Physical Ergonomics of the Infantry Helmet
The protective helmet currently in use by infantry soldiers in the U.S. Army was developed in the early 1970s and was last tested for its ergonomic suitability in 1976 (McManus et al., 1976). In the intervening 20 years, crucial advances have taken place in the technologies of ballistic protection and in the design of displays for military use. In particular, display improvements have been made in the capability to enhance vision under low-light conditions by the use of photo multiplication technology. The size and weight of such devices have been greatly reduced-making it possible to provide a highly portable night-vision apparatus for use by dismounted soldiers. A secondary benefit of such equipment is that it can be made sensitive to infrared frequencies. This means that objects that radiate in the infrared range can be discerned by the appropriately equipped soldier in low ambient light conditions. Furthermore, high-intensity infrared sources now can be used for communication purposes or to illuminate targets without revealing intent or position to hostile forces that are not so equipped. In short, technical means now are available to conduct many operations that once were restricted to daylight execution.
In addition to image enhancement, electronic technologies have improved the capability to convey both real scenic images (e.g., television pictures), symbol sets (e.g., maps and instrument readings), and narrative text on a miniature opaque screen.
The combination of these technical advances has made it possible to contrive small, multipurpose, multimode displays that can be mounted on a headband or a helmet. Applications of this capability have been undertaken for crew members in both fixed and rotary wing aircraft and in some surface vehicles. The step toward providing similar benefit for the dismounted soldier was a reasonable and
logical extension of the applications already shown to be successful. However, the research, development, test, and evaluation work that has been done on air crew (Rash et al., 1987) and tanker helmet-mounted displays (Nelson, 1994) is not particularly relevant to the problems associated with the use of such equipment by dismounted soldiers. The weight of the equipment can be externally supported in a vehicle, and personal mobility is not usually a requirement for such vehicle-borne personnel. Consequently, the design of a helmet-mounted display for the dismounted soldier presents a set of new problems for engineers and program managers.
ERGONOMICS DESIGN CRITERIA
The infantry soldier differs from the other fighting personnel because he must physically carry all of his equipment. That means that weight is a design criterion for the helmet-mounted display (Kibler, 1975). Although serious effort has been given to minimizing the weight of the helmet-mounted display for the Land Warrior System, it still represents some increment over what the soldier must carry now-and that constitutes an ergonomic drawback. From this perspective, an increment of more than approximately 1 kilogram on the helmet and possibly 2 kilograms in a back or side pack would generate measurable impairment to the endurance of the soldier (Woodson and Conover, 1964).
In addition to sheer weight, other key criteria are as follows:
Weight distribution. The central issue in weight distribution is the possibility of a shift in the center of gravity-both with respect to the head alone and to the body as a whole. Any shift in the weight distribution on the head away from its normal center of gravity can increase the likelihood of whiplash-type injuries to the neck (Jones et al., 1972). Some muscle strain will accompany any displacement-and the strain will be proportional to the lever force on the neck (i.e., the force vector composed of the weight and the angle of displacement). Changes in overall bodily center of gravity impair balance during movement. The compounded problem in this area comes from the fact that the body has reflexes and highly overlearned muscular compensation responses when moving at different speeds. Thus, whereas the ideal location of any weight on the head might be centered on the vertical line connecting the body's center of gravity and the head's center of gravity, the situation is complicated by the postural changes as one shifts from an erect, standing position to a slow walk, to a fast walk, to a jog, and to a run. In combat, when the soldier may be in a partial crouch while moving as quickly as possible, the problem of maintaining balance will require extensive muscular accommodations with the cost of added stress and fatigue.
Protection. If a new helmet is developed for the dismounted soldier, all standard protection tests will need to be conducted (Perry, 1994). Ballistic protection is a primary consideration. The capabilities of the current helmet are good for fragments and general debris, but it will not deflect a direct hit from a standard
military rifle round (Corona et al., 1974). There are several facets to the protection criterion that suggest substantial inquiries by empirical means. One is the evaluation of new materials that might be superior to that currently used, Kevlar. For example, some preliminary tests of Aramid as a Kevlar substitute have already been conducted (North Atlantic Treaty Organization, 1995). In any case, the 20-year interval from the adoption of the current helmet leads to the thought that some advances in materials or concepts are likely. Such advances have taken place in the design of protective headgear for various sports (Hurt and Thom, 1993). The differences in the headgear worn by bicyclists and that worn by football players is instructive. For example, football helmets are designed for sustained use, whereas cyclists' helmets are designed to serve in only a single incident (Vetter et al., 1987).
Given the long interval since the last redesign of the infantry helmet, it may be useful to consider what standards of ballistic protection are valid in present day war-fighting environments (see, e.g., Edwards and Kash, 1995).
Freedom of head movement. In combat settings, the individual soldier must be able to visually scan the entire scenic surround for threats and targets. To do so efficiently and with minimum exposure, the head must be free so that head-neck movements are totally unimpaired. Even if the shape of the helmet itself does not restrict such movement (U.S. Army Human Engineering Laboratory, 1973; Scheetz et al., 1973), care must be exercised with respect to any attachments to the helmet. Of particular concern should be the use of wires or cables that provide electronic connections from the helmet-mounted display with other equipment such as remote gun sights and navigation gear such as the global position system and computer equipment. Wires or cables can become snagged on other items of carried gear as well as on external objects such as vegetation. Various distributions of the cabling connections should be tried under field conditions to determine what arrangement is the least likely to give the soldier problems in head movement.
Helmet position stability. In electro-optical systems in which the eye is positioned a few centimeters from a small display, there is little or no tolerance for any shift in the relative location of the display with respect to the eye. In terms of physical ergonomics, this means that the helmet cannot be free to move on the wearer's head-or if some movement is allowed, resettling of the helmet must be very easy and quickly accomplishable by the wearer. A potentially detrimental trade-off is presented by the prospect that restrictions on helmet-to-head movement will mean the use of some kind of harness that will add weight and possibly be quite uncomfortable to the wearer. Very little research has been done on headgear restraints. The modest data that are available indicate that an adequate design solution will be difficult to achieve when mobility factors are also taken into account.
Microclimate control. Various options exist with respect to the arrangement of support for the helmet or helmet liner (Fonseca, 1974). Webbing, foam padding, liquid-filled pads, and other materials provide some reasonable alterna-
tives. However, those that might fulfill the need for weight distribution and positional stability are likely to be deficient with respect to air circulation inside the helmet. It appears likely that the optimally stabilized helmet will add to the heat load of the wearer. This may not be too debilitating except that some wearers will produce quantities of sweat that can impair vision. A possible counteraction would be to equip the helmet with some form of cooling capability; however, the trade-off is added weight.
User acceptance. User acceptance of a newly designed helmet depends in part on the actual quality of the fit (Robinette, 1993) and freedom from discomfort (Mozo et al., 1995). However, research on user acceptance of innovations strongly suggests that users conduct a form of subjective cost-benefit analysis in forming their reactions. Another important variable is the user's sense of participation or influence on the configuration of the innovation. And group effects such as the expressed attitudes of fellow workers can be strong (Coch and French, 1948). Engaging prospective users early and often in the development process is one way to promote user acceptance of the final product. This procedure is even more effective if participation is shared across the workforce-or if a large, representative sample of users is engaged in the early evaluation stages of innovation development. The crucial factor is whether or not the benefits objectively outweigh the costs to the user. For the helmet-mounted display, penalties in the form of added weight and physical stress are inevitable. For acceptance to be achieved, it must be clearly evident to the individual soldier that the device is highly beneficial-not just with respect to the likelihood of mission accomplishment-but also with respect to survivability. If the helmet-mounted display is perceived as providing a better chance of completing an engagement without being killed or wounded, the soldier will tolerate some added stress.
The lack of system-specific research on helmets and helmet attachments for dismounted soldiers means that the configuration of the Land Warrior System will probably be based on the judgments of the design engineers and extrapolations from adjacent research areas. The lack of direct comparability between the research on helmets and helmet-mounted displays for vehicular crew members and the conditions that are experienced by the dismounted soldier provides a strong argument for a comprehensive program of laboratory research augmented by small-scale simulated engagement tests.
The laboratory tests should be mainly directed to the helmet per se and the liner and support cushioning materials. Small-scale operational simulation means the use of a design prototype or a succession of prototypes to outfit a small unit, such as a fire team, which is then put through some exercises that are roughly comparable to an engagement situation. The purpose of tests of this kind is to identify particular problems-not to generate solutions. For example, if the helmet attachments project to the front of the soldier's face, will he adjust his
movements to avoid impacts between the device and the ground? Also, the researchers should acquire a fair notion of what features of the helmet and its attachments give rise to negative feelings or complaints on the part of the wearers. Ultimately, such-small scale field tests should result in a user-centered configuration.
DESIGN AND DEVELOPMENT GUIDELINES
There is very little theory to guide the program of research on the physical ergonomics of the helmet and its attachments. However, human factors design principles can be interpreted to fit the specific ergonomic issues posed by the proposed system:
The helmet and its attachments should be as light weight as possible while fulfilling the objective of providing ballistic protection.
The attachments, including cables and wires, should extend as short a distance as possible from the head and body of the soldier.
Any attachments to the helmet should be easily removable by the soldier in a field environment.
The helmet should be cushioned so that the contours automatically adjust to the conformation of the wearer's head.
The positioning of the helmet should be as stable as possible while allowing free head movement and either air circulation around the head, miniaturized climate control, or both.
The base of engineering design and test data for the development of a helmet-mounted (or hand-carried) display subsystem to be used by dismounted soldiers is not sufficient for the purpose. A program of system-specific laboratory research and prototype testing could significantly enhance the likelihood that the Land Warrior ensemble will meet all military criteria.
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