Predicting physiological capacity of human load carriage – A review

Highlights

• Physiological limits in occupational settings have previously been investigated.

• Existing workload limit guidelines are likely not appropriate for load carriage.

• A model to predict occupational load carriage performance has been proposed.

• Able to provide rapid assessments to inform task and personnel management.

• The predictive tool would be invaluable for planning in military and firefighting.

Abstract

This review article aims to evaluate a proposed maximum acceptable work duration model for load carriage tasks. It is contended that this concept has particular relevance to physically demanding occupations such as military and firefighting. Personnel in these occupations are often required to perform very physically demanding tasks, over varying time periods, often involving load carriage. Previous research has investigated concepts related to physiological workload limits in occupational settings (e.g. industrial). Evidence suggests however, that existing (unloaded) workload guidelines are not appropriate for load carriage tasks. The utility of this model warrants further work to enable prediction of load carriage durations across a range of functional workloads for physically demanding occupations. If the maximum duration for which personnel can physiologically sustain a load carriage task could be accurately predicted, commanders and supervisors could better plan for and manage tasks to ensure operational imperatives were met whilst minimising health risks for their workers.

Introduction

Load carriage is a core requirement of personnel serving in various physically demanding occupational settings, particularly military and firefighting (Louhevaara et al., 1986, Ruby et al., 2002, Knapik et al., 2004). In conducting military operations personnel are often required to conduct dismounted patrols and approach marches carrying heavy loads (e.g. >30 kg) over long distances (e.g. >4 km) and for prolonged durations (e.g. >4 h) to successfully execute mission requirements (Dean, 2004, Knapik et al., 2004, Henning et al., 2011). The composition of load for military personnel varies across missions but often includes protective ensemble (e.g. body armour, helmet), combat specific equipment (e.g. weapon systems, ammunition, power sources, radio) and sustainment stores (e.g. food and water) (Johnson et al., 1995, Orr, 2010). Both structural and wildfire fighters are also required to carry heavy loads for sustained periods (Eglin and Tipton, 2005, Cuddy et al., 2007). Wildfire fighters often hike to fire fronts inaccessible by vehicle whilst carrying specialist equipment (e.g. axes, rakes) and sustainment stores (e.g. food, water) with total loads in excess of 20 kg (Rodríguez-Marroyo et al., 2012). During structural fire suppression, personal protective clothing and self-contained breathing apparatus weighing approximately 15–20 kg (Louhevaara et al., 1986, Taylor et al., 2011) is often worn in conjunction with the carriage of specialist equipment (e.g. method of entry tools, fire suppression tools), resulting in total external loads approaching 30 kg (von Heimburg et al., 2006).

Across occupations, there are differences in job tasks and the composition and body region upon which load is borne (e.g. back, hands, head). However there is a universal need for personnel serving in many physically demanding occupations to be capable of moving their body mass plus an external load. In order to plan for and manage tasks, the maximum amount of time that personnel can sustain a given work rate without fatigue, or maximal acceptable work duration, is required. Wu and Wang (2001) first described ‘maximum acceptable work duration’ in examining workload limits for physically demanding jobs. The term refers to workloads that can be sustained by an individual in a physiologically steady state and which will not cause fatigue or discomfort. The maximum acceptable work duration model relates relative task intensity (i.e. % $\dot{\mathrm{V}}$ O_2max) and work duration (Wu and Wang, 2001, Wu and Wang, 2002). Other researchers have investigated allied concepts related to physical workload tolerance in occupational settings (e.g. industrial, manual handling) (Saha et al., 1979, Jorgensen, 1985, Ilmarinen and Tuomi, 1992). The recommendations from this body of work have fundamentally focused on the physical workloads that can be sustained in a safe manner hence the reference to ‘acceptable’ workloads. The current authors suggest extending this work by developing a maximum acceptable workload duration model for load carriage could be an operationally relevant tool for physically demanding occupations such as military and firefighting. Personnel in these occupations are often required to perform very physically demanding tasks following prolonged tasks with little or no recovery between tasks. In the military context this may represent engaging the enemy following a long approach march and for firefighters a high-rise building casualty evacuation following prolonged fire suppression. Therefore, understanding the duration for which personnel can sustain tasks ‘safely’ or without undue fatigue could serve as critical information for operation/mission planners and those commanding personnel on the ground.

Existing guidelines for workload duration predictions are based upon cycle ergometry (Wu and Wang, 2001, Wu and Wang, 2002), unloaded walking/running data (Saha et al., 1979) or modelling (Bink, 1962). The correlation between these laboratory-derived and modelled guidelines and actual tolerance time in occupational settings is unclear but it is suggested these guidelines may overestimate work duration (Jorgensen, 1985). The inappropriate management of firefighting and military personnel may risk the health and well-being of the personnel themselves, their colleagues and those they are endeavouring to protect. Working longer shifts/undertaking longer missions than personnel are capable of, reduced rest breaks and increased work rates could all reduce physical performance and/or degrade decision making ability. Load carriage related fatigue is attributed with changing tactics during the first World War (Lothian, 1921) and recent evidence suggests that load carriage related fatigue is still an issue in modern warfare (Dean, 2004). Whilst there are limited studies examining the effect of load carriage on physiological capacity ($\dot{\mathrm{V}}$ O_2max) and/or tolerable workload duration there is evidence to suggest that both are reduced (Raven et al., 1977, Louhevaara et al., 1986, DeMaio et al., 2009). The results from this load carriage work however, do not allow for the establishment of evidence-based maximum acceptable work duration guidelines for load carriage tasks in the field. The U.S. Army provides guidance on sustainability for load carriage tasks based upon rate of energy expenditure (Field Manual 21-18) (Department of the Army, 1990). However the empirical basis (i.e., experimental or predicted data) for this load carriage performance prediction tool is not well defined and the validity is yet to be tested.

Several studies with an interest in physically demanding occupations have shown correlations between load carriage performance and $\dot{\mathrm{V}}$ O_2max, muscular strength, body fat and body weight (Rayson et al., 2000, Bilzon et al., 2001a, Williams and Rayson, 2006, Ricciardi et al., 2007). Whilst these findings are informative, they do not provide clear guidance pertaining to the ability of personnel to undertake specific load carriage tasks or the duration that they can sustain their performance in the field. There is considerable research investigating the energy cost of load carriage (Louhevaara et al., 1986, Bilzon et al., 2001a, Scott and Christie, 2004, Crowder et al., 2007), however it is not always possible to apply this previous work to current (and future) occupational load carriage requirements given differences in conditions (e.g. footwear, load distribution, terrain surface and worker populations). Practitioners can also estimate the energy cost of load carriage tasks, using metabolic equivalent tables (Ainsworth et al., 2000), heart rate derived energy expenditures (Rissanen et al., 2007) and predictive models (Pandolf et al., 1977, Epstein et al., 1987, Santee et al., 2001). These tools vary in both their ease of use and their accuracy. As per load carriage performance correlates, energy cost alone does not provide clear guidance to mission planners and commanders as to the likely duration that personnel can sustain operational tasks involving load carriage. If maximum acceptable work duration is under-estimated then operations may be unnecessarily constrained. Conversely, if maximum acceptable work duration is over-estimated, then commanders and supervisors may be unwittingly exposing personnel to excessive work demands, potentially leading to adverse health outcomes and/or compromise the operation.

This review article will explore the establishment of an evidence-based maximum acceptable work duration model for physically demanding occupations involving load carriage. In doing so, this review will assess the strengths and weaknesses of the concept and identify scientific gaps that would improve the utility of the proposed model.