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International Archivesof Occupational andEnvironmental Health ISSN 0340-0131 Int Arch Occup Environ HealthDOI 10.1007/s00420-011-0661-4

Physiological work demands of Spanishwildland firefighters during wildfiresuppression

Jose A. Rodríguez-Marroyo, Jorge López-Satue, Raul Pernía, Belén Carballo,Juan García-López, Carl Foster & JoséG. Villa

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ORIGINAL ARTICLE

Physiological work demands of Spanish wildland firefightersduring wildfire suppression

Jose A. Rodrıguez-Marroyo • Jorge Lopez-Satue •

Raul Pernıa • Belen Carballo • Juan Garcıa-Lopez •

Carl Foster • Jose G. Villa

Received: 14 March 2011 / Accepted: 24 May 2011

� Springer-Verlag 2011

Abstract

Purpose The aim of this study was to analyze the phys-

iological demands and thermal strain of wildland fire-

fighters during real wildfire suppression.

Methods The response of core temperature and heart rate

(HR) were analyzed in 200 wildland firefighters during

wildfire suppression activities of different duration: \1 h

(n = 52), 1–3 h (n = 70), 3–5 h (n = 44), and [5 h

(n = 34). The exercise workload (TRIMP), the physio-

logical strain index (PSI), and cumulative heat strain index

(CHSI) were calculated using the time spent in different

intensity zones, the HR, and core temperature.

Results Mean HR was significantly higher (P \ 0.05) in

wildfires \1 h (133 ± 2 bpm) and 1–3 h (128 ± 1 bpm)

versus 3–5 h (120 ± 3 bpm) and [5 h (116 ± 32 bpm).

The time spent in higher intensity zones increased

(P \ 0.05) when wildfire duration increased. TRIMP

accumulation increased with wildfire duration (54.9 ± 3.2,

167.4 ± 5.9, 296.0 ± 8.3, 511.7 ± 12.8 in \1, 1–3, 3–5,

and [5 h, respectively). Neither core temperature

(37.4 ± 0.1�C) nor PSI (4.5 ± 0.2) were influenced by

wildfire duration. The CHSI increased (p \ 0.05) in the

following order: \1 h (104 ± 23), 1–3 h (1,396 ± 275),

3–5 h (4,586 ± 387), and [5 h (10,703 ± 710).

Conclusions The results demonstrate the high work strain

sustained by Spanish wildland firefighters during wildfire

suppression. Both workload and CHSI increased with the

wildfires duration although the pace of work was faster in

wildfires of a shorter duration.

Keywords Heart rate � Core temperature � Exercise

intensity � Workload � Thermal strain

Introduction

Wildfire firefighting is an extremely demanding occupa-

tion. The average daily energy consumption, estimated

using doubly labeled water, is 10–11 MJ day-1

(2,400–2,600 kcal day-1) (Ruby et al. 2002). Wildfire

firefighting and clean-up activities include tasks such as

hiking, building fire lines, brush removal, chainsaw work,

and setting backfires (Budd et al. 1997a; Ruby et al. 2002;

Heil 2002). All of these activities require work with hand

tools that weight between 3 and 20 kg (e.g., Pulaskis,

McLeods) including chainsaws, backpack pumps, swatters,

and shovels. These tasks are usually performed in difficult

conditions unique to wildland firefighting including inhal-

ing smoke (Wegesser et al. 2009), high environmental

temperatures (Budd et al. 1997b; Budd 2001), the weight of

the personal protective equipment (*6 kg) (Ruby et al.

2002), and working on steep terrain (Brotherhood et al.

1997a). However, other than one report on heart rate and

core temperature responses in wildland firefighters during

training exercises (Budd et al. 1997b), few reports on

activity levels using accelerometry (Heil 2002; Cuddy

et al. 2007, 2008), and a number of reports on remarkably

J. A. Rodrıguez-Marroyo (&) � B. Carballo � J. Garcıa-Lopez �J. G. Villa

Department of Physical Education and Sports,

Institute of Biomedicine (IBIOMED),

University of Leon, 24071 Leon, Spain

e-mail: j.marroyo@unileon.es

J. Lopez-Satue � R. Pernıa

Empresa de Transformacion Agraria (TRAGSA),

Madrid, Spain

C. Foster

Department of Exercise and Sport Science,

University of Wisconsin-La Crosse, La Crosse, WI, USA

123

Int Arch Occup Environ Health

DOI 10.1007/s00420-011-0661-4

Author's personal copy

normal core temperatures during wildland firefighting

(Budd et al. 1997b; Cuddy et al. 2008), there is a limit

direct physiological monitor data (i.e., HR and core tem-

perature) during real wildfires.

In order to develop preparation strategies, to optimize

performance, and to promote the health of wildland

firefighters, more information about the physical chal-

lenges experienced by wildland firefighters is needed.

Many studies have analyzed the physiological strain of

structural firefighters during fire suppression activities

using heart rate (Smith et al. 2001; Bos et al. 2004;

Eglin et al. 2004; Eglin and Tipton 2005; von Heimburg

et al. 2006; Angerer et al. 2008; Richmond et al. 2008)

and core temperature (Eglin et al. 2004; Eglin and

Tipton 2005; Angerer et al. 2008). However, compara-

tively little information about the physiological chal-

lenges associated with wildfire firefighting is available

(Budd et al. 1997b; Budd 2001; Ruby et al. 2002; Heil

2002; Cuddy et al. 2007). In addition, most of the

studies that have been carried out with wildland fire-

fighters have been conducted under simulated conditions

(Budd et al. 1997b; Budd 2001) with few studies con-

ducted during real wildfire suppression (Ruby et al.

2002; Cuddy et al. 2008). Therefore, the aim of this

study was to analyze the physiological demands and

thermal strain of wildland firefighters during real wildfire

suppression according to the duration of firefighting

activity. We hypothesized that an increase in wildfire

duration would lead to an increase in both exercise load

and thermal strain. Nevertheless, increasing wildfires

duration can be assumed to be associated with a decrease

in both work intensity and core temperature because of

the longer recovery breaks.

Methods

Subjects

One hundred and sixty wildland firefighters volunteered

and took part in this study. They were recruited from

different helitack crews located in different work areas

within Spain. Before the beginning of each season, all of

them successfully passed selection tests imposed by the

qualified authority (Ministry of Rural, Marine and Natural

Environment). The main anthropometric and physiologi-

cal characteristics of the subjects are shown in Table 1.

Written informed consent was obtained from the subjects

before starting the study. The study protocol was devel-

oped in accordance with the guidelines of the Helsinki

conference for research on human subjects and was

approved by the Ethics Committee of the University of

Leon, Spain.

Experimental design

The study was carried out over four consecutive seasons,

from 2006 to 2009. In each of the seasons (i.e., June–

October), the study was divided into two parts to quantify

the effort exerted during wildfire suppression by the sub-

jects. In the first part, all firefighters performed incremental

exercise tests in the laboratory to assess their exercise

capacity and to determine the heart rate (HR) at which their

ventilatory (VT) and respiratory compensation (RCT)

thresholds occurred. The second part of the study consisted

of individual monitoring of the HR and core temperature

response during each wildfire suppression analyzed, in

order to analyze the physiological strain of wildfire sup-

pression, based on HR and temperature data. In total, the

number of wildfire suppression trials monitored was 2,400,

representing an average of 15 fires/subject.

Laboratory testing

All subjects performed a treadmill exercise test (Bruce

1971) (PowerJog M30, Sport Engineering Ltd, Birming-

ham, UK) to assess their aerobic fitness at the onset of the

season (i.e., month of June). The test started with a speed of

1.7 mph and a slope of 10%. The treadmill speed and grade

were incremented every 3 min. Heart rate via radiotelem-

etry (Polar Team, Polar Electro Oy, Kempele, Finland),

electrocardiograph (Schiller AG, Baar, Switzerland), and

breath-by-breath gas exchange (Medical Graphics System

CPX-Plus, Medical Graphics Corporation, St. Paul, MN,

USA) were continuously analyzed throughout the exercise.

Table 1 Anthropometric characteristics and physiological values

measured during treadmill exercise testing (n = 160)

Mean ± SEM

Age (years) 25.2 ± 0.4

Body mass (kg) 75.8 ± 0.8

Height (cm) 176.5 ± 0.5

BMI (kg m-2) 24.3 ± 0.2

Sum 6 skin folds (mm) 85.6 ± 2.9

VO2max (ml kg-1 min-1) 56.2 ± 0.9

HRmax (bpm) 190 ± 1

VO2 RCT (ml kg-1 min-1) 44.7 ± 0.7

% VO2max RCT 79.8 ± 0.8

HR RCT (bpm) 170 ± 1

VO2 VT (ml kg-1 min-1) 27.5 ± 0.5

% VO2max VT 49.2 ± 0.8

HR VT (bpm) 135 ± 2

VO2max maximum oxygen consumption; BMI body mass index;

HRmax maximal heart rate; VT ventilatory threshold; RCT respiratory

compensation threshold; %VO2max percentage of VO2max at which VT

and RCT occur

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The highest VO2 obtained during the last 30 s of exercise

test was accepted as VO2max. The VT was determined from

V slope method (Beaver et al. 1986) in combination with

the break point of the ventilatory equivalent for O2 against

VO2. The RCT was identified by the break-points of the

ventilatory equivalent for CO2 and the end tidal CO2

concentration against VO2. Subsequently, three intensity

zones were identified according to the HR values

corresponding to the VT and RCT (Lucıa et al. 2003;

Rodrıguez-Marroyo et al. 2009): zone 1 (Z1), below VT

(low intensity exercise); zone 2 (Z2), between VT and RCT

(moderate intensity exercise); and zone 3 (Z3), above the

RCT (high intensity exercise). These zones were used to

determine the exercise load (TRIMP) as a method for

integrating work duration and intensity. Although this

approach has been used to calculate the exercise load in

athletes (Foster et al. 2001; Lucıa et al. 2003; Billat et al.

2003; Rodrıguez-Marroyo et al. 2009), a recent study has

extended this approach to occupational work physiology

(Takken et al. 2009). We calculated the TRIMP by multi-

plying the time spent in Z1, Z2, and Z3 by the constants 1,

2, and 3, respectively, the TRIMP score being obtained by

summating the results of the 3 phases (Foster et al. 2001).

Wildfire suppression

Two hundred wildfires were analyzed, for a total of 2,400

person-fires of observation. Wildfires were classified into

four categories according to their duration:\1 h (n = 52),

between 1 and 3 h (n = 70), between 3 and 5 h (n = 44),

and [5 h (n = 34). During wildfire suppression, all wild-

land firefighters wear the same regulation protective gear

(*6 kg) that includes a fireproof overall (65% viscose,

30% nomex, and 5% kevlar), helmet, goggles, gloves, face

neck shroud, mid-calf leather boots, 100% cotton short

sleeve undershirt and underpants, and backpack (*3 kg,

principally with water and food). The participating fire-

fighters wore a HR monitor (Polar Team, Polar Electro Oy,

Kempele, Finland) with their HR recorded every 5 s and a

temperature data logger (Termoregister TR-51A, T and

Do., Nagano, Japan) to measure the air temperature every

10 s during the analyzed wildfires. However, relative

humidity was not measured in this study. Core body tem-

perature was measured and recorded continuously using a

JonahTM intestinal temperature capsule (VitalSenses, Mini

Mitter Co., Inc, Bend, OR, USA) that has been validated

previously (McKenzie and Osgood 2004). We only recor-

ded the core body temperature when the firefighter had

ingested the capsule at least 3 h before the beginning of

wildfire suppression activities (Laursen et al. 2006). This

was done because if the sensor is swallowed near the work

period, it may remain in the stomach and be influenced by

the temperature of ingested liquid and food (Byrne and Lim

2007). The physiological strain index (PSI) and cumulative

heat strain index (CHSI) were calculated according to

Tikuisis et al. (2002), using a modified version of the

equation of Moran et al. (1998), and Frank et al. (2001),

respectively:

PSI unitsð Þ ¼ 5 � CT� CT0ð Þ � 39:5� CT0ð Þ�1h i

þ 5 � HR� 60ð Þ � HRmax � 60ð Þ�1h i

where CT and HR are the mean core body temperature (�C)

and mean heart rate (bpm) recorded during the wildfire

suppression activity, respectively. CT0 is the baseline core

body temperature, and HRmax is the subject’s maximal

heart rate measured in the laboratory during incremental

exercise.

The CHSI unitsð Þ ¼Xt

0

hb� HR0 � t

" #

� 10�3�Z t

0

CT � dt� CT0 � t

24

35

where Rhb is the accumulation of heart beats over the

period t (min) during which the index is calculated, HR0 is

the initial lowest heart rate (bpm), CT is the core body

temperature (�C), and CT0 is the baseline CT. Although

rectal temperature is typically used to calculate PSI and

CHI (Moran et al. 1998; Frank et al. 2001; Tikuisis et al.

2002; Hadid et al. 2008; Petruzzello et al. 2009), it is not

easy to use in the field (Byrne and Lim 2007). Therefore,

different authors have used an ingestible telemetric tem-

perature sensor to calculate these variables (Byrne et al.

2006; Hostler et al. 2009). It has been shown that gastro-

intestinal temperature provides a valid measurement of CT

compared with rectal temperature (McKenzie and Osgood

2004; Byrne and Lim 2007).

Statistical analyses

The results are expressed as mean ± SEM. The

Kolmogorov–Smirnov test was applied to ensure a

Gaussian distribution of the results. The analyzed param-

eters were compared using ANOVA test with repeated

measures. When a significant F value was found, Bonfer-

roni’s test was applied to establish significant differences

between means. Values for P \ 0.05 were considered

statistically significant. The relationship between variables

was determined by using the Pearson correlation coeffi-

cient (r).

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Results

Heart rate

Maximal and mean HR observed during wildland fire-

fighting were 173 ± 1 and 125 ± 1 bpm, respectively.

Mean HR, expressed as a percentage of maximal HR was

66.4 ± 0.6% HRmax. Significant differences (P \ 0.05) in

mean HR between shorter and longer wildfires (\3 and

[3 h) were observed (Table 2). This trend toward a

decreasing relative intensity with longer duration efforts

was observed when we compared the mean % HRmax

during wildfires \1 h (70.8 ± 0.8%), between 1 and 3 h

(67.0 ± 0.4%), between 3 and 5 h (63.3 ± 1.3%), and

[5 h (61.6 ± 0.9%).

Intensity zones

The percentage time spent in Z1, Z2, and Z3 for all wild-

fires together derived from laboratory testing was

66.5 ± 1.2, 26.4 ± 0.9, and 6.4 ± 0.5%, respectively.

When wildfire duration increased, the percentage time

spent in Z1 increased and the percentage time spent in Z2

and Z3 decreased. The lowest (P \ 0.05) percentages of

time in Z1 were found in wildfires \3 h. However, these

wildfires presented the highest (P \ 0.05) percentages in

Z2 and Z3 (Fig. 1a). When we analyzed the total time spent

in the different zones, we found a significant increase of the

time spent in Z1 and Z2 when the wildfire duration

increased (Fig. 1b). Surprisingly, the lowest (P \ 0.05)

time spent in Z3 was observed in wildfires \1 h, poten-

tially because these wildfires are intrinsically easier to

fight.

Workload

The TRIMP score significantly increased (P \ 0.05) with

wildfire duration (Table 2). However, when we calculated

the TRIMP h-1, this behavior was opposite (Table 2). We

found significant differences in TRIMP h-1 (P \ 0.05)

between the wildfires\3 versus[3 h, with the higher rate

of TRIMP accumulation reflecting the higher intensity

during relatively shorter fires.

Core temperature

Neither air (27.7 ± 0.7�C) nor core (37.4 ± 0.1�C) tem-

perature was influenced by the wildfires duration (Table 3).

In the same way, the PSI (an index of combined

Table 2 Total duration, heart rate (HR), and exercise load (TRIMP) analyzed according to the wildfire duration (mean ± SEM)

Total wildfire

duration (min)

Maximal

HR (bpm)

Mean

HR (bpm)

TRIMP TRIMP h-1

Wildfires below 1 h 36.3 ± 1.7*�� 172 ± 1 133 ± 2�� 54.9 ± 3.2*�� 90.4 ± 2.3��

Wildfires 1–3 h 118.6 ± 4.0�� 173 ± 1 128 ± 1�� 167.4 ± 5.9�� 85.9 ± 1.2��

Wildfires 3–5 h 227.7 ± 4.2� 173 ± 1 120 ± 3 296.0 ± 8.3� 77.5 ± 1.6

Wildfires above 5 h 420.3 ± 14.3 174 ± 2 116 ± 2 511.7 ± 12.8 73.9 ± 1.6

* Significant differences with wildfire between 1 and 3 h (P \ 0.05)� Significant difference with wildfire between 3 and 5 h (P \ 0.05)� Significant difference with wildfire [5 h (P \ 0.05)

0

10

20

30

40

50

60

70

80

90

Per

cent

age

of to

tal t

ime

(%)

below 1 h between 1-3 h between 3-5 h above 5 h

†‡†‡

†‡†‡

†‡ †‡

(a)

0

50

100

150

200

250

300

350

Zone 1 Zone 2 Zone 3

Zone 1 Zone 2 Zone 3

Tot

al ti

me

(min

)

*†‡

†‡

‡

*†‡†‡

‡

*†‡

(b)

Fig. 1 Percentage (a) and total time (b) spent in the three HR

intensity zones defined during laboratory treadmill testing. Values are

mean ± SEM. Zone 1 exercise intensity below ventilatory threshold

(VT); Zone 2 exercise intensity between VT and the respiratory

compensation threshold (RCT); Zone 3 exercise intensity above RCT.

*Significant difference with wildfire between 1 and 3 h (P \ 0.05);�significant difference with wildfire between 3 and 5 h (P \ 0.05);�significant difference with wildfire [5 h (P \ 0.05)

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cardiovascular and thermal stress) was similar during all

wildfires (4.5 ± 0.2). However, the CHSI, a potentially

more sensitive index of combined cardiovascular and

thermal stress, significantly increased (P \ 0.05) with the

wildfire duration (Table 3). We found significant relation-

ship between the CHSI and the TRIMP (r = 0.88,

P \ 0.001).

Discussion

The main finding of this study was that the HR (Table 2)

and core temperatures (Table 3) of wildland firefighters

during real wildfires did not reflect high levels of cardio-

vascular and thermal strain. Critical core temperatures

were not found during any of the fires (38.5 ± 0.2 and

37.4 ± 0.1�C were maximal and mean core temperatures,

respectively), possibly due to the low thermal stress

endured by wildland firefighters (36.5 ± 0.8 and 27.7 ±

0.7�C were maximal and mean air temperature). Similarly,

the HRs obtained were similar to those recommended by

the ACSM (1998) to improve the cardiorespiratory fitness

of subjects who are unfit (55–64% HRmax). The mean and

maximal HR found did not exceed 150 and 180 bpm,

respectively, the limit established for workloads considered

acceptable in the workplace (Wu and Wang 2001). How-

ever, the use of mean HR to determine the exercise

intensity of activities requiring variable levels of effort, as

is the case for wildfire firefighting, does not provide a

faithful reflection of the effort exerted by the subjects.

Therefore, other methods have been proposed that establish

different intensity zones according to the HR recorded in

incremental tests (Foster et al. 2001; Lucıa et al. 2003;

Rodrıguez-Marroyo et al. 2009).

The exercise intensities found in this study (62–71%

HRmax) were lower than those observed in rescue simula-

tions (*85–90% HRmax) (Eglin et al. 2004; von Heimburg

et al. 2006; Richmond et al. 2008). However, the tasks

involved in structural firefighting may involve both greater

exercise intensities than those carried out by wildland

firefighters and a greater heat load and the weight of per-

sonal protective gear. Structural firefighters endure mean

environmental temperatures of between 46 and 200�C

(Smith et al. 2001; Angerer et al. 2008), while the tem-

perature range analyzed in this study was 27.0 ± 0.6�C.

Similarly, the equipment carried by structural firefighters is

heavier (*23 kg) than that used by wildland firefighters

(*6 kg), provoking an additional effort and an increase in

cardiovascular strain (Richmond et al. 2008). The HRs

obtained in this study were also lower than those found by

other authors for wildland firefighters (Budd et al. 1997b).

Budd et al. (1997b) reported a mean HR of *157 and

*149 bpm during the construction of firelines with

experimental fires and without fires, respectively. Possibly,

the methodological differences between studies influenced

the results obtained. For example, Budd et al. (1997b) only

analyzed the work phases, whereas in the present study,

HR was analyzed throughout the fire, including both work

and recovery phases. Moreover, it is important to note that

differences in wildland firefighter characteristics (e.g., age

and physical fitness level) can influence the HR response.

Brotherhood et al. (1997b) observed lower HR’s in fitter

wildland firefighters than the less fit while they performed a

specific task.

The mean HR and the percentage of total time spent in

Z2 and Z3 decreased as the duration of fires rose (Table 2

and Fig. 1). The increased work that wildland firefighters

must accomplish in fires of longer duration could imply

greater muscle fatigue, limiting the ability to work at

greater intensities. Previous studies carried out for cyclists

have reported a reduction in exercise intensity (i.e., mean

HR and percentage of total time spent in Z3) as competi-

tion duration increased (Rodrıguez-Marroyo et al. 2009),

including reductions in Z2 and Z3 time per stage in the

relatively longer Tour de France versus the shorter, but

more intense, Vuelta a Espana (Lucıa et al. 2003).

Table 3 Air and core temperature (�C), physiological strain index (PSI), and cumulative heat strain index (CHSI) analyzed according to the

wildfire duration (mean ± SEM)

Maximal air

temperature

Mean air

temperature

Maximal core

temperature

Mean core

temperature

PSI CHSI

Wildfires below

1 h

34.1 ± 2.3� 28.1 ± 1.6 37.8 ± 0.3 37.6 ± 0.2 4.5 ± 0.2 104 ± 23*��

Wildfires 1–3 h 33.5 ± 1.2� 26.7 ± 1.1 38.3 ± 0.1 38.0 ± 0.1 4.5 ± 0.3 1,396 ± 275��

Wildfires 3–5 h 36.4 ± 1.7 27.1 ± 0.9 38.4 ± 0.2 38.0 ± 0.2 4.7 ± 0.3 4,586 ± 387�

Wildfires above

5 h

41.9 ± 2.9 27.6 ± 1.2 38.4 ± 0.1 37.8 ± 0.3 3.9 ± 0.5 10,703 ± 710

* Significant difference with wildfire between 1 and 3 h (P \ 0.05)� Significant difference with wildfire between 3 and 5 h (P \ 0.05)� Significant difference with wildfire [5 h (P \ 0.05)

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However, it is more plausible that this behavior is due to

the suppression strategies used. Generally, fires of longer

duration are characterized by more intense flames, obliging

the wildland firefighters to work at a larger relative distance

from the flames (indirect attack). This creates lower

physical demands and implies less thermal, mental, and

physical stress than a direct attack (Budd et al. 1997a).

Furthermore, during the initial phases of a fire, the wildland

firefighters work with greater intensity in order to gain

control the fire, acting directly against the flames or con-

structing firelines using different hand tools (e.g., Pulaskis,

McLeods, swatters, backpack pumps), all of which require

higher workrates and an increase in the HR (Brotherhood

et al. 1997a). After the fire has been controlled, the use of

hand tools decreases and the tasks carried out are lighter

(e.g., mop-up) (Budd et al. 1997b). This latter type of task

represents *55, 60, 75, and 80% of the total time in

wildfires of \1, 1–3, 3–5, and [5 h duration, respectively

(Fig. 1a).

When we analyzed the times spent in the different

intensity zones, it was observed that time spent in Z1, Z2,

and Z3 (Fig. 1b) rose as the duration of the wildfire

increased. The greater intrinsic severity of wildfires of

longer duration (i.e., those with the highest maximal air

temperatures) dictated that the wildland firefighters took

longer to control them and were obliged to carry out a

greater quantity of demanding activities. This increased

volume of work at high intensities also resulted in an

increase in TRIMP as wildfire duration rose (Table 2).

Although, to compare TRIMP scores in wildland fire-

fighters with those obtained in athletes can be inaccurate,

due to the wildfires characteristics, this may still help to

understand the work demands of these subjects. The mean

TRIMP recorded in this study (*260) was similar to that

reported for marathon events (*300) and slightly below

those observed in professional cyclists in different races

(*330) (Lucıa et al. 2003; Rodrıguez-Marroyo et al. 2009)

and in police officers during mountain bike patrols (*360)

(Takken et al. 2009). The TRIMP analyzed in wildfires of

[5 h duration (*510) were similar to those found for

professional cyclists in the hardest stages of the Tour of

France and the Tour of Spain (*500) (Lucıa et al. 2003;

Rodrıguez-Marroyo et al. 2009). Further, this result rep-

resented over half the weekly training load of Kenyan elite

runners (*800 TRIMP wk-1) (Billat et al. 2003). These

data reflect the high demands placed on wildland fire-

fighters when suppressing wildfires, although the demand is

based more on duration than on intensity.

It is possible that the workload intensities analyzed in

this study have been overestimated due to the effects of

dehydration or hot environments on HR (Achten and

Jeunkendrup 2003) and to the upper body requirements

(Bos et al. 2004; Eglin and Tipton 2005). However, some

authors have indicated that in these circumstances HR

still represents the best indicator of cardiovascular strain

(Angerer et al. 2008; Richmond et al. 2008). The reduction

in heat dissipation combined with excessive sweating due

to the protective gear of wildland firefighters and the

radiant heat of the fire potentially create high levels of

thermal stress and could cause an increase in cardiovas-

cular strain (Smith et al. 2001; Angerer et al. 2008).

However, neither the environmental nor the core temper-

atures observed in this study indicate that the subjects

experienced high levels of thermal strain (Table 3). Con-

sequently, the HR behavior was apparently primarily due to

the work carried out by the subjects. The low environ-

mental temperatures (27.0 ± 0.6�C) and the work:rest ratio

(1:2) observed during suppression of wildfires helped to

prevent an increase in thermal strain. The core tempera-

tures found in this study were consistent with those

reported in other studies of wildland firefighters

(*37–38�C) (Budd et al. 1997b; Budd 2001; Cuddy et al.

2007) and are lower than those observed in structural

firefighters (*38.5–39�C) (Eglin et al. 2004; Richmond

et al. 2008).

Moran et al. (1998) proposed the use of PSI, based on

the HR and core temperature of the subjects, as a tool for

determining thermal strain. These authors established a

scale of 0–10 for measuring this index, where 10 was

associated with a high PSI. In all the wildfires analyzed, the

subjects of this study obtained a score close to a moderate

PSI (Table 3). Similar results were obtained by Hadid et al.

(2008) when they simulated a military march of 155 min in

a hot environment (*4.7 and 5.5 PSI units at 35 and 40�C,

respectively), and by Frank et al. (2001) and Hostler et al.

(2009) when different insulation gear (flame-retardant and

chemical-resistant coveralls) was assessed in the laboratory

under different conditions (*4–5 PSI units). Higher

values were found by Petruzzello et al. (2009) in simula-

tions of structural fires (*6 PSI units). However, the use

of CHSI instead of PSI has also been proposed to assess

thermal strain since it is more comprehensive (Frank et al.

2001). Our results show that a significant increase

(P \ 0.05) in CHSI occurred when the duration of fires

increased (Table 3). This behavior was similar to that

observed for the TRIMP, and a high correlation was found

between both variables (r = 0.88, P \ 0.001). The values

obtained for wildland firefighters in the fires of [3 h

duration were higher than those found by other authors for

half-marathon races (*2,800 units) (Byrne et al. 2006).

In conclusion, this study is the first to combine HR and

core temperature measures to classify work demands dur-

ing real wildfire suppression. Despite the fact that the core

temperatures obtained did not exceed the values considered

critical (*40�C) (Montain et al. 1994), wildfire duration

together with the exercise carried out resulted in a high

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CHSI. The latter index appears to be more sensitive than

PSI for assessing thermal strain under conditions of varying

duration. The results obtained demonstrate the high inten-

sities and workload sustained by wildland firefighters in the

course of their work. Although the highest workloads were

encountered in wildfires of longer duration, the pace of

work was faster in wildfires of a shorter duration (Table 2).

These data provide direction for the development of spe-

cific approaches to physical training. Thus, training ses-

sions that simulate workload of *260 TRIMP could be

more specific to replicate wildfire suppression demands.

Using pace of work of *80 TRIMP h-1 or an exercise

intensity between 60 and 70% HRmax can also help to

simulate working conditions.

Acknowledgments This work has been funded by Empresa de

Transformacion Agraria, SA. (TRAGSA), Mutua Fraternidad–Mupr-

espa and Empresa de Gestion Medioambiental, SA. (EGMASA). This

study was conducted with the collaboration of the Area de Defensa

contra Incendios Forestales, Direccion General de Medio Natural y

Polıtica Forestal, Ministerio de Medio Ambiente y Medio Rural y

Marino, Spain.

Conflict of interest The authors declare that they have no conflict

of interest.

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