Metabolic demands of a simulated smoke-diving drill

Abstract

The main goal of this study was to update the Finnish smoke-diving drill (FSDD) and to measure the physical strain of and recovery from the drill. Furthermore, the aim was to compare the physical strain of contract and professional firefighters and effect of floor materials. The associations between aerobic capacity and physical strain were also studied. The updates made included an added hose pull task and updating the equipment used. Heart rate (HR), oxygen consumption (V̇O₂), and blood lactate concentration ([La-]) of 32 professional and 5 contract firefighters were measured before, during, and 10 and 30 min after the updated drill. The mean HR during the drill was 78% and V̇O₂ 59% of maximum. HR and [La-] had not recovered to baseline levels after 30-minute recovery period. Physical strain was higher among contract firefighters and [La-] accumulation on rough floor surfaces. Better aerobic capacity was associated with reduced physical strain.

Practitioner summary:

The purpose of this study was to update the Finnish smoke-diving drill. This paper describes the process of updating the drill, and the experimental measurements regarding the metabolic demands of the updated drill. The updates made included adding a hose pull task and updating the equipment used during the drill.

Keywords:

• Firefighting
• oxygen consumption
• heart rate
• recovery
• aerobic capacity

Introduction

Firefighting and rescue work is physically demanding and requires adequate aerobic and muscular capacity to safely operate at emergency sites. Therefore, in many countries, operative firefighters routinely undergo physical employment tests (PETs) to ensure that their physical capacity is sufficient for the demands of their work. Physical employment standards (PESs) are based on analysis of the work, identifying the critical physically demanding tasks and determining their physical workload. The purpose of establishing PESs is to ensure that employees can perform their work tasks without imposing undue stress on themselves or others (Tipton, Milligan, and Reilly 2013). Individual PETs are selected on the basis of established PESs. The measures of a good PET are reliability (i.e. the test produces consistent results on different occasions), validity (i.e. the test measures what it is meant to be measured), feasibility, and safety (Payne and Harvey 2010).

In Finland, a smoke-diving drill is used as a task simulation exercise in addition to mandatory aerobic and muscular PETs. The goal of the drill is to complement the assessment of work ability in job-related tasks. The Finnish smoke-diving drill (FSDD) was developed in the 90s by Louhevaara et al. (1994) and the purpose of the simulation drill is to evaluate the physical work capacity of firefighters in tasks that simulate actual smoke-diving. These tasks include walking without and with two rolls of hose, stair climbing, hammering a truck tire, going under and over obstacles, and hose rolling. Not completing the drill within the fixed time limit of 14.5 minutes or a mean heart rate (HR) above 90% of the maximal HR (HR_max) are considered to be indicators of low physical work capacity (Louhevaara et al. 1994).

Similar simulations of smoke-diving-related tasks are also widely used in other countries to test the physical abilities of incumbent firefighters and firefighter applicants. These include the firefighting simulation test in the UK (Siddall et al. 2018; Stevenson et al. 2019) and the simulated firefighter work circuit in Canada (Dreger and Petersen 2007). Simulations have consisted of carrying equipment, dragging a hose, climbing stairs or ladders, hammering or striking an object, and evacuating a rescue mannequin (e.g. Dreger and Petersen 2007; Mamen et al. 2021; Plat, Frings-Dresen, and Sluiter 2010; Siddall et al. 2018; Williams-Bell et al. 2009).

Passing a simulation test requires a maximal oxygen consumption (V̇O_2max) of 34–45 ml/kg/min depending on the simulation (e.g. Dreger and Petersen 2007; Mamen et al. 2021; von Heimburg et al. 2013). Short execution time in firefighting and rescue simulations is associated with better aerobic fitness (Blacker et al. 2016; Elsner and Kolkhorst 2008; Lindberg et al. 2013; Siddall et al. 2018; Stevenson et al. 2019; von Heimburg, Rasmussen, and Medbø 2006, von Heimburg et al. 2013; Williams-Bell et al. 2009), and absolute V̇O_2max (l/min) has shown to be a better determinant of performance in firefighting and rescue simulations than body weight-relative V̇O_2max (ml/kg/min) (Blacker et al. 2016; Lindberg et al. 2013; von Heimburg, Rasmussen, and Medbø 2006, von Heimburg et al. 2013; Williams-Bell et al. 2009). Indeed, higher V̇O_2max has also been significantly associated with lower cardiac strain in FSDD (Louhevaara et al. 1994). In addition to aerobic fitness, greater muscular strength and endurance (Lindberg, Oksa, and Malm 2014; Michaelides et al. 2008, 2011; Nazari et al. 2018; Norris et al. 2021; Rhea, Alvar, and Gray 2004; Williford et al. 1999), as well as anthropometric characteristics, mainly greater height and muscle mass (Skinner et al. 2020; von Heimburg, Rasmussen, and Medbø 2006; Williford et al. 1999) and less body fat (Michaelides et al. 2008, 2011; Norris et al. 2021; Siddall et al. 2018; Skinner et al. 2020; Williford et al. 1999), are associated with better performance in firefighting and rescue simulations. Although physical capacity is associated with simulation test performance, these tests may not alone be sufficiently accurate to determine whether a firefighter has adequate physical fitness for firefighting duties (Stevenson et al. 2019).

FSDD is widely used across the country to evaluate physical work capacity in job-related tasks. However, since the drill was established over 30 years ago, the equipment and working methods have evolved. For example, lighter composite air cylinders are used in place of steel air cylinders, and hoses are updated to smaller and lighter ones. The smoke-diving gear worn during the drill has also developed over these three decades, and it affects the physical strain and heat stress experienced during the drill. Therefore, the drill and equipment used during the drill needed to be reassessed to match present-day occupational demands and to update tasks, if necessary. The purpose of this study was to re-evaluate and update FSDD tasks and equipment. Furthermore, the aim was to describe the metabolic demands of and recovery from the updated drill, to compare the physical strain of contract and professional firefighters, and to compare floor materials. Associations between aerobic capacity and physical strain during the FSDD were also examined.

Materials and methods

Participants

A total of 37 Finnish firefighters (34 males and 3 females) participated in this study. The volunteer participants were recruited through a contact person from three selected fire departments across Finland. Table 1 describes the study participants of whom 32 were professional and 5 were contract firefighters. Measurements were taken at three different fire stations in Finland. Inclusion criteria were eligibility for smoke-diving and no health conditions that could affect drill performance (e.g. infectious diseases or musculoskeletal problems). In Finland, all firefighters eligible for smoke-diving are required to undergo regular health screening. Participants of all ages and fitness levels were invited to participate in this study to obtain a wide perspective of the metabolic demands of the updated FSDD.

Table 1. Characteristics of all professional and contract firefighters. An activity rating of 7 corresponds to over 3 hours of strenuous exercise per week.

	All firefighters (n = 37)		Professional (n = 32)		Contract (n = 5)
	Mean (SD)	Range	Mean (SD)	Range	Mean (SD)	Range
Age (years)	36.1 (9.0)	21–56	37.4 (8.5)	24–56	27.8 (8.5)	21–42
Height (cm)	177.7 (6.9)	167–196	177.5 (6.1)	167–196	179.4 (7.0)	169–188
Weight (kg)	82.1 (8.5)	68–107	82.8 (8.6)	68–107	77.4 (7.2)	70–89
Activity rating (0–10)	6.9 (0.8)	5.0–8.5	7.0 (0.8)	5.0–8.5	6.6 (0.5)	6.0–7.0
V̇O2max^a (ml/kg/min)	48.6 (7.3)	37.1–67.0	48.6 (7.4)	37.1–67.0	48.6 (6.9)	39.0–56.7
V̇O2max^a (l/min)	3.94 (0.59)	2.92–5.43	3.96 (0.59)	2.92–5.43	3.78 (0.65)	3.00–4.65
HRmax^b (bpm)	190 (7)	174–206	190 (7)	174–206	193 (4)	188–200

^a V̇O_2max: maximal oxygen consumption.

^b HR_max: maximal heart rate.

Information on the participants’ HR_max and V̇O_2max were obtained from their respective fire departments. V̇O_2max was estimated based on submaximal bicycle ergometer test if maximal V̇O_2max test results were not available from the preceding 5 years. HR_max was obtained from a maximal exercise test or reported self-measured HR_max. Any missing measured HR_max or V̇O_2max information was substituted with values calculated on the basis of the following equations: $${\text{HR}}_{\mathit{\text{max}}}(\text{bpm})=208-0.7\times \text{age}$$ (Tanaka, Monahan, and Seals 2001)

  $$\begin{array}{l}{\text{VO}}_{2\mathit{\text{max}}}(\text{ml}/\text{kg}/\text{min})=56.363+1.921\times \text{activity}\hspace{0.17em}\hspace{0.17em}\text{rating}\\ -0.381\times \text{age}-0.754\times \text{body mass}\hspace{0.17em}\text{index}\\ +10.987\times \text{sex}\hspace{0.17em}\hspace{0.17em}(\text{male}=1,\text{\hspace{0.33em} female}=0)\end{array}$$

(Jackson et al. 1990)

Calculated HR_max was used for 18 participants and Non-Exercise V̇O_2max value was calculated for two participants.

Study design

Before the measurements began, the participants signed their informed consents and answered questionnaires on their health status and background and anthropometric information. Participants were instructed to avoid strenuous physical exercise and alcohol consumption for 24 hours, caffeinated products for 4 hours, and nicotine products for 2 hours prior to the measurements. The study protocol was approved by the Ethics Committee of the hospital district of Helsinki and Uusimaa (No. HUS/1503/2020).

The FSDD consists of five different, consecutive tasks (see ‘Updated Finnish smoke-diving drill’). The starting and finishing times from all tasks were recorded. The participants sat on a chair for 10 minutes before starting the drill (baseline) and for 30 minutes after completing the drill (recovery). HR was measured continuously from the start of the baseline measurement until the end of the recovery period. Oxygen consumption (V̇O₂), carbon dioxide production (V̇CO₂), minute ventilation (VE), and the respiratory exchange ratio (RER, V̇CO₂ divided by V̇O₂) were measured during the last five minutes of the 10-minute baseline measurement, throughout the drill, and during the initial 10 minutes of the 30-minute recovery period as a single recording. The rating of perceived exertion (RPE) was assessed after each of the five tasks. Blood lactate concentration ([La-]) was measured before and after the completion of the drill, and after 10 and 30 minutes of recovery.

Finnish smoke-diving drill (FSDD) and equipment

Evaluating and updating the Finnish smoke-diving drill (FSDD)

The FSDD was re-evaluated and updated during 11 workshops over two years (November of 2020 to September of 2022). The workshop participants were researchers, professionals in the field of firefighting, and exercise physiologists. The goal was to assess the present FSDD and to update and evaluate the tasks and equipment, if deemed necessary. The workshops consisted of analysing and discussing the available literature and the areas in need of development, discussing the present version of the drill (what to add and what to retain), piloting the added new task, organising and planning measurements, and finally, presenting and discussing the measurement results and deciding on the final form of the updated FSDD.

The FSDD consists of five tasks (Louhevaara et al. 1994), one of which was updated. Based on the available literature of firefighting and rescue simulations used elsewhere (Blacker et al. 2016; Dreger and Petersen 2007; Elsner and Kolkhorst 2008; Lindberg et al. 2013, Lindberg, Oksa, and Malm 2014; Mamen et al. 2021; Michaelides et al. 2008, 2011; Nazari et al. 2018; Norris et al. 2021; Plat, Frings-Dresen, and Sluiter 2010; Rhea, Alvar, and Gray 2004; Siddall et al. 2018; Skinner et al. 2020; Stevenson et al. 2019; von Heimburg, Rasmussen, and Medbø 2006, von Heimburg et al. 2013; Williams-Bell et al. 2009; Williford et al. 1999) and through cooperation with the firefighting field, a hose pull task was added to the beginning of the Task 4 (going under and over obstacles). The going under and over obstacles task itself was also modified. The drill and the updated fourth task are described in detail below. Other changes consisted of updating the equipment used during the FSDD (composite air cylinder instead of a steel cylinder, and 42-mm instead of 76-mm hoses) to match the equipment used today.

Equipment

The participants wore full smoke-diving gear and self-contained breathing apparatus (SCBA, Dräger, Lübeck, Germany), including a composite air cylinder (300 bar, 6.8 l). However, instead of an SCBA face mask, a Hans Rudolph facemask (Hans Rudolph 7450 Series V2, Hans Rudolph Inc, Shawnee, KS, USA) was worn for the measurement of breathing gases. The total weight of the equipment worn was 22.3 kg.

Updated Finnish smoke-diving drill (FSDD)

The FSDD consists of five tasks that simulate smoke-diving (Louhevaara et al. 1994). All the tasks have fixed maximal completion times. If a task is completed before the end of the fixed time limit, the participant waits until the end of the time limit before starting the next task. Total maximal time for completing the drill is 14.5 minutes. The participants were instructed to complete the tasks at a habitual work rate, not as fast as possible. All study participants were familiar with the FSDD but the updated fourth task was new to them and they performed it for the first time. The tasks were:

• Task 1: Walking without and with two rolls of hose. Participants walked 100 m without a hose and 100 m carrying two rolls of hose (20 m, 42 mm) each weighing 5–6 kg. Time limit for completing Task 1 was 4 minutes.

• Task 2: Stair climbing. Participants ascended and descended stairs, until a total vertical ascent of 20 m was reached. Time limit for completing Task 2 was 3.5 minutes.

• Task 3: Hammering a truck tire. Participants moved a truck tire (weight 47 kg) lying on its side 3 m by striking it with a sledgehammer. Time limit for completing Task 3 was 2 minutes.

• Task 4: Hose pull and going over and under obstacles. Participants moved a 20 m hose under three obstacles placed 2 m apart (Figure 1a). The width of the obstacles was 150 cm, the height of the first and third obstacle was 60 cm, and the height of the second obstacle was 90 cm. The total length of the obstacle course from the starting point to the end point was 8 m. A 47-kg truck tire was attached to one end of the hose and a 20-kg plate was attached 2 m from the other end. Once the end point of the obstacle course was reached, the participants pulled the hose until the truck tire reached the end point (20 m of pulling in total, Figure 1b). After pulling, the participants went back to the starting point, moving under all the obstacles without the hose. Finally, the participants completed two rounds of the obstacle course, going over the first and third obstacle and under the second obstacle back and forth. Time limit for completing Task 4 was 3 minutes in the original version of the drill. The time limit used for the updated fourth task was three minutes, but the participants were allowed to continue if they exceeded this time limit. If the time limit was exceeded, they participants continued to the last task with no rest.

• Task 5: Hose rolling. Participants rolled a 20-m, 42-mm hose in their hands. Time limit for completing Task 5 was 2 minutes.

Figure 1. (a) Going under obstacles, (b) hose pull.

The FSDD was performed in the fire truck hall on a concrete floor. The ambient temperature in the hall was 20.4 ± 2.5 °C. In two fire stations, the floor surface was rough and in one fire station the floor was coated, making it smoother.

Measurements

Breathing gases

Expired and inspired breathing gases were measured breath-by-breath by a portable Metamax 3B cardiopulmonary exercise testing system (Cortex Biophysik GmbH, Leipzig, Germany) during baseline, the FSDD and 10-minute recovery. Metamax 3B was connected via Bluetooth to Metasoft Studio software (Cortex Biophysik GmbH, Leipzig, Germany), where V̇O₂ (ml/kg/min and l/min), V̇CO₂ (l/min), VE (l/min), and RER were calculated. The Hans Rudolph facemask was attached to the participant’s face at the beginning of the baseline measurement. Breathing gases were measured continuously for the last five minutes of the baseline measurement, the FSDD and initial 10 minutes of the recovery period. The O₂ and CO₂ sensors were calibrated according to the manufacturer’s instructions once a week and sensor adjustment was done before every measurement. Single-use, calibrated turbines (Cortex Biophysik GmbH, Leipzig, Germany) were used for the measurements and the turbine was changed before each new participant.

Heart rate (HR)

HR was measured during baseline, the FSDD and 10-minute recovery using the Polar H10 HR sensor (Polar Electro Ltd, Kempele, Finland) and during baseline, the FSDD and 30-minute recovery using Bodyguard 2 (Firstbeat Technologies Ltd, Jyväskylä, Finland), which was attached to the participant’s chest by two single-use ECG electrodes (BlueSensor VL-00-S, Ambu A/S, Ballerup, Denmark). Polar H10 was connected via Bluetooth to the Metasoft Studio software. The Polar H10 and Bodyguard 2 were attached to the participant’s chest before the baseline measurement. HR was measured by Polar H10 continuously for the last five minutes of the baseline measurement, the FSDD and initial 10 minutes of the recovery period, and by Bodyguard 2 from the beginning of the 10-minute baseline measurement until the end of the 30-minute recovery using. HR was measured with two devices, because the Polar H10 could be connected to the Metasoft Studio software and measured online during the drill. The Bodyguard 2 measurement was done to register HR throughout the whole recovery period.

Perceived exertion (RPE)

The participants were asked to rate their physical exertion immediately after completion of each task using a standardised 6–20 RPE scale (Borg 1970). The participants were instructed to evaluate the exertion of the whole body.

Blood lactate concentration ([La-])

[La-] was measured from the fingertip before the drill began but after sitting for 10 minutes, immediately after Task 5 was completed, and after 10- and 30-minute recovery. A drop of capillary blood was placed on a test strip (BM-Lactate, Roche, Basel, Switzerland) and lactate concentration was analysed with Accutrend Plus analyser (Roche, Basel, Switzerland).

Data analysis

The mean baseline values of HR and breathing gases were taken during the last minute of the 10-minute baseline measurement. The mean values of the FSDD were calculated separately for each task and the average for the whole drill was calculated without the resting periods between tasks. The mean recovery values were taken from minutes 9–10 and 29–30 of the recovery period. HR was obtained from the Polar measurements at all time points except for the 30-minute recovery. The breathing gas measurements of one participant were excluded due to abnormally low values resulting from the mask being too loose. The relative V̇O₂ in relation to V̇O_2max (%V̇O_2max) and HR in relation to HR_max (%HR_max) values were calculated for the whole drill (without resting periods) and each of the five tasks.

Statistical analyses

The statistical analyses were carried out using IBM SPSS Statistics for Windows, Version 27.0 (IBM, Armonk, NY, USA). The data were tested for normal distribution using the Shapiro–Wilk test. Natural log transformation was carried out for the variables that did not follow normal distribution before performing the statistical analyses. The differences between baseline and 10-minute V̇O₂, VE, and RER were tested using the paired samples t-test, and the differences between HR and [La-] at baseline, during the 10-minute recovery and during the 30-minute recovery were tested using repeated measures ANOVA and Bonferroni adjustment. To examine the effects of floor surface friction on physical strain, measurements taken on participants who performed the drill on rough floor (n = 22) were compared to those participants who performed the drill on smooth floor (n = 15) using the independent samples t-test. The independent samples t-test was used to analyse the differences between the physical strain of the professional firefighters and that of the contract firefighters. Correlations between background and anthropometric characteristics (age, height, weight, and V̇O_2max), execution time, and physical strain during the drill (relative and absolute mean V̇O₂, %V̇O_2max, mean HR, %HR_max, and [La-]) were studied using the Pearson correlation coefficient. A partial correlation was run to determine the relationship between relative and absolute V̇O_2max and physical strain during the drill (relative and absolute mean V̇O₂, %V̇O_2max, mean HR, %HR_max, and [La-]) while controlling for age. The level of statistical significance was set at p < 0.05. All the results are presented as mean ± SD.

Results

The mean execution time of the whole FSDD was 13 min 56 s ± 40 s, and the proportion of working time was 70.3 ± 6.9% (9 min 49 s ± 1 min 18 s). Table 2 presents all the physiological parameters measured during the FSDD. The mean HR during the whole drill without recovery periods was 147.1 ± 12.4 bpm (77.5 ± 6.1%HR_max) and V̇O₂ was 28.6 ± 3.0 ml/kg/min (59.2 ± 7.8%V̇O_2max). The corresponding values for the whole drill including rest periods were 145.3 ± 12.4 bpm (76.5 ± 6.0%HR_max) and 27.5 ± 2.9 ml/kg/min (57.1 ± 7.6%V̇O_2max), respectively. Peak HR elicited during the drill was 174.2 ± 9.7 bpm (91.7 ± 4.1%HR_max). The most strenuous task was Task 4 (hose pull and going over and under obstacles), during which the mean HR was 164.8 ± 10.8 bpm (86.8 ± 4.9%HR_max) and V̇O₂ 35.2 ± 4.6 ml/kg/min (73.1 ± 10.9%V̇O_2max). [La-] after the drill was 7.3 ± 2.3 mmol/l. Table 3 presents the values measured at baseline and after 10 and 30 minutes of recovery. None of the measured variables had recovered to baseline levels after 10- or 30-minute recovery. An example of changes in HR and V̇O₂ throughout the whole measurement (baseline, the FSDD and recovery) from one participant can be seen in Figure 2.

Figure 2. Heart rate and oxygen consumption of one participant averaged in 30-second time intervals during baseline, Finnish smoke-diving drill and 10-minute recovery.

^aTask 1: walking without and with two rolls of hose; Task 2: stair climbing; Task 3: hammering a truck tire; Task 4: hose pull and going over and under obstacles; Task 5: hose rolling.

Table 2. Physiological parameters measured during Finnish smoke-diving drill, mean (SD).

	Walking without and with two rolls of hose (Task 1)	Stair climbing (Task 2)	Hammering a truck tire (Task 3)	Hose pull and going over and under obstacles (Task 4)	Hose rolling (Task 5)	Whole drill
Duration (s)	158.9 (17.1)	152.4 (15.2)	49.2 (26.4)	159.6 (43.3)	70.4 (13.2)	836.1 (40.4)
HR^a (bpm)	117.7 (12.6)	137.5 (12.7)	153.9 (14.8)	164.8 (10.8)	157.7 (13.3)	147.1 (12.4)
HR^a (%max)	62.0 (6.3)	72.4 (6.3)	81.0 (7.1)	86.8 (4.9)	83.1 (7.0)	77.5 (6.1)
V̇O2^b (l/min)	1.69 (0.20)	2.36 (0.28)	2.06 (0.35)	2.87 (0.40)	2.17 (0.35)	2.34 (0.28)
V̇O2^b (ml/kg/min)	20.8 (2.8)	28.9 (3.2)	25.1 (3.7)	35.2 (4.6)	26.6 (3.7)	28.6 (3.0)
V̇O2^b (%max)	43.3 (6.9)	60.1 (8.0)	52.3 (9.9)	73.1 (10.9)	55.3 (9.4)	59.2 (7.8)
VE^c (l/min)	38.5 (6.1)	53.8 (8.5)	66.1 (11.7)	88.0 (15.1)	76.6 (12.6)	67.0 (10.4)
RER^d	0.78 (0.05)	0.85 (0.05)	0.99 (0.07)	1.02 (0.05)	1.10 (0.04)	0.94 (0.04)
[La-]^e (mmol/l)						7.3 (2.3)
RPE^f	9.1 (1.5)	11.4 (1.7)	13.0 (2.0)	15.1 (1.9)	12.2 (2.0)

^a HR: heart rate.

^b V̇O₂: oxygen consumption.

^c VE: ventilation.

^d RER: respiratory exchange ratio.

^e [La-]: blood lactate concentration.

^f RPE: perceived exertion.

Table 3. Physiological parameters measured at baseline and 10- and 30-minute recovery after Finnish smoke-diving drill, mean (SD).

	Baseline	10-min recovery	30-min recovery
HR^a (bpm)	75.3 (7.1)	98.6 (9.3)***	86.8 (8.5)***$$$
V̇O2^b (l/min)	0.46 (0.06)	0.54 (0.07)***
V̇O2^b (ml/kg/min)	5.7 (0.9)	6.6 (0.9)***
VE^c (l/min)	13.8 (2.2)	18.3 (3.1)***
[La-]^d (mmol/l)	1.5 (0.4)	5.1 (1.8)***	3.2 (0.9)***$$$

* Statistically significant difference from baseline value, p < 0.001.

^$ Statistically significant difference from 10-min recovery value, p < 0.001.

^a HR: heart rate.

^b V̇O₂: oxygen consumption.

^c VE: ventilation.

^d [La-]: blood lactate concentration.

When comparing the effects of smooth and rough floor on physical strain, there were no differences between the conditions in relative or absolute V̇O₂ or HR. However, task duration was significantly longer on the rough floor when all tasks were combined (610.1 ± 74.3 s vs. 560.6 ± 79.8 s, p = 0.031) and in Task 3 (64.5 ± 21.9 vs. 26.8 ± 13.0 s, p = 0.000), but the durations of the other tasks did not differ. [La-] was significantly higher on the rough floor after the drill (8.4 ± 2.0 vs. 5.6 ± 1.5 mmol/l, p = 0.000) and remained higher after both the 10- (5.8 ± 1.7 vs. 4.0 ± 1.4 mmol/l, p = 0.002) and the 30-minute (3.5 ± 0.8 vs. 2.6 ± 0.9 mmol/l, p = 0.003) recovery.

The contract firefighters were significantly younger than the professional firefighters (p = 0.025), but did not differ in terms of height, weight, or V̇O_2max (Table 1). The whole drill and task durations did not differ, but physical strain was higher among the contract personnel. The contract firefighters also had higher relative V̇O₂ (31.4 ± 2.7 vs. 28.1 ± 2.8 ml/kg/min, p = 0.021), and HR (159.7 ± 3.2 vs. 145.1 ± 12.2 bpm, p = 0.012) during the drill, as well as higher relative strain, i.e. %V̇O_2max (65.6 ± 11.0% vs. 58.2 ± 6.8%, p = 0.046) and %HR_max (82.8 ± 3.2% vs. 76.6 ± 6.1%, p = 0.032). There were no differences in terms of RPE or [La-].

Better aerobic fitness was related to lower physical strain during the drill. Table 4 presents the results of the partial correlation between V̇O_2max and physical strain during the drill while controlling for age. Higher relative and absolute V̇O_2max were associated with lower HR, %V̇O_2max and %HR_max. Higher relative V̇O_2max was also associated with lower [La-] after the FSDD. Higher absolute V̇O_2max was associated with higher absolute V̇O₂ during the drill and, correspondingly, higher relative V̇O_2max was associated with higher relative V̇O₂. The time taken to perform the FSDD (without resting periods) was not associated with age, height, weight, V̇O_2max, or mean physical strain during the drill. However, age was associated with mean and peak HR (r = −0.43, p = 0.008 and r = −0.44, p = 0.006, respectively), %HR_max (r = −0.37, p = 0.026), and absolute V̇O₂ (r = −0.49, p = 0.003) during the drill.

Table 4. Pearson correlation coefficients controlled for age between maximal oxygen consumption and physical strain during Finnish smoke-diving drill.

	1.	2.	3.	4.	5.	6.	7.	8.
1. V̇O2max^a (ml/kg/min)	1.00	0.80***	0.53***	0.29	–0.72***	–0.48**	–0.50**	–0.37*
2. V̇O2max^a (l/min)		1.00	0.21	0.56***	–0.73***	–0.57***	–0.54***	–0.20
3. V̇O2^b (ml/kg/min)			1.00	0.52**	0.19	0.08	0.03	0.09
4. V̇O2^b (l/min)				1.00	0.10	–0.11	–0.04	0.25
5. %V̇O2max^c					1.00	0.59***	0.56***	0.55***
6. HR^d (bpm)						1.00	0.90***	0.41*
7. %HRmax^e							1.00	0.40*
8. [La-]^f (mmol/l)								1.00

*p < 0.05.

**p < 0.01.

***p < 0.001.

^a V̇O_2max: maximal oxygen consumption.

^b V̇O₂: oxygen consumption.

^c %V̇O_2max: oxygen consumption in relation to maximal.

^d HR: heart rate.

^e %HR_max: heart rate in relation to maximal.

^f [La-]: blood lactate concentration.

Discussion

Updated Finnish smoke-diving drill (FSDD)

The FSDD was updated to meet present-day occupational demands by incorporating a hose pull task and updating the equipment (hoses and air cylinders) used during the drill. The updates were made based on available literature and cooperation between the researchers, exercise physiologists, and the field of firefighting. A hose pull task was added because the original version of the FSDD did not contain crawling and pulling/dragging tasks, which are frequent in real-life smoke-diving. Based on comments from the workshops and study participants, the updates that were made to the FSDD have improved the drill and its correspondence to actual smoke-diving scenarios.

Previous study regarding the FSDD was by Louhevaara et al. (1994), where the measured mean HR was 140 bpm (range 102–175 bpm) and 76%HR_max (range 59–93%). Louhevaara et al. (1994) calculated a V̇O₂ estimate for the drill, which was 26 ml/kg/min (56%V̇O_2max). The present study of the drill presented similar levels of cardiac strain (147 bpm, 78%HR_max) and V̇O₂ (29 ml/kg/min, 59%V̇O_2max), despite the added new hose pull task at the beginning of Task 4 (going over and under obstacles). However, the equipment used during the drill was changed to smaller hoses and a lighter SCBA air cylinder (composite instead of steel). Also, the smoke-diving gear worn during the drill has developed in the last 30 years. Together these changes have reduced the load carried by firefighters and the heat stress they experience during the drill. The FSDD is meant to be performed at a habitual work rate, not as fast as possible, and the similarity between the study results indicate that the study participants succeeded in pacing themselves to reduce cardiac strain during the added hose pull task. Further proof of this is the fact that the time taken to perform the test drill was not associated with V̇O_2max, meaning that individuals with better aerobic fitness did not complete the tasks any faster than individuals with lower aerobic fitness.

All the participants in this study managed to complete the updated FSDD within the predetermined time limit of 14.5 minutes. However, 7 of 37 participants (19%) exceeded the three-minute time limit in the updated hose pull and going over and under obstacles task. The execution times in this task ranged from 1 minute 20 seconds to 4 minutes 29 seconds. After analysis of the results, the length of the hose pull was shortened to 15 metres to reduce time and space requirements. With this change, a theoretical execution time was calculated by subtracting a quarter of the execution time of the hose pull. The shorter hose pull distance would theoretically have enabled all the study participants to perform the task within four minutes (range of theoretical execution times from 1 min 11 s to 3 min 44 s) and therefore, the time limit of Task 4 was extended to four minutes and the whole drill time limit to 15.5 minutes. As the FSDD is an exercise that Finnish firefighters perform regularly, all the participants were familiar with the drill except for the added hose pull task. It is possible that the execution times of Task 4 will shorten once the task and performance techniques become more familiar to the firefighters.

Metabolic demands of and recovery from updated smoke-diving drill

The mean HR during the updated FSDD was 147.1 ± 12.4 bpm (77.5 ± 6.1%HR_max) and the mean V̇O₂ was 28.6 ± 3.0 ml/kg/min and 2.34 ± 0.28 l/min (59.2 ± 7.8%V̇O_2max). The HR and V̇O₂ measured during the drill in the present study corresponded with the values measured during real-life fire suppression tasks (Horn et al. 2013, 2015; Lusa et al. 1993; Sothmann et al. 1992), indicating that the metabolic demands of the FSDD accurately represent the metabolic strain in actual smoke-diving tasks.

The FSDD tasks are designed so that they simulate a real-life smoke-diving scenario. The intensity of the tasks increases during the first four tasks and the hose rolling (Task 5) acts as a ‘cool-down’ exercise. This was evident in HR, which increased throughout Tasks 1–4 but decreased during the hose rolling task. The changes in V̇O₂ in turn were not as linear. V̇O₂ was highest during the updated hose pull and going over and under obstacles task (Task 4) but declined during hose rolling, similarly to HR. However, V̇O₂ decreased during the hammering task (Task 3) from that during the stair climbing (Task 2). The decline of mean V̇O₂ during the hammering task is likely due to it being the shortest of all the tasks. When a person starts to exercise, there is a 10- to 20-second delay in the rise of pulmonary V̇O₂ before the deoxygenated blood from the working muscles reaches the lungs for gas exchange (Burnley and Jones 2007). HR in turn exhibits an almost immediate response to exercise (Poole and Jones 2012). The differences between the V̇O₂ and HR kinetics might explain the discrepancy in V̇O₂ and HR responses during the hammering task, as the mean duration of the task was only 49 seconds. Moreover, the hammering task is high in intensity and requires muscular effort, especially from the muscles of the upper body, which is why the proportion of anaerobic energy production is likely to be significant in this task. This is supported by RER rising to 0.99 during the hammering task. RER remained above 1.0 throughout the final tasks to eliminate the H⁺ produced in anaerobic energy processes through the exhalation of CO₂.

Physiological recovery was monitored for 30 minutes after the FSDD was completed. Neither V̇O₂, VE, nor RER recovered to baseline levels after 10 minutes of recovery. Furthermore, neither HR nor [La-] had recovered after 30 minutes of recovery. This was evident even though HR and V̇O₂ were slightly elevated at baseline, which was likely to be caused by the measurement situation or anticipation of performing the drill. V̇O₂ remained 16% higher than baseline levels after 10 minutes and HR 15% higher after 30 minutes of recovery. The Finnish guidelines for smoke-diving recommend 20 to 30 minutes recovery after a smoke-diving task before starting a new one (Finnish Ministry of the Interior 2007). The results of the current study support an even longer recovery period for HR and [La-] to recover to resting levels after firefighting tasks. However, the rate of removal of [La-] can be accelerated by doing light exercise (Hermansen and Stensvold 1972). In the case of consecutive smoke-diving tasks, performing light exercise in between tasks can be recommended to speed up the rate of lactate removal.

Comparison of floor materials

The friction of the floor material affected physical strain during the FSDD. In this study, the measurements were conducted in three different fire stations and the use of different testing environments could be considered a limitation. However, this enabled us to compare physical strain on rough and smooth floor materials. Finnish firefighters perform the FSDD regularly at their respective fire stations, and therefore, the testing environments are not always equal. The results of the present study demonstrated that performing the drill on a rough floor surface did not increase physical strain (V̇O₂ or HR) significantly. However, the duration of the hammering task and [La-] after the drill were greater when the drill was performed on a rough floor. This indicates that the floor material might affect physical strain and especially increase the proportion of anaerobic energy production despite no differences in V̇O₂ or HR. In practice, it is impossible to standardise floor material to be the same in all fire stations, but this aspect needs to be taken into consideration when interpreting the results. Therefore, it is recommended that firefighters always perform the drill at the same location to permit monitoring intraindividual changes in physical work capacity and fitness over time. Nevertheless, the floor material did not affect mean HR or %HR_max during the drill, which are the parameters used when evaluating cardiac strain during the drill.

Comparison of contract and professional firefighters

Comparison of contract and professional firefighters revealed no differences in task durations during the drill, but physical strain was higher among the contract personnel. Professional firefighters may have more experience and routine in working techniques and handling the equipment, which increases economy of movement and reduces physical strain. The contract firefighters were also younger compared to professional firefighters, but there were no differences in height, weight, or V̇O_2max, further corroborating the finding that the differences between the contract and professional personnel were caused by work experience. Moreover, in terms of the whole study population, older age was associated with lower physical strain (mean and peak HR, %HR_max, and absolute V̇O₂) during the drill. HR_max is largely predicted by age (Tanaka, Monahan, and Seals 2001), which could explain the lower mean and peak HR, but in connection with lower mean %HR_max and V̇O₂, these results indicate that age and work experience reduce physical strain during the drill. Older firefighters are also likely to have more experience in performing FSDD, and therefore, to have developed effective techniques and pacing in these specific tasks.

Associations between aerobic fitness and physical strain

Although aerobic fitness was not associated with FSDD execution time, higher V̇O_2max was associated with lower physical strain during the drill (HR, %HR_max and %V̇O_2max). Higher V̇O_2max was also associated with less lactate accumulation, meaning that individuals with better aerobic fitness could rely more on aerobic energy production and less on anaerobic processes when performing the FSDD. This is an important finding, as the goal of PESs is to ensure that a firefighter can perform their work tasks safely, without undue physical strain (Tipton, Milligan, and Reilly 2013). Typical authentic firefighting and rescue activities include work bouts of a few minutes followed by periods of less work, resulting in fluctuations in physical strain (Ensari et al. 2017; Horn et al. 2013). The FSDD simulates a real-life firefighting scenario, using tasks that simulate firefighting and rescue activities, at a pace that represents habitual work rate. Many other firefighting simulations are performed as quickly as possible to test firefighters’ maximal physical capacity (e.g. Dreger and Petersen 2007; Mamen et al. 2021; Siddall et al. 2018; Stevenson et al. 2019; von Heimburg et al. 2013). The evaluation of the FSDD is based on %HR_max rather than on execution time, to enable evaluation of the cardiac strain of firefighting and rescue work and therefore, cardiac strain in authentic firefighting and rescue duties.

Strengths and limitations

This study had several strengths, including a wide range of physiological measurements and study participants of various ages and fitness levels, to determine the overall metabolic demands of the updated FSDD. In addition, the research group involved in the evaluation and update of the FSDD was diverse, consisting of experienced researchers, professionals in the field of firefighting, and exercise physiologists. However, this study also had some limitations. The measurements were conducted in three different fire stations, and although there are specific guidelines regarding the equipment used during the drill, the equipment and the environmental conditions of the measurement sites might have varied slightly. In addition, it was not feasible to do a maximal exercise test for all study participants, and therefore, we opted to use the existing health screening test results for HR_max and V̇O_2max. A calculated HR_max value was used for almost half of the study participants and V̇O_2max was estimated with a submaximal V̇O_2max test or a maximal V̇O_2max test performed within 5 years, which might have affected the reliability of the calculated %HR_max and %V̇O_2max values.

One goal of the study was to compare the physical strain of the contract and professional firefighters, but unfortunately, the number of volunteer contract firefighters was low. Therefore, the comparison of the contract and professional firefighters may not be reliable, as the small sample may not be representative of all contract firefighters. The study participants were predominantly male, with only three female participants. This did not enable comparison of the genders. Nevertheless, the occupational physical demands in firefighting are the same, regardless of gender, and the PETs used in Finland do not have separate standards for males and females. Therefore, gender comparison was not relevant for the aims of this study. All the female participants in this study passed the updated version of the FSDD.

Conclusions

The FSDD was first developed in the 1990s (Louhevaara et al. 1994) and the drill has not been updated since then. Yet, the equipment and working methods have evolved in the past 30 years, and therefore, the main goal of this study was to update the FSDD and to measure the physical strain of and recovery from the drill. The updates that were made to the FSDD included an added hose pull task, extending the total time limit of the hose pull and going under and over obstacles task to four minutes, and updating the equipment used. In Finland, the FSDD is used alongside the aerobic and muscular physical employment tests to assess firefighter’s work ability in job-related tasks. The updated version better depicts occupational demands of firefighting tasks and consequently, is better suitable for assessing work ability of firefighters.

Disclosure statement

The authors report there are no competing interests to declare.

Data availability statement

The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.

Additional information

Funding

This work was supported by the Fire Protection Fund under Grant SMDno-2019-983 and the Finnish Institute of Occupational Health. The funders played no role in the study design, data collection or analysis, the decision to publish, or the preparation of the manuscript.