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Article

Exploration of Heart Rate Recovery After Maximal Treadmill and Three-Minute All-Out Shuttle Tests in Firefighters

by
Benjamin J. Mendelson
*,
Kyle T. Ebersole
*,
Scott D. Brau
and
Nathan T. Ebersole
Human Performance & Sport Physiology Laboratory, School of Rehabilitation Sciences & Technology, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
*
Authors to whom correspondence should be addressed.
Fire 2025, 8(1), 20; https://doi.org/10.3390/fire8010020
Submission received: 2 December 2024 / Revised: 3 January 2025 / Accepted: 6 January 2025 / Published: 8 January 2025
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Abstract

:
The purpose of this study was to compare heart rate recovery (HRR) after a maximal treadmill (MAX-TM) and three-minute all-out (3MT) test between firefighters (FF) and a control (CON) group. Nine male CON and nine male FF participants completed height (m), weight (kg), body fat percent (BF%), normalized handgrip (GRIPNORM, kg/kg), and MAX-TM with direct gas analysis to capture aerobic capacity (VO2PEAK, mL/kg/min). A shuttle-sprint 3MT was used to measure critical velocity (CV, m/s) and D′ (m). Non-linear models determined HR decay (HRRτ), HR asymptote (HR∞), and HR amplitude (HRamp). Two-way GROUP (FF vs. CON) by TEST (MAX-TM vs. 3MT) repeated measures ANOVAs indicated a significant TEST (F = 7.004, p = 0.018) effect on HRamp. When divided by VO2PEAK classification (FITNESS), a significant TEST effect was observed (F = 7.661, p = 0.014) on HRamp. VO2PEAK was significantly related to CV (r = 0.583, p = 0.011), GRIPNORM (r = 0.668, p = 0.002), and BF% (r = −0.890, p < 0.001). Complete autonomic nervous system recovery may depend on the intensity of task demands and cardiorespiratory fitness.

1. Introduction

Firefighting is widely recognized as strenuous work, and responding to emergencies can require a significant cardiorespiratory demand. As a result, sudden cardiac death (SCD) is consistently a leading cause of line of duty death [1]. To safely meet job demands, it has been recommended that firefighters maintain an aerobic capacity (VO2PEAK) of 42.0–45.0 mL/kg/min [2,3]. Higher cardiorespiratory fitness has been inversely associated with cardiometabolic risk factors and linked to improved performance of firefighting tasks and efficiency of air usage [4,5,6]. However, many SCDs may occur after an emergency call, suggesting that cardiovascular recovery may also be a significant consideration for firefighter health and workability [1,7]. Heart rate recovery (HRR) is a common, non-invasive measure of post-exercise autonomic nervous system (ANS) recovery, and it can be impacted by exercise intensity. Specifically, higher-intensity exercise may result in a slower HRR due to increased activity of the sympathetic branch of the ANS [8]. As such, investigations of performance and recovery from high-intensity exercise are needed in active-duty firefighters.
The three-zone model to classify exercise intensity proposed by Gaesser and Poole has gained interest in occupational or tactical populations [9]. The model defines distinct zones of exercise intensity: moderate, heavy, and severe. Exercise in the moderate- and heavy-intensity zones is marked by increases in heart rate (HR), oxygen consumption (VO2), and blood lactate (LA−) concentrations, yet a steady state can be reached in either zone. Sustained exercise in the severe-intensity zone leads to increases in HR, VO2, and LA− until maximal exertion is reached [9,10]. As a result, the delineation between heavy and severe intensity has been labeled “critical” intensity, or the highest intensity at which predominately oxidative energy pathways can fuel sustained exercise [11]. Measurement of the critical intensity has been accomplished in running, or critical velocity (CV), and cycling, or critical power (CP) [12,13,14,15,16]. While, in theory, the CV can be maintained for an indefinite amount of time without fatigue, it has been reported that exercise at the CV may be sustained for up to 60 min [15,17].
To date, the CV or CP has been researched predominately in sport–athlete populations, yet it has a growing interest in occupational populations. It has been indicated that a higher CV is associated with a higher VO2PEAK, and it has been linked to faster time to complete tactical skills in military special operators and astronauts [14,18,19]. Furthermore, measurement of the CV and CP allows for calculation of the finite anaerobic work capacity that can be accomplished at work rates greater than the CV or CP, labeled D′ for running and W′ for cycling [20,21]. It is possible that the CV and D′ measures can provide unique insight into metabolic efficiency during physical work in firefighter or other occupational populations, and they have previously been linked to aerobic capacity [10,22,23].
Measurement of the CV via all-out testing can provide a field test that requires minimal equipment and incorporates agility components, and it may be representative of job demands [12,14,24]. The three-minute all-out test (3MT) provides a time-efficient field test that has been validated to measure the CV and D′ in running and CP and W′ in cycling [16,21]. The test requires individuals to produce an all-out sprint effort at the onset of the test and maintain the effort for the duration of the test, which can have practical applicability to the fire service. Firefighters experience significant increases in HR and physiological arousal from hearing alarm tones, which then can extend into prolonged periods of cardiovascular workload while responding to emergencies [25,26,27]. Furthermore, shuttle-sprint adaptations of the 3MT have been developed to be used for populations that require agility components [12,14]. To date, the use of a shuttle-sprint 3MT to measure CV and D′ in active-duty firefighters has not been explored. However, the rapid-onset nature of the 3MT provides a unique field test to investigate cardiovascular response and recovery after severe-intensity activity. Further, as ANS recovery may be a key consideration in the overall cardiorespiratory health of firefighters, a comparison of HRR between active-duty firefighters and non-firefighters may provide unique insight into the recovery abilities of firefighters. As such, the purpose of this study was to investigate the HRR after a firefighter-specific maximal exertion treadmill test protocol and shuttle sprint 3MT between active-duty firefighters and compared to a recreationally active control group.

2. Materials and Methods

2.1. Subjects

A sample of nine male non-firefighter control (CON) and nine male active-duty career structural firefighters (FF) volunteered to participate. All FF participants were municipal firefighters from a large metropolitan midwestern city in the United States. Each participant was screened for eligibility and signed a written informed consent form prior to data being collected. To be deemed eligible for this study, participants needed to be incorporating steady state running (i.e., 30 min of steady state) at a frequency of 1–2 sessions per week, have previous exposure to sprinting and change of direction ability, not be diagnosed with any cardiovascular, metabolic, or neuromuscular condition, and have no surgery due to a musculoskeletal injury within the last 12 months. FF participants were required to be medically cleared for full duty at the time of participation.

2.2. Procedures

Data collection was completed across two laboratory testing sessions separated by at least 48 h. Test session 1 included the maximal treadmill test (MAX-TM), and test session 2 included the repeated-sprint three-minute all-out test (3MT). In each session, participants were fitted for a Zephyr Bioharness 3 wireless physiological status monitor (Medtronic, Annapolis, MD, USA) at a sampling rate of 250 Hz, which captured HR (bpm) during (1) a five-minute seated rest period prior to any test data being collected, (2) the exercise test completed during the session, and (3) a five-minute seated rest completed after the exercise test. At the first test session, anthropometric data, including height (m), body weight (BW, kg), and body fat percentage (BF%) estimated from the Jackson and Pollock 3-site skinfold measure, were collected prior to the MAX-TM [28]. In addition, maximal handgrip strength using a hydraulic hand dynamometer (Lafayette Instrument, Lafayette, IN, USA) was measured prior to the MAX-TM. Participants completed two trials using each arm in a standing position with the elbow bent at approximately 90° of flexion. The highest trial on each hand was recorded, and both the right and left arm values were summed and normalized to body weight (GRIPNORM, kg/kg).

2.2.1. Maximal Treadmill Test

The maximal treadmill test (MAX-TM) followed the protocol designed by the International Association of Firefighters and the International Association of Fire Chiefs Joint Wellness Fitness Initiative [29]. Participants completed this protocol on a motorized treadmill (Woodway USA Inc., Waukesha, WI, USA) beginning at a 3 mph walk for three minutes at a 0% incline grade, followed by the treadmill velocity increasing to 4.5 mph for a one-minute stage. Subsequent stages progressed in one-minute intervals, alternating between increases in treadmill incline by 2% or treadmill velocity by 0.5 mph. While this protocol is designed to be submaximal and terminated when the participant reaches 85% of their age-predicted maximal HR [29], for the purposes of achieving maximal exertion, participants continued increasing the stages until at least two of the following three criteria were achieved: (a) meeting or exceeding the predicted maximal HR (MHR = 208 − 0.7 × age) for more than 15 s, (b) participant rating of perceived exertion was ≥10 on Foster’s Session RPE 0–11 scale [30], and/or (c) volitional termination by the participant due to fatigue [31]. During the MAX-TM, expired air was measured directly using a calibrated metabolic analysis system (PARVO TrueOne 2400, ParvoMedics, Inc., Sandy, UT, USA), and data were expressed in 15 s averages. Maximal oxygen consumption (VO2PEAK, mL/kg/min) during the MAX-TM was represented by the highest 15 s average volume of oxygen (VO2, mL/kg/min) throughout the entire test. Upon test completion, participants returned to a seated position for a five-minute rest period, during which HR was recorded continuously.

2.2.2. Three-Minute All-Out Test

The 3MT has been validated for running [21] to determine critical velocity. Previous running-based 3MT protocols have used running tracks or flat ground courses, and the measurement of running speed has been completed using GPS-tracking wearable technology [18,21]. However, Saari et al. [12] validated a shuttle-sprint test format that used 30 m sprints with switchbacks, which may have greater applicability for athletes or occupational groups that require agility and change of direction movements (Figure 1). To maximize the applicability of these results to practitioners working with sport or occupational athletes that may require repeated sprint efforts, the repeated sprint 3MT protocol validated by Saari et al. [12] was used (Figure 1).
The 3MT was carried out on a basketball court for all participants. Cones were set up at 10 m increments to create a lane that measured 30 m long. The lane was measured at five meters wide to guide participants against taking wide turns when performing switchbacks. Participants began the session with a five-minute seated rest to collect resting HR measures. After the rest period, participants were directed to the sprint lane to perform a standardized dynamic warm-up that was designed to increase HR and blood flow, prepare participants for sprint efforts, and familiarize the participants with the sprint course. Once the warm-up was completed, participants returned to a seated position to allow for their HR to decrease, and a baseline blood lactate value was measured. When lactate collection was completed, participants were directed to the start of the course and given a countdown to begin the 3MT.
Participants were instructed to sprint the entire length of the 30 m lane, followed by a switchback and immediate subsequent 30 m sprints for the entirety of the three-minute trial. Participants were not told how much time had elapsed to avoid pacing, and strong verbal encouragement was provided. All 30 m sprints were hand-timed by the same researcher (B.J.M) using a digital stopwatch. Participants were alerted when the three-minute time had elapsed and returned to a seated position to complete a five-minute seated recovery period while HR was measured continuously.
Each sprint time was converted to meters per second to calculate approximate sprint velocity. The average velocity of the final 30 s of the 3MT determined the CV (m/s) [12]. To calculate the anaerobic work capacity (D′, m), the following equation was used:
D′ = 150 s × (S150s − CV)
where S150s is equal to the average velocity in the first 150 s of the 3MT.

2.2.3. Heart Rate Recovery Measures

Data from the Zephyr Bioharness 3 captured in the five-minute pre- and post-test seated rest periods from both the MAX-TM and 3MT were used to identify baseline and HRR data. The average HR of the five-minute pre-test rest period was used to establish the resting HR for the MAX-TM (HRREST-TM) and 3MT (HRREST-3MT) tests. To determine HRR, the immediate post-exercise HR (HR0) was identified, and then HR data were expressed in 30 s averages (HR30, HR60, HR90,…, HR300) for the entire five-minute post-test recovery period. Each 30 s average was then converted to a percentage of maximal HR to normalize the data to each participant and eliminate the effect of age, as the FF group was found to be significantly older than the CON group (Table 1). To determine the HRR profile of each participant, the HR data after both the MAX-TM and 3MT tests were plotted separately, and a non-linear least squares model was fitted using the “nls” formula from R Statistical Software [v2023.06.0+421; Posit Software, 2022] to determine the parameters of the equation
H R = H R ∞ + H R a m p e − t H R R τ
where HR∞ (% MHR) is the asymptotic value of HR, HRamp (% MHR) represents the difference between HR0 and HR∞, t is time (s), and HRRt (% MHR) represents the decay constant of the entire recovery period [32]. Each model was assessed for goodness of fit using the “aomisc” package from R Statistical Software, which calculates the Pseudo-R2 introduced by Schabenberger and Pierce [33], and results indicated that a high amount of variance in HR was accounted for in the MAX-TM (R2 = 0.969 ± 0.024) and 3MT (R2 = 0.964 ± 0.029) data.

2.2.4. Blood Lactate Measurement

Blood lactate (LA−, mmol/L) was measured via finger prick using a handheld portable blood lactate monitor (Nova Biomedical Lactate Plus, Nova Biomedical, Waltham, MA, USA). During the MAX-TM test, lactate was tested after the five-minute pre-test resting period (LA−PRE-TM) and immediately post-test (LA−POST-TM). During the 3MT, baseline LA− was tested after the dynamic warm-up (LA−PRE-3MT) to control for the contribution of the warm-up to the total LA− concentration post-test. Post-test LA− measurement (LA−POST-3MT) was collected after the five-minute recovery period. All lactate measures were conducted by the same researcher (B.J.M).

2.2.5. Statistical Analysis

Data were visually inspected for normality prior to analysis. Independent samples t-tests were used to identify significant differences in anthropometric test results as well as VO2PEAK, CV, and D′ between the FF and CON groups. Two-way (GROUP × TEST) repeated measures analyses of variance (ANOVA) tests were run to identify significant differences in HRPEAK and HRR parameters (HR∞, HRamp, and HRRt) between the FF and CON groups from the MAX-TM and 3MT tests. After visual inspection of the data and obtaining the outcome of the GROUP × TEST ANOVA, a follow-up analysis was conducted to investigate the impact of cardiorespiratory fitness level on HRR. As such, two-way (FITNESS × TEST) repeated measures ANOVA tests were run with the sample divided into groups based on aerobic capacity classification using the American College of Sports Medicine (ACSM) age-based normative values [34]. The higher-fitness group (HF) was those who achieved a VO2PEAK that was rated as “Good” or higher, while the lower-fitness group (LF) was those who achieved a “Fair” or worse rating. Pearson-product correlations were run to identify significant relationships between BF%, VO2PEAK, GRIPNORM, CV, D′, and HRR parameters of the MAX-TM and 3MT tests. Correlation coefficients were interpreted using the following parameters: very weak: <0.20; weak: 0.20–0.39; moderate: 0.40–0.59; strong: 0.60–0.79; or very strong: >0.80 [35]. Statistical analysis was conducted using IBM Statistical Package for the Social Sciences, version 29.0.1.0 (IBM SPSS Statistics for Windows, Version 29.0. Armonk, NY: IBM Corp), and data visualizations were created using R Statistical Software. An alpha level of 0.05 determined statistical significance.

3. Results

Descriptive statistics for anthropometric tests as well as the results of the MAX-TM and 3MT tests are presented in Table 1 for the CON and FF groups. Using the ACSM guidelines of classification for BF%, participants were rated as Very Lean (n = 3), Excellent (n = 5), Good (n = 4), Fair (n = 2), or Poor (n = 4). Additionally, using ACSM classifications for VO2PEAK, participants were rated as Good (n = 9), Fair (n = 5), Poor (n = 3), and Very Poor (n = 1). Thus, the HF group resulted in n = 9 participants and the LF group resulted in n = 9 participants [34].
Means and standard deviations of baseline and peak HR and post-test LA− values are presented in Table 2 for the FF and CON groups. Means and standard deviations of HRR parameters when the sample was divided by GROUP and FITNESS are presented in Table 3. Results of a two-way ANOVA indicated that there was no significant main effect of TEST (F1,16 = 2.783, p = 0.115) or interaction (F1,16 = 0.696, p = 0.417) between GROUP and TEST on HRPEAK, yet there was a significant main effect of GROUP (F1,16 = 7.515, p = 0.014); specifically, the CON group had significantly higher HRPEAK values than the FF group in both the MAX-TM and 3MT tests. An independent samples t-test indicated that the CON group was significantly younger (t16 = −3.226, p = 0.002) than the FF group. As such, recovery HR data were normalized to percentage of max to calculate the HRR parameters to control for the effect of age on maximal HR.
Plots of the mean normalized average 30 s HR values during the five-minute post-test rest periods are represented in Figure 2. Means and standard deviations of the two-way GROUP × TEST ANOVA of HRR parameters indicated there was no significant main effect of TEST (F1,16 = 0.088, p = 0.711), GROUP (F1,16 = 1.524, p = 0.235) or interaction between TEST and GROUP (F1,16 = 1.551, p = 0.231) on HRRτ. Similarly, there was no significant main effect for TEST (F1,16 = 0.685, p = 0.420), GROUP (F1,16 = 0.144, p = 0.709), or interaction between TEST and GROUP (F1,16 = 2.956, p = 0.105) on HR∞. Finally, there was no significant main effect for GROUP (F1,16 = 0.110, p = 0.745) or interaction between TEST and GROUP (F1,16 = 1.984, p = 0.178) on HRamp, yet there was a significant main effect of TEST (F1,16 = 7.004, p = 0.018); specifically, the 3MT produced significantly lower HRamp compared to the MAX-TM. These results indicated that when comparing the FF to CON groups, both groups had similar HRRτ and HR∞ between the MAX-TM and 3MT tests, and both groups had significantly lower HRamp after the 3MT when compared to the MAX-TM. The LA− results indicated no significant interaction between TEST and GROUP on LA−POST (F1,16 = 0.000, p = 0.992). A significant main effect of TEST (F1,16 = 20.020, p < 0.001) and GROUP (F1,16 = 6.119, p = 0.025) was found on LA−POST; specifically, the FF group had significantly lower post-test blood lactate concentration after both the MAX-TM and 3MT tests than the CON group, yet both groups had significantly higher post-test blood lactate after the 3MT compared to the MAX-TM.
When the groups were separated by fitness level (FITNESS x TEST), the results of the two-way repeated measures ANOVAs indicated that there was no significant main effect of TEST (F1,16 = 0.093, p = 0.764), FITNESS (F1,16 = 0.024, p = 0.879), or interaction between TEST and FITNESS (F1,16 = 2.546, p = 0.130) on HRRτ. There was no significant main effect of TEST (F1,16 = 0.721, p = 0.408) or FITNESS (F1,16 = 3.444, p = 0.082) or significant FITNESS x TEST interaction (F1,16 = 3.936, p = 0.065) on HR∞. Regarding the HRamp, there was a significant main effect of TEST (F1,16 = 7.661, p = 0.014); specifically, the HRamp after the 3MT was significantly lower than the HRamp after the MAX-TM. There was no significant main effect of FITNESS (F1,16 = 3.326, p = 0.087) or interaction between TEST and FITNESS (F1,16 = 3.669, p = 0.073). The LA− results indicated no significant interaction between TEST and FITNESS on LA−POST (F1,16 = 1.312, p = 0.269) or main effect for FITNESS (F1,16 = 2.293, p = 0.149). A significant main effect of TEST was found on LA−POST (F1,16 = 21.662, p < 0.001), indicating that both the HF and LF groups had significantly higher post-test blood lactate concentration after the 3MT when compared to the MAX-TM.

Correlation Results

Correlation results indicated strong positive relationships between VO2PEAK and GRIPNORM (r = 0.668, p = 0.002) and CV (r = 0.583, p = 0.011). Furthermore, strong significant negative relationships were found between BF% and CV (r = −0.733, p = 0.001) and GRIPNORM (r = −0.635, p = 0.005), while there was a very strong significant negative relationship between BF% and VO2PEAK (r = −0.890, p < 0.001). There were no significant relationships found between BF%, VO2PEAK, GRIPNORM, CV, and D′ and HRR parameters from the MAX-TM or 3MT.

4. Discussion

The purpose of the current investigation was to investigate HRR between a maximal treadmill test and a three-minute all-out sprint test in a sample of active-duty firefighters and a non-firefighter control group. The results of this study indicated that when comparing firefighters to a recreationally active control, the HRR profiles may not significantly differ from each other. A non-significant trend emerged that higher-fit (HF) individuals were able to recover to a lower percentage of maximal HR after both tests. Furthermore, the significant interaction between test type and fitness level suggests that, together, cardiorespiratory fitness and exercise intensity can impact post-exercise ANS recovery [36].
Cardiorespiratory fitness has been indicated to be a key component of health and performance for firefighters due to high cardiorespiratory strain and risk of SCD from job demands [7,26,37]. It is possible that higher cardiorespiratory fitness may positively impact parasympathetic nervous system (PSNS) reactivation after severe-intensity exercise [8,38]. The rapid onset of intensity demands during the 3MT may better represent the response to an emergency call for firefighters; thus, aerobic capacity may benefit post-call HRR in addition to meeting job demands during the task. Furthermore, the relationships between VO2PEAK, CV, BF%, and GRIPNORM suggest that improved body composition and muscular qualities of force output and oxygen utilization may positively impact an individual’s highest sustainable work rate. Prior work has suggested that VO2PEAK, BF%, and grip strength are related to performance of firefighting tasks [39,40,41], and these results suggest that CV may also be an important performance marker to consider.

4.1. Post-Exercise ANS Recovery

Post-exercise HRR includes a fast and slow phase. The fast phase is characterized by reactivation of the vagal nerve and PSNS activity, while the slow phase encompasses the clearance of metabolites (e.g., LA−, H+) and gradual withdrawal of the sympathetic nervous system (SNS) [42]. In the current study, no significant relationships were found between body composition, handgrip, VO2PEAK, CV, and D′ and the HRR parameters of the MAX-TM or 3MT. However, prior work has found significant relationships that suggest that aerobic capacity may be a key component of ANS recovery. Cornell et al. [32] examined the impact of cardiorespiratory fitness, based on the National Fire Protection Association (NFPA)-recommended 42.0 mL/kg/min, on HRR parameters after a maximal treadmill test in active-duty firefighters (N = 37, 39.14 ± 8.91 yrs). Results of that investigation indicated that when controlling for age and body mass index (BMI, kg/m2), a higher VO2PEAK was significantly related to lower HR∞, greater HRamp, and faster HRRτ. In the present study, the normalization of HR values as a percentage of MHR was used to control for maximal HR differences due to age. An interesting finding that emerged from the current study was that the HRamp was found to be significantly lower after the 3MT compared to the MAX-TM in both GROUP and FITNESS comparisons. This would suggest that task intensity may play a key role in ANS recovery.
The all-out nature of the 3MT requires production of near-maximal sprint efforts at the onset of the test, which requires recruitment of large motor units and glycolytic type II muscle fibers to produce maximal power [43]. Furthermore, the 3MT is designed to deplete finite anaerobic stores within the first 150 s of the test, and it has been indicated that a significant build-up of metabolites can occur during the short time frame of the test [12,21,44]. The results of this study support that regardless of whether groups were separated by GROUP or FITNESS, the post-exercise blood lactate concentrations after the 3MT were significantly higher than after the MAX-TM test. Thus, it is possible that the 3MT demanded a greater build-up of metabolites compared to the MAX-TM, which impaired vagal reactivation, created a greater demand for metabolite clearance, and inhibited SNS withdrawal, which is associated with the slow phase of HRR [36,45]. A similar finding was reported by Ebersole et al. [46], who investigated differences in ANS recovery via HRR and HRV in active-duty firefighters (N = 37, 39.00 ± 9.00 yrs) after a maximal treadmill test and a submaximal step test. The findings indicated that firefighters experienced significantly higher HR∞ and slower HRRτ after a maximal treadmill task compared to a submaximal step test, further indicating that task intensity may be a key consideration for recovery strategies in the fire service.
It has been recently reported that emergency calls in the fire service may have differing magnitudes of intensity. Marciniak et al. [26] investigated the cardiovascular workload of emergency calls using Edward’s Training Impulse (eTRIMP, AU). The findings indicated that emergency calls that included fire suppression tasks demanded a significantly higher (p < 0.001) cardiovascular workload (74.33 ± 59.84 AU) than fire calls without fire suppression (8.48 ± 7.28 AU) and medical calls (7.67 ± 6.31 AU). Furthermore, the 3MT provides a method to account for rapid acceleration and deceleration and changes of motion that are not captured in a treadmill assessment yet may be important for occupational performance [23]. These results would suggest that tasks that require a rapid onset of severe-intensity activity, even if shorter in duration, may have greater recovery needs that allow for the clearance of metabolites and restore parasympathetic control of the ANS. An area for future research is to investigate HRR following job-specific tasks and identify the role of task intensity and physical fitness in post-task recovery.

4.2. CV as a Performance Measure

This study provides a novel examination of CV using a shuttle-sprint 3MT in active-duty firefighters. The results suggest that a higher VO2PEAK is related to a higher CV, which agrees with prior studies that have found moderate to strong significant positive relationships between CV and VO2PEAK in sport–athlete populations [14,24,47]. This is a critical finding for firefighters, as cardiorespiratory fitness has been indicated to be related to greater efficiency in time to complete firefighting tasks and use of air from a self-contained breathing apparatus during non-burn and fire suppression tasks [5,6,41,48]. The CV represents the upper limit of the heavy-intensity domain [9,20] or the highest intensity at which primarily oxidative pathways are used for energy production [10,11,13]. While research on CV in occupational populations is limited, Hoffman et al. [18] reported strong negative relationships between CV and time to complete running-based tasks during a special operations tactical assessment. It is possible that measurement of CV can provide insight into the overall work capacity of firefighters, as it may be closely linked with aerobic capacity and, more specifically, the maximal rate of work where oxidative metabolism is used for energy.
The results from this study also agree with prior work that has suggested that body composition, muscular strength, and cardiorespiratory fitness are impactful for firefighter performance. Numerous studies have indicated that a lower BF% [40,49], higher handgrip strength [40,49], and greater cardiorespiratory fitness [39,50,51,52,53] are related to a faster time to complete firefighting tasks in simulated environments. Higher BF% has also been linked to lower VO2PEAK [41,54,55] and a faster time to exhaustion on a maximal treadmill test in firefighters, particularly at intensities greater than 85% of MHR [56]. A proposed mechanism for this has been that at higher intensities of activity, body fat may serve as a non-functional load that increases the relative neuromuscular demand of a task [57]. This mechanism may help to explain the negative relationship between BF% and CV in this study, as prior work in firefighters has also indicated that higher BF% may be related to greater neuromuscular fatiguability in a stair climb task [58]. While a study by Cornell et al. [59] identified that higher BF% was significantly related to a lower change in HR during the first minute after completing a five-minute submaximal step test, no significant relationships emerged from these results between CV, BF%, and muscular strength and HRR parameters. This may suggest that maintaining an adequate body composition, muscular strength, and CV can be impactful for cardiorespiratory fitness and work capacity, specifically regarding the upper limit of oxygen utilization for energy. However, more work may be needed to identify the impact of these factors on recovery from maximal exertion.
The results of this investigation provide support for the use of the 3MT as a potential field test for firefighters. The 3MT has been used as a field test in sport–athlete and tactical or occupational populations, with a rationale being that CV and D′ provide unique metrics to design training programs to improve aerobic or anaerobic abilities [13,14,18,23,43,60,61]. Measurements of the CV and D′ measures have been used to design HIIT training programs, with a consistent finding that the CV may be more responsive to training [60,61,62]. Given the strong significant relationships between CV, BF%, and GRIPNORM and VO2PEAK, a promising area for further research is to investigate the ability of these parameters to accurately predict VO2PEAK in active-duty firefighters, as the equipment to test these are portable and can allow for multiple individuals to test at the same time [23,24]. A model to predict VO2PEAK from CV and sex was reported by Dexheimer et al. [24], yet they used a flat-track 3MT protocol instead of a shuttle-sprint protocol. The sample size of the current investigation did not allow for a regression model to be run with adequate statistical power, but further research should be conducted to identify if prediction is viable. An additional area for future research is to examine the relationships between CV and D′ derived from a 3MT and other commonly used measures of firefighter performance, such as time to complete sequential tasks or air management, which have both been linked to cardiorespiratory fitness [5,6,48].

4.3. Limitations

The sample sizes of the CON and FF groups in this study were small, which may limit the overall generalizability. However, this study serves as a pilot investigation to establish application of the 3MT to the fire service. The CON group was required to have a running background, yet none were true running-based athletes, so the applicability of these results may be limited to running-based sports. While it has been reported that approximately 4.4% of all US firefighters are female [63], the FF group in this study did not include any females. Future researchers should seek to replicate these protocols with a sample of male and female firefighters. Compared to age-based normative values, this sample had a relatively high VO2PEAK and low BF%, which may not be the case in all occupational settings. As such, an area for future research includes development of submaximal testing of CV that could be used for less-trained populations or those with other comorbidities, such as obesity, which has a high prevalence in the fire service [64,65]. In addition, the demands of firefighting extend beyond physical ones, and future investigations should consider concurrently examining psychological factors related to HRR.

5. Conclusions

The novelty of this study is the comparison of HRR between a maximal treadmill test and a 3MT in active-duty firefighters. Cardiorespiratory fitness has been consistently indicated to be an important factor in the efficiency of performing firefighting tasks and recovery from exercise. These results support that aerobic capacity may be beneficial to critical velocity, as measured through the 3MT. However, full autonomic nervous system recovery may be impacted by task intensity and build-up of metabolic byproducts of severe-intensity work. Measurement of critical velocity using a 3MT may be a useful, non-invasive field test for firefighters that can provide insight into aerobic and anaerobic components of work capacity.

Author Contributions

Conceptualization, B.J.M. and K.T.E.; methodology, B.J.M. and K.T.E.; formal analysis, B.J.M.; data curation, B.J.M. and K.T.E.; investigation, B.J.M., K.T.E., S.D.B. and N.T.E.; writing—original draft preparation, B.J.M. and K.T.E.; writing—review and editing, S.D.B. and N.T.E. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki and approved by the Institutional Review Board (or Ethics Committee) of the University of Wisconsin-Milwaukee (Protocol Number 23.058, approved on 1 December 2022).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy reasons.

Acknowledgments

We would like to acknowledge the City of Milwaukee Fire Department for supporting this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Diagram of the three-minute all-out 30 m shuttle sprint test (3MT).
Figure 1. Diagram of the three-minute all-out 30 m shuttle sprint test (3MT).
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Figure 2. Plot of mean normalized 30 s average heart rate (HR) recovery (% of HR max) during the five-minute post-test recovery period. Plot (A) represents the CON (◯) group versus FF (+) post-MAX-TM test. Plot (B) represents the CON group versus FF group post-3MT. Plot (C) represents the HF (Δ) versus the LF (♢) group post-MAX-TM test. Plot (D) represents the HF group versus the LF group post-3MT.
Figure 2. Plot of mean normalized 30 s average heart rate (HR) recovery (% of HR max) during the five-minute post-test recovery period. Plot (A) represents the CON (◯) group versus FF (+) post-MAX-TM test. Plot (B) represents the CON group versus FF group post-3MT. Plot (C) represents the HF (Δ) versus the LF (♢) group post-MAX-TM test. Plot (D) represents the HF group versus the LF group post-3MT.
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Table 1. Means ± SD results for the FF and CON groups from anthropometric, normalized handgrip (GRIPNORM), MAX-TM, and 3MT test results. a = significantly different from CON group (p < 0.05).
Table 1. Means ± SD results for the FF and CON groups from anthropometric, normalized handgrip (GRIPNORM), MAX-TM, and 3MT test results. a = significantly different from CON group (p < 0.05).
FFCON
Age (yrs)34.00 ± 7.43 a24.56 ± 4.48
Height (m)1.79 ± 0.021.78 ± 0.14
Weight (kg)91.62 ± 16.8183.57 ± 14.94
Percent Body Fat (%)16.27 ± 7.2513.07 ± 4.33
VO2PEAK (mL/kg/min)46.27 ± 8.9649.27 ± 4.72
GRIPNORM (kg/kg)1.13 ± 0.251.36 ± 0.29
CV (m/s)3.05 ± 0.463.20 ± 0.51
D′ (m)103.75 ± 44.23127.65 ± 51.76
Table 2. Means ± SD for baseline heart rate (HRREST), peak heart rate (HRPEAK), baseline blood lactate (LA−PRE), and post-test blood lactate (LA−POST) for FF and CON participants in the MAX-TM and 3MT tests. * = significantly higher than the MAX-TM (p < 0.05).
Table 2. Means ± SD for baseline heart rate (HRREST), peak heart rate (HRPEAK), baseline blood lactate (LA−PRE), and post-test blood lactate (LA−POST) for FF and CON participants in the MAX-TM and 3MT tests. * = significantly higher than the MAX-TM (p < 0.05).
MAX-TM3MT
FFCONFFCON
HRREST (bpm)75.56 ± 12.0371.56 ± 9.7682.56 ± 11.1682.00 ± 13.32
HRPEAK (bpm)183.22 ± 9.30193.00 ± 6.42181.89 ± 4.62189.00 ± 8.23
LA−PRE (mmol/L)1.37 ± 0.533.32 ± 3.814.47 ± 1.296.05 ± 2.38
LA−POST (mmol/L)9.86 ± 2.8212.44 ± 2.0312.37 ± 2.18 *14.94 ± 2.90 *
Table 3. Means ± SD for HRR parameters post-test from the MAX-TM and 3MT Tests. The table presents the data separated into the two groupings that were used for the two-way repeated measures ANOVAs: by population (FF versus CON) and by high (HF) and low (LF) fitness levels. All HRR parameters were calculated based on normalized post-test heart rate (% MHR). ‡ = MAX-TM significantly different from 3MT (p < 0.05).
Table 3. Means ± SD for HRR parameters post-test from the MAX-TM and 3MT Tests. The table presents the data separated into the two groupings that were used for the two-way repeated measures ANOVAs: by population (FF versus CON) and by high (HF) and low (LF) fitness levels. All HRR parameters were calculated based on normalized post-test heart rate (% MHR). ‡ = MAX-TM significantly different from 3MT (p < 0.05).
MAX-TM3MT
FFCONFFCON
HR Decay (HRRτ, % MHR)139.46 ± 54.76143.84 ± 29.75126.08 ± 38.63165.57 ± 62.33
HR Asymptote (HR∞, % MHR)48.39 ± 5.7149.97 ± 8.1952.39 ± 4.5748.57 ± 9.04
HR Amplitude (HRamp, % MHR) ‡50.56 ± 5.0449.05 ± 8.1943.56 ± 4.8146.91 ± 8.74
HFLFHFLF
HR Decay (HRRτ, % MHR)151.16 ± 64.31132.14 ± 26.66133.46 ± 42.11158.20 ± 64.31
HR Asymptote (HR∞, % MHR)45.16 ± 7.5453.20 ± 2.8049.49 ± 6.8851.47 ± 7.83
HR Amplitude (HRamp, % MHR) ‡53.67 ± 7.1845.92 ± 2.8545.97 ± 4.9244.51 ± 8.97
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Mendelson, B.J.; Ebersole, K.T.; Brau, S.D.; Ebersole, N.T. Exploration of Heart Rate Recovery After Maximal Treadmill and Three-Minute All-Out Shuttle Tests in Firefighters. Fire 2025, 8, 20. https://doi.org/10.3390/fire8010020

AMA Style

Mendelson BJ, Ebersole KT, Brau SD, Ebersole NT. Exploration of Heart Rate Recovery After Maximal Treadmill and Three-Minute All-Out Shuttle Tests in Firefighters. Fire. 2025; 8(1):20. https://doi.org/10.3390/fire8010020

Chicago/Turabian Style

Mendelson, Benjamin J., Kyle T. Ebersole, Scott D. Brau, and Nathan T. Ebersole. 2025. "Exploration of Heart Rate Recovery After Maximal Treadmill and Three-Minute All-Out Shuttle Tests in Firefighters" Fire 8, no. 1: 20. https://doi.org/10.3390/fire8010020

APA Style

Mendelson, B. J., Ebersole, K. T., Brau, S. D., & Ebersole, N. T. (2025). Exploration of Heart Rate Recovery After Maximal Treadmill and Three-Minute All-Out Shuttle Tests in Firefighters. Fire, 8(1), 20. https://doi.org/10.3390/fire8010020

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