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A methodology for measuring the physiological strain of enhanced soldiers : the 1998 Soldier Combat System Enhancement Study

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Abstract

The prime objective of the 1998 Soldier Combat System Enhancement Study was to assess, develop and verify methods to evaluate the physiological performance of dismounted soldiers with basic or enhanced capabilities conducting routine operations in the tropics. Core temperature, mean skin temperature and heart rate are appropriate measures for evaluating the physiological burden of soldier combat system enhancements. Current techniques for measuring mean skin temperature and heart rates are adequate. The measurement of core temperature using rectal thermistors has significant limitations, especially during vigorous activities. Studies of the hydration status of soldiers can be conducted using relatively straightforward methods to determine water intake, weight loss, urine production, and total sweat rate by weight differences. For field studies of hydration, there may be no need to analyse urine for sodium; specific gravity is more easily measured and appears to provide adequate information on hydration status. The robustness of the Metamax used for VO2 measurements was demonstrated and provided real time measurements of oxygen consumption, and of metabolic stress associated with activities.

Executive Summary

The 1998 Soldier Combat System Enhancement Study (Phase 2 of Land125) focussed on developing analytical methods to assess the principal aspects of human factors and technology enhancements for future soldiers. The prime objective of this study was to assess, develop and verify methods to measure the physiological performance of enhanced soldiers conducting routine infantry operation in the tropics. The study was conducted at the High Range Training Area (HRTA) and Land Command Battle School (LCBS) at Tully in northern Queensland. Three clothing systems were evaluated for their impact on physiological strain: the standard Disruptive Pattern Combat Uniform (DPCU), the prototype Chemical Biological Combat Suit (CBCS) and Combat Body Armour (CBA). During the fighting patrol, observation post and assault operations, various indices of thermal strain were measured: body core temperature, skin temperature, heart rate, work rate and hydration status. The performance of enhanced soldiers in CBA and in CBCS was compared with that in DPCU. Core temperature, mean skin temperature and heart rate were determined to be appropriate measures for evaluating the physiological burden of soldier combat system enhancements. Current techniques for measuring skin temperature and heart rate are adequate; measurement of core temperature using rectal thermistors has significant limitations, especially during vigorous activities such as assaults. Studies of the hydration status of soldiers can be conducted using relatively straightforward methods. Water intake, weight loss, urine production and total sweat rate can be determined simply by weight differences. In field studies of hydration there appears to be no need to analyse urine for sodium; specific gravity is easily measured and provides adequate information on hydration status. Wearing CBA in hot/dry climates may increase total sweat rate and therefore water requirements but may not add significantly to the heat strain of soldiers. Wearing CBA during operations in hot/wet climates may increase water requirements and heat strain in soldiers. Wearing CBCS may not add to the thermal strain of soldiers operating in hot/dry climates; the effects of wearing CBCS in hot/wet climates need to be investigated. Several recommendations for future trials arise from the study. Because of the nature of many physiological indices of strain, it is imperative that experimental conditions are replicated as fully as possible between treatments. This requires more use of laboratory trials to evaluate soldier combat system enhancements. Balanced order, within-subjects experimental designs are necessary to minimise the impact of nonquantifiable, confounding factors (carry-over effects and day-to-day differences in levels of environmental or metabolic stress). It is of paramount importance that scientists and commanders agree on the scientific protocol to be followed in future studies of Land125. This protocol should not be varied without the agreement of both parties. In particular, there should be prior agreement on and commitment to, key experimental parameters (time of day, duration, route and speed of patrol) by both organisations. There must be discussion between DSTO and the Section commander during the course of the experiment and the role of a Subject Matter Expert should be expanded to maximise standardisation of the task. A DSTO observer should be included within trials to minimise data loss and document experimental details. For improved measures of thermal strain, the development of alternative indices for core temperatures (gastrointestinal radio pill, insulated skin temperature) should be hastened. The DSTO observer can be used to conduct regular thermometry checks, especially following physically vigorous events. For heart rate measurements, hardwired output or individualised frequency in the telemetry should be investigated. The effects of soldier enhancement on indices of physiological strain can be measured by using time averaging across the entire operation, or by using change scores between selected events, eg. onset and completion of task. Time averaging can be suitable for variables that change rapidly or are sensitive to factors that are not constant and outside experimental control. Absolute values of parameters are often not suitable for evaluating differences in strain imposed by different treatments but are essential for revealing actual levels of the physiological burden experienced by soldiers and should be used in conjunction with time averaging or change scores.

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