering Center partners to make the equipment better. Protecting hand function is a critical contribution to soldier readiness. Fifty years ago, the USARIEM senior scientist, Ralph Goldman, pointed out that modern day man is less prepared to work in the cold than cave dwelling ancestors because we have been so effective at avoiding the cold using modern technologies in heated shelters. However, at some point in warfare, soldiers “have to leave the shelter with a good clothing system and depend on and conserve their own metabolic heat production for survival.”9 USARIEM clothing manikin test technologies continue to be a crucial component for ensuing the soldier is equipped with the best possible clothing systems to optimize performance and provide protection in the cold.