Utilizing multimodal high-intensity interval training for a firefighter training academy during the COVID-19 pandemic

Abstract

BACKGROUND:

Firefighters typically undergo a 16–24-week training academy during which they perform a variety of traditional exercise programs such as cardiovascular, resistance, and concurrent training. Because of limited facility access, some fire departments seek alternative exercise programs, such as multimodal high-intensity interval training (MM-HIIT), which essentially combines resistance and interval training.

OBJECTIVE:

The primary purpose of this study was to assess the effect of MM-HIIT on body composition and physical fitness in firefighter recruits who completed a training academy during the coronavirus (COVID-19) pandemic. A secondary purpose was to compare the effects of MM-HIIT to previous training academies that implemented traditional exercise programs.

METHODS:

Healthy and recreationally-trained recruits (n = 12) participated in 2-3 days/week of MM-HIIT for 12 weeks and had several components of body composition and physical fitness measured before and after the program. Because of COVID-19-related gym closures, all MM-HIIT sessions were performed outdoors at a fire station with minimal equipment. These data were retroactively compared to a control group (CG) that previously completed training academies with traditional exercise programs.

RESULTS:

Subjects in the MM-HIIT group significantly improved several components of body composition and fitness, including fat mass, fat-free mass, body fat percentage, aerobic capacity, and muscular endurance. Moreover, there were no significant differences for any dependent variable when MM-HIIT was compared to the CG.

CONCLUSION:

These results suggest that MM-HIIT may serve as an effective substitute for traditional concurrent training paradigms that are typically used for firefighter academies.

Keywords

Tactical strength and conditioning time-efficient training minimum equipment training circuit training

1 Introduction

Firefighting is a physically demanding occupation that requires regular exercise training to ensure that fitness levels are sufficient to match job demands [1 –5]. The acute physiological effects of firefighting are well documented, as research has indicated that various firefighting operations stimulate high blood lactate concentrations (6–13 mmol·L^–1) [6], heart rates (188–190 bpm) [7], and oxygen consumption (33.6–49.0 ml·kg^–1·min^–1) [8]. Therefore, it is logical that those with higher levels of fitness tend to perform better during simulated fire suppression drills [4] and are at significantly less risk for cardiovascular disease [9] and musculoskeletal injuries [10]. Together, this information implies that physical preparation, and the resulting increase in fitness, may benefit the health, performance, and survival of firefighters. Because fire ground tasks require utilization of the phosphagen (e.g., ladder raise), glycolytic (e.g., load carriage), and oxidative (e.g., stair climb) energy systems, it is recommended that physical preparation for firefighters include a blend of cardiovascular, circuit, and resistance training [1, 2].

In most cases, the physical preparation process for firefighters begins with a 16–24-week training academy, during which the recruits learn basic occupational tasks (e.g., hose advance, victim rescue, and fire suppression) while performing a variety of exercise programs [11 –17]. Previous investigations into firefighter academies have indicated that normal academy training (i.e., no physical preparation intervention) [14], traditional resistance training (RT) [16], high-intensity functional training (HIFT) [11, 15], and concurrent training [12 , 17] all improve the physical fitness of firefighter recruits. Despite these positive effects, some firefighter academies do not provide physical preparation for their recruits [14, 18], which likely stems from a lack of time and resources. In turn, it is critical for fitness professionals to design training interventions that can be performed with minimal equipment and without gym access.

Serving as a hybrid between resistance and interval training, HIFT involves multi-planar, whole-body resistance exercises that are performed for specific set durations (e.g., as many repetitions in 60 seconds) or session durations (e.g., as many rounds as possible for 15 minutes) [19]. The primary advantage of HIFT is that it incorporates several ‘functional’ patterns of movement (e.g., pushing, pulling, and squatting), which allows it to fit the specific movement demands for a variety of populations [19, 20]. For instance, Sempf & Thienes [20] argued that HIFT should be included in the physical preparation of firefighters because it may improve aerobic fitness, anaerobic capacity, and muscular strength in a time-efficient manner. Provided proper exercise selection, HIFT may also involve training patterns of movement that emulate job-specific tasks such as forcible entry, crawling in a confined space, and victim rescue [20]. It is noteworthy that Sempf & Thienes’ description of HIFT as a training style, with prescribed work-to-rest ratios (e.g., 30:15 s), departs from the official definition of HIFT, as Feito et al. [19] specifically wrote that HIFT does not include prescribed rest intervals. Although similar to HIFT, the work-to-rest ratio style of RT is better characterized as multimodal high-intensity interval training (MM-HIIT), which was first described by Buckley et al. [21]. Regardless of semantic differences, HIFT and MM-HIIT both require minimal equipment and time [19 –21], and may serve as effective styles of physical preparation that can be conducted at a fire station or drill yard when a gym is not accessible.

The onset of the coronavirus (COVID-19) global pandemic, and subsequent lockdowns associated with it, caused several commercial gyms and wellness centers to close in order to follow social distancing guidelines [22]. In turn, researchers recommended that fitness enthusiasts [23] and athletes [24] partake in convenient forms of physical activity such as aerobic, bodyweight, and low-intensity RT either outdoors or inside of a home gym. Data from a recent survey confirmed that recreational lifters followed this advice, as 82% of respondents indicated that they maintained their participation in an RT program during the COVID-19 pandemic by utilizing high-repetition and/or bodyweight training [25]. Considering the unusual circumstances, it was necessary for strength and conditioning professionals to develop dynamic and creative RT programs for their tactical athletes by using alternative training methods, such as MM-HIIT or HIFT, during the pandemic.

To our knowledge, there is no available research regarding the physical preparation of recruits while navigating the complications of the COVID-19 pandemic (e.g., gym closures and social distancing). Hence, the primary purpose of the current study was to assess the effect of a 12-week MM-HIIT program on various measurements of physical fitness in a population of recruits during a training academy. Uniquely, training took place outdoors at the academy drill yard, with minimal equipment, and while maintaining social distance (greater than six feet apart). We hypothesized that body mass (BM), fat mass (FM), and body fat percentage (BF%) would significantly decrease while fat-free mass (FFM), aerobic capacity, back flexibility, muscular strength, and muscular endurance would significantly increase following the 12-week MM-HIIT program. A secondary purpose of the current study was to assess the effect of MM-HIIT on physical fitness compared to previous firefighter academies that were able to use traditional training approaches. We hypothesized that change scores for physical fitness would not significantly differ between MM-HIIT and traditional training.

2 Materials and methods

2.1 Experimental design

This was an observational, cohort design that assessed the effect of a 12-week MM-HIIT program on the body composition and physical fitness of 12 recreationally-trained recruits. Subjects visited a corporate wellness center for physical fitness testing on two occasions: before (pre) and after (post) the 12-week MM-HIIT intervention. The data collected during these visits included BM, FM, FFM, BF%, estimated maximal oxygen uptake (est-VO₂max), sit-and-reach, bicep strength, and push-up repetitions to failure. Due to the COVID-19 pandemic, all MM-HIIT sessions took place outdoors at the academy drill yard with minimal equipment and subjects spaced greater than six feet apart. This training intervention took place in the Fall of 2021 between the months of August and November. Because the subjects were training to become professional firefighters, they also performed daily physical activity for occupational tasks (e.g., rucking for wildland firefighting) and unplanned bouts of exercise (e.g., pull-ups for disciplinary reasons).

2.2 Subjects

Twelve recreationally-trained, healthy recruits (11 males, 1 female) who were enrolled in a training academy volunteered for this study (experimental group; EG), and the characteristics for the subjects are displayed in Table 1. As a research team, we were not involved in the recruitment process, as the local fire department was completely in charge of how many people were hired to partake in the training academy. Thus, we were not able to conduct an a priori power analysis for any of the dependent variables measured in this study. The subjects completed a physical activity survey during their first laboratory visit, which revealed that they were currently participating in cardiorespiratory exercise (15–90 min; 3–5 days/week) and RT (30–60 min; 3-4 days/week). Each subject received medical clearance from a physician which indicated that they were free of cardiovascular disease, metabolic disease, and had no orthopedic injuries that would limit their ability to perform physical activity. Subjects were informed about the potential risks of the MM-HIIT program, and they signed an informed consent to allow the use of pertinent data for potential publication. This protocol was approved by the Human Subjects Research Review Board (Protocol code 22-06X, approved 03 March 2022) and was done in accord with the Declaration of Helsinki of 1964 and its later amendments.

Table 1
Baseline physical characteristics for the experimental group (EG) that underwent 12 weeks of multi-modal high-intensity interval training and the control group (CG) that underwent 12 weeks of traditional physical preparation that is typically implemented during training academies. Data are displayed as means±standard deviation

EG (n = 12) CG (n = 12) p-value Cohen’s d 95% CI

Age (years) 27.6±7.2 25.6±6.3 0.48 0.30; small [–0.51, 1.10]

Height (cm) 177.3±6.9 176.0±5.8 0.63 0.20; small [–0.60, 1.00]

Body mass (kg) 79.9±11.4 79.2±6.3 0.84 0.08; small [–0.72, 0.88]

BF% 16.8±6.1 14.8±4.6 0.37 0.38; small [–0.44, 1.18]

est-VO₂max (ml·kg^–1·min^–1) 48.6±6.2 48.9±7.0 0.91 0.05; small [–0.85, 0.76]

	EG (n = 12)	CG (n = 12)	p-value	Cohen’s d	95% CI
Age (years)	27.6±7.2	25.6±6.3	0.48	0.30; small	[–0.51, 1.10]
Height (cm)	177.3±6.9	176.0±5.8	0.63	0.20; small	[–0.60, 1.00]
Body mass (kg)	79.9±11.4	79.2±6.3	0.84	0.08; small	[–0.72, 0.88]
BF%	16.8±6.1	14.8±4.6	0.37	0.38; small	[–0.44, 1.18]
est-VO₂max (ml·kg^–1·min^–1)	48.6±6.2	48.9±7.0	0.91	0.05; small	[–0.85, 0.76]

EG and CG were matched based on age, height, body mass, body fat percentage (BF%), and estimated relative maximal oxygen consumption (est-VO₂max). CI = Confidence interval.

2.3 Control group

To compare the effect of MM-HIIT to previous training academies, we sampled a control group (CG) from a data set that contained 50+ individuals who were previously enrolled in our traditional training academies that were not confined by COVID-19-related restrictions. One researcher, who was blinded to names and corresponding data, followed recently published methodology by Chizewski et al. [11] and identified est-VO₂max-matched controls for subjects in the EG. If multiple matches were found, they were further matched based on their age, height, weight, and BF%. As shown in Table 1, there were no significant differences between EG and CG at baseline. For context, those in the CG participated in a 12-week exercise program during a training academy between 2010 and 2019, for which traditional physical preparation was implemented at a corporate wellness center with a variety of modalities: barbells, dumbbells, machines, battle ropes, kettlebells, and cardiovascular machines (e.g., stationary bikes). Like previous training academy studies [12 , 17], subjects in the CG performed a combination of resistance and cardiovascular exercise three days per week for approximately 60 minutes per session.

2.4 Physical fitness testing

To measure physical fitness, we used the Microfit Fas-2 system (Microfit, California, USA) because it was approved by the local fire department labor union and had been used to collect data for past training academies and professional firefighters at this specific fire department. Like Chizewski et al. [11], two researchers conducted the fitness assessments throughout the study, and the same tester performed the pre- and post-MM-HIIT assessments on the same group of subjects (i.e., 6 and 6) to maintain congruity between the tester and the subject. Furthermore, to ensure the consistency of data collection across pre- and post-MM-HIIT assessments, testing took place at the same time of day (7–9 am), and the subjects followed the same pre-test guidelines for both assessments: No strenuous physical activity for 48 hours, abstain from alcohol for 24 hours, drink ample fluids for 24 hours before the test to ensure normal hydration, avoid caffeine and nicotine for 3 hours, consume a small meal/snack 1-2 hours before, and wear loose fitting exercise clothing that they would typically train in. The specific test battery performed for this study, which was also used for the CG during their training academies, was as follows:

2.4.1 3-site skinfold

Before the skinfold (SKF) measurement, subjects stood on the Microfit floor scale wearing their socks and gym clothes to provide a measurement for BM. When measuring the male subjects, the researcher used their forefinger and thumb to pinch the subcutaneous fat of the chest, abdomen, and thigh [26]. In contrast, the triceps, suprailliac, and thigh [27] were measured for the female subject. Measurements were taken in duplicate, and the average of the skinfolds was used to estimate BF%. A third measurement was taken if the difference between duplicate measurements was greater than 2 mm [17]. The value of BF% was used to calculate FM and FFM for each subject. Pilot data from our lab revealed high interrater reliability between the two testers for this study (r = 0.95).

2.4.2 Submaximal cycle ergometer test

Following a self-selected 2-3-minute warm-up, subjects were fitted with a heart rate (HR) monitor (Polar, H10, California, USA) that was worn around the chest. Next, seat height was adjusted so that the subject had approximately 5–10° of knee flexion at the bottom of each pedal stroke. Although subjects were instructed to maintain a cadence of 50–60 RPM, an electronically-braked cycle ergometer (Monark, LC4, Varberg, Sweden) was used to accommodate for any variation in pedaling cadence (i.e., power was constant during each stage). The initial stage was performed at 50 Watts (W), and workload was increased by 25–50 W, as stages progressed. Each stage lasted 2-3 minutes to allow HR to reach steady state, and the subject completed as many stages as necessary to achieve ∼80% of estimated maximal HR (220-age). The researchers used two HRs between 110–150 bpm to extrapolate to an est-VO₂max, and the test-retest reliability coefficient for this method was r = 0.95 [17]. To accommodate for the limitations of submaximal VO₂max testing with HR data, we also recorded and analyzed the highest workload achieved during the test (High-WL) and the steady-state HR associated with this workload (High-HR).

2.4.3 Sit-and-reach

Subjects sat upright with their legs fully extended so their hamstrings were flat on the floor. Having removed their shoes, the subjects placed their feet approximately six inches apart with their heels resting against the sit-and-reach apparatus. They were instructed to point their toes upward, so their feet were in a neutral position. While keeping their legs extended, and with one hand placed over the other, the subjects were instructed to lean forward and reach as far as possible. The electronic device required the subject to hold their end range of motion for four seconds [28]. Subjects were given three attempts, and the highest measurement was recorded for analysis. The test-retest reliability coefficient for this method was r = 0.94 [17].

2.4.4 Bicep strength test

Subjects stood on the Microfit scale and grasped the biceps curl bar with a supinated grip so their hands were approximately as wide as their shoulders [28]. The length of the tether was adjusted so that the subject’s elbows were flexed at approximately 90°. Further, the subject was instructed to stand tall with a slight bend in their knees and with their eyes straight ahead. When ready, the subject performed a maximal effort isometric contraction for ∼4 seconds, and peak force was recorded by the Microfit system. They were given three attempts with 30 seconds of rest between them, and the highest measurement was recorded for analysis. Pilot data from our lab indicated a high test-retest reliability coefficient for this test (r = 0.96).

2.4.5 Push-up test

To assess local muscular endurance, subjects completed as many push-ups as possible until failure or volitional termination. For this assessment, the male subjects began in a prone position with their feet together, back straight, elbows fully extended, and hands slightly wider than their shoulders. The female subject adopted a similar starting position except with her knees on the floor and legs together. The subjects completed a successful push-up by descending until their sternum contacted a four-inch foam block and ascending until their elbows were almost fully extended (i.e., slight bend). Going at their own pace, subjects completed full-range-of-motion push-ups until they voluntarily stopped, reached concentric muscular failure, could not maintain proper form, or rested for longer than approximately one second between consecutive repetitions. The test-retest reliability coefficient for this method was r = 0.93 [17].

2.5 Physical training program

MM-HIIT took place 2-3 days per week at the academy drill yard, and each training session was separated by 48–96 hours. Each session of MM-HIIT was facilitated by two researchers who both hold certifications from the National Strength and Conditioning Association (e.g., Certified Strength and Conditioning Specialist) and American College of Sports Medicine (e.g., Certified Exercise Physiologist). Before the training session began, subjects completed a 5–10-minute dynamic warm-up that consisted of multi-planar mobility, stability, and calisthenics as previously described [20]. As shown in Table 2, subjects completed 2-3 rounds of 12 total-body exercises with various work-to-rest ratios (e.g., 30:15 s) and volume was gradually decreased as the week progressed. During each working set, subjects were encouraged to complete as many repetitions as possible, so their Ratings of Perceived Exertion (RPE) was between 8 and 9 at the end of each set [29]. However, for weeks 4, 8, and 12, they were instructed to finish each set with an RPE between 4 and 6, so that much less fatigue was incurred during the set. Exercises were arranged in clusters of three with a general grouping of total-body, lower-body, and upper-body movements. Prescribed rounds (e.g., 2-3) were performed consecutively for each cluster before moving on to the next cluster, and 60 seconds of rest was provided during these transitions. After MM-HIIT was completed, subjects performed 5–10 minutes of static stretching that focused primarily on the shoulder, hip, and ankle joints. Due to training academy commitments and expectations, the subjects also performed two weekly sessions of moderate intensity continuous training (MICT), during which they jogged for 30–60 minutes as a team.

Table 2
A visual display of the multimodal high-intensity interval (MM-HIIT) training program

Week Monday Wednesday Friday

1 2 rounds 2 rounds 2 rounds Cluster #1

30:15 s 25:15 s 20:15 s High knees in place

2 2 rounds 2 rounds 2 rounds Versa log zercher squat

35:15 s 30:15 s 25:15 s Dumbbell bent over row

3 2 rounds 2 rounds 2 rounds Cluster #2

40:15 s 35:15 s 30:15 s Medicine ball slam

4 3 rounds 3 rounds 3 rounds Dumbbell lateral lunge

20:15 s 20:15 s 20:15 s Push up w/ shoulder tap

5 2 rounds 2 rounds 2 rounds Cluster #3

35:10 s 30:10 s 25:10 s Jumping jacks

6 2 rounds 2 rounds 2 rounds Versa log single leg RDL

40:10 s 35:10 s 30:15 s Dumbbell push press

7 2 rounds 2 rounds 2 rounds Cluster #4

45:10 s 40:10 s 35:10 s Medicine ball chop/lift

8 3 rounds 3 rounds 3 rounds Versa log burpee + deadlift

20:10 s 20:10 s 20:10 s Dumbbell biceps curl

9 2 rounds 2 rounds 2 rounds

40:10 s 35:10 s 30:10 s

10 2 rounds 2 rounds 2 rounds

45:10 s 40:10 s 35:10 s

11 2 rounds 2 rounds 2 rounds

50:10 s 45:10 s 40:10 s

12 3 rounds 3 rounds 3 rounds

20:10 s 20:10 s 20:10 s

Week	Monday	Wednesday	Friday
1	2 rounds	2 rounds	2 rounds	Cluster #1
	30:15 s	25:15 s	20:15 s	High knees in place
2	2 rounds	2 rounds	2 rounds	Versa log zercher squat
	35:15 s	30:15 s	25:15 s	Dumbbell bent over row
3	2 rounds	2 rounds	2 rounds	Cluster #2
	40:15 s	35:15 s	30:15 s	Medicine ball slam
4	3 rounds	3 rounds	3 rounds	Dumbbell lateral lunge
	20:15 s	20:15 s	20:15 s	Push up w/ shoulder tap
5	2 rounds	2 rounds	2 rounds	Cluster #3
	35:10 s	30:10 s	25:10 s	Jumping jacks
6	2 rounds	2 rounds	2 rounds	Versa log single leg RDL
	40:10 s	35:10 s	30:15 s	Dumbbell push press
7	2 rounds	2 rounds	2 rounds	Cluster #4
	45:10 s	40:10 s	35:10 s	Medicine ball chop/lift
8	3 rounds	3 rounds	3 rounds	Versa log burpee + deadlift
	20:10 s	20:10 s	20:10 s	Dumbbell biceps curl
9	2 rounds	2 rounds	2 rounds
	40:10 s	35:10 s	30:10 s
10	2 rounds	2 rounds	2 rounds
	45:10 s	40:10 s	35:10 s
11	2 rounds	2 rounds	2 rounds
	50:10 s	45:10 s	40:10 s
12	3 rounds	3 rounds	3 rounds
	20:10 s	20:10 s	20:10 s

Weeks 4, 8, and 12 served as recovery weeks where volume and effort were intentionally reduced (left). Also, an example of a MM-HIIT exercise routine that was used during the study (right). As a general template, each cluster consisted of a total-body or body-weight cardio exercise, an upper-body exercise, and a lower-body exercise. For each session, subjects completed 2-3 rounds of each cluster with various work-to-rest ratios (30:15 s) before resting for 60 s and moving on to the next cluster. RDL = Romanian deadlift; DL = deadlift.

2.6 Statistical analyses

Independent samples t-tests were conducted to determine if there were statistically significant differences at baseline between EG and CG for the following dependent variables: age, height, BM, SKF-BF%, and est-VO₂max (ml·kg^–1·min^–1). The Type 1 error rate for simultaneous comparisons was controlled using the Sidak-Bonferroni Procedure (α= 1–(1–α_FW)^1/c, where α_FW is the familywise alpha level of 0.05 and c = 5 (the number of dependent variables compared in the prior analysis). Therefore, the corrected alpha level was 0.010. Paired-samples t-tests were performed to detect if there were statistically significant differences between pre- and post-training in the EG for the following dependent variables: body composition (BM, FFM, FM and SKF-BF%), aerobic capacity [est-VO2max (L·min^–1), est-VO2max (ml·kg^–1·min^–1), high-WL, and high-HR], back flexibility, biceps strength, and push-ups. Independent samples t-tests were performed to detect differences in the percent change (%Δ) values between EG and CG for the 11 dependent variables previously listed. The Type 1 error rate for simultaneous comparisons was controlled during the paired-samples and independent samples t-tests using the Sidak-Bonferroni Procedure (α= 1–(1–α_FW)^1/c, where α_FW is the familywise alpha level of 0.05 and c = 11 (the number of dependent variables compared in each analysis). Therefore, the corrected alpha levelwas 0.005.

The assumption of normality was checked using the Shapiro-Wilk test for all t-tests. If the assumption was violated (p < 0.05) for the paired samples and one-sample t-tests, the Wilcoxon signed-rank test was used to determine if there were statistically significant differences between the means. If the assumption was violated (p < 0.05) for the independent samples t-tests, the Mann-Whitney U test was used to determine if there were statistically significant differences between the means. The assumption of homogeneity of variance was checked for the independent samples t-tests using the Levene’s Test of Equality of Variances. If the assumption was violated (p < 0.05), indicating that the group variances were unequal, an adjusted t-statistic based on the Welch method was used to compare the means. The effect sizes for all t-tests were calculated and reported as Cohen’s d and were interpreted as small (d ≈ |0.25|), medium (d ≈ |0.5|), or large (d ≈ |0.8|) [30]. These interpretations are purely subjective, as the effect size calculation and subsequent reporting simply allows for stronger comparisons between studies in the literature [31]. The 95% confidence interval (CI) for the effect sizes was also calculated and reported. The statistical package JASP (Version 0.16.1, Amsterdam, The Netherlands) was used to conduct all analyses.

3 Results

Baseline physical characteristics for the EG and CG are displayed in Table 1. There were not statistically significant between-group differences at baseline for all five variables (p > 0.010). Several measures of physical fitness improved significantly from pre- to post-training in the EG (p < 0.005), including SKF-BF%, FFM, FM, est-VO₂max (L·min^–1), est-VO₂max (ml·kg^-1·min^–1), and push-ups (Table 3). There were no statistically significant differences in the % Δ values between EG and CG (Table 4).

Table 3
Mean values for all body composition and physical fitness dependent variables as measured by Microfit for the subjects that underwent 12 weeks of multimodal HIIT training (MM-HIIT), respectively (n = 12). Data are displayed as mean±standard deviation

Pre-MM-HIIT Post-MM-HIIT p-value Cohen’s d 95% CI

Body mass (kg) 79.9±11.5 78.7±9.5 0.27 –0.34; small [–0.91, 0.25]

BF% 16.8±6.1 12.3±4.1^* <0.001 –1.60; large [–2.45, –0.71]

FM (kg) 13.5±5.5 9.7±3.3^* 0.001 –1.24; large [–1.99, –0.46]

FFM (kg) 66.5±10.0 69.0±8.8^* <0.001 1.53; large [0.67, 2.36]

est-VO₂max (ml·kg^–1·min^–1) 48.6±6.2 60.5±4.9^* <0.001 2.10; large [1.06, 3.13]

est-VO₂max (L·min^–1) 3.22±0.54 4.17±0.62^* <0.001 2.43; large [1.27, 3.57]

High-WL (watts) 181±36 206±24 0.020 0.78; large [0.12, 1.42]

High-HR (bpm) 148±7 145±6 0.126 –0.48; medium [–1.07, 0.13]

Sit-and-reach (cm) 42.5±9.5 42.6±9.1 0.27^a 0.37; small [–0.25, 0.77]

Biceps strength (kg) 50.0±7.7 52.8±6.4 0.04 0.64; medium [0.01, 1.25]

Push-up repetitions 41±11 54±8^* <0.001 1.35; large [0.54, 2.13]

	Pre-MM-HIIT	Post-MM-HIIT	p-value	Cohen’s d	95% CI
Body mass (kg)	79.9±11.5	78.7±9.5	0.27	–0.34; small	[–0.91, 0.25]
BF%	16.8±6.1	12.3±4.1^*	<0.001	–1.60; large	[–2.45, –0.71]
FM (kg)	13.5±5.5	9.7±3.3^*	0.001	–1.24; large	[–1.99, –0.46]
FFM (kg)	66.5±10.0	69.0±8.8^*	<0.001	1.53; large	[0.67, 2.36]
est-VO₂max (ml·kg^–1·min^–1)	48.6±6.2	60.5±4.9^*	<0.001	2.10; large	[1.06, 3.13]
est-VO₂max (L·min^–1)	3.22±0.54	4.17±0.62^*	<0.001	2.43; large	[1.27, 3.57]
High-WL (watts)	181±36	206±24	0.020	0.78; large	[0.12, 1.42]
High-HR (bpm)	148±7	145±6	0.126	–0.48; medium	[–1.07, 0.13]
Sit-and-reach (cm)	42.5±9.5	42.6±9.1	0.27^a	0.37; small	[–0.25, 0.77]
Biceps strength (kg)	50.0±7.7	52.8±6.4	0.04	0.64; medium	[0.01, 1.25]
Push-up repetitions	41±11	54±8^*	<0.001	1.35; large	[0.54, 2.13]

^*Significantly different compared to pre-training (p < 0.005), BF% = body fat percent, CI = confidence interval, FM = fat mass, FFM = fat-free mass, est-VO₂max = estimated maximal oxygen uptake, High-WL = highest workload achieved during submaximal cycle ergometry test, High-HR = highest heart rate achieved during submaximal cycle ergometry test. ^aNormality was violated, and the Wilcoxon signed-rank test was used to determine if there were statistically significant differences between the means.

Table 4

A comparison of percent change (% Δ) for several measurements of physical fitness between the experimental group (EG; n = 12) that underwent 12 weeks of multimodal high-intensity interval training and the control group (CG; n = 12) that underwent 12 weeks of traditional physical preparation that is typically implemented during training academies. Each subject had their percent change determined, and the mean of these percent changes was calculated. Data are displayed as means±standard deviation

	EG % Δ	CG % Δ	p-value	Cohen’s d	95% CI
Body mass (kg)	–1.2±4.5%	–3.2±3.4%	0.24	0.50; medium	[–0.32, 1.30]
BF%	–25.4±11.6%	–13.4±18.3%	0.07	–0.78; large	[–1.61, 0.06]
FM (kg)	–26.0±13.6%	–16.1±17.8	0.14	–0.63; medium	[–1.44, 0.20]
FFM (kg)	4.3±3.7%	–0.6±4.4%	0.01	1.21; large	[0.32, 2.07]
est-VO₂max (ml·kg^–1·min^–1)	30.9±14.1%	24.2±14.9%	0.27	0.46; medium	[–0.35, 1.27]
est-VO₂max (L·min^–1)	25.6±13.9%	24.9±12.9%	0.89	0.06; small	[–0.75, 0.86]
High-WL (watts)	17.2±22.5%	23.3±16.1	0.45	–0.31; small	[–1.11, 0.50]
High-HR (bpm)	–1.97±4.29	–0.21±3.0	0.25	–0.49; medium	[–1.29,0.33]
Sit-and-reach (cm)	1.5±12.6%	9.4±15.1%	0.23^a	–0.30; small	[–0.65, 0.16]
Biceps strength (kg)	6.6±9.9%	9.5±20.5%	0.71^a	–0.10; small	[–0.36, 0.52]
Push-up repetitions	38.9±39.1%	40.3±25.0%	0.67^a	–0.11; small	[–0.53, 0.35]

BF% = body fat percentage; CI = confidence interval; cm = centimeters; est-VO₂max = estimated maximal oxygen consumption; FM = fat mass; FFM = fat-free mass; kg = kilograms; L = liter; min = minute; ml = milliliter; High-WL = highest heart rate achieved during submaximal cycle ergometry test; High-HR = highest workload achieved during submaximal cycle ergometry test. ^aNormality was violated, and the Mann-Whitney U test was used to determine if there were statistically significant differences between the means.

4 Discussion

The primary finding of the current study is that a 12-week MM-HIIT program had a positive effect on body composition and several components of physical fitness in recruits who participated in a training academy during the COVID-19 pandemic. Moreover, data from this research suggest that the 12-week MM-HIIT program had similar effects on physical fitness compared to previous academies that used traditional training methods (e.g., concurrent training with regular gym access). These findings bear practical significance for fitness professionals who work with tactical athletes and seek to improve several components of fitness with limited resources, gym access, and time.

In support of our original hypotheses, subjects in the EG increased their FFM, while FM and BF% both decreased. Indeed, losing fat and gaining muscle simultaneously has been reported elsewhere in the RT literature [32, 33]. However, because SKF analysis is a two-component model for measuring body composition [34], and not a direct measure of muscle mass [35], these outcomes should be considered with caution. Although statistically significant differences were not detected between the EG and CG, subjects in the EG increased their FFM by several magnitudes more than the CG, which is reflected by the large effect size (d = 1.21; Table 4). However, due to the high degree of variability between recruits and the occupational demands of each training academy, it is difficult to declare that the perceivable difference in FFM were caused by the MM-HIIT program. It is noteworthy that authors from previous training academy studies have reported similar decreases in BF% by using SKF analysis (2.5–5.5%) [12 , 17]. In other words, the literature generally agrees that recruits improve their body composition after performing a variety of exercise programs during training academies (e.g., concurrent, HIFT, and RT). Future research can focus on training interventions to help firefighters maintain these positive adaptations, as they tend to gain weight and increase their body mass index as their careers progress [36].

The present data indicate that MM-HIIT helped increase cardiovascular fitness, as est-VO₂max significantly increased for subjects in the EG. As shown in Table 4, the percent increases did not differ from the CG, suggesting that traditional and non-traditional training techniques similarly influence aerobic capacity during training academies. Both of these findings support our hypotheses. Taken as an absolute value, the post-MM-HIIT data point (60.5·ml·kg^–1·min^–1) is much higher than previously reported est-VO₂max following various training academy programs (41.1–46.9 ml·kg^–1·min^–1) [11 , 17]. However, when considered as a percent increase (+25%), the current data is nearly identical to previously reported improvements in est-VO₂max (+25–28%) [12, 17], but is still considerably larger than others (+6.4–11.6%) [11, 15]. Disparities between studies are likely explained by differences in the est-VO₂max protocol employed (e.g., submaximal cycle ergometer, forestry step test, and 1.5-mile run test), which reflects that tactical populations may not have access to, or clearance to perform, maximal laboratory-based aerobic assessments. Despite the unusually high est-VO₂max, we contend that our subjects increased their cardiovascular fitness as reflected by a large (d = 0.78; Table 3), albeit non-significant (p = 0.02; Table 3), increase in High-WL (25W) and a non-significant decrease in High-HR (3 bpm) during the cycle ergometer test [37]. Given the inverse relationship between VO₂max and cardiovascular disease risk factors in firefighters [38], it is important to maintain cardiovascular fitness beyond the training academy.

For upper-body muscular endurance, data revealed that the EG significantly improved their push-up performance, and their percent increase did not differ from the CG. Both results supported our hypotheses. These outcomes are consistent with previous studies that reported significant increases in push-up performance during a training academy (25–37%) [11 , 17]. In contrast, Hollerbach et al. [15] reported a non-significant increase in push-up performance (4.4 reps) following a 10-week HIFT program. This divergence from the typical increase in push-up performance could be explained by smaller sample size (n = 7), or the fact that their training sessions were unsupervised [15]. In other populations, there is evidence that HIFT/MM-HIIT increased squat endurance performance [20], sit-up performance [39, 40], bench press repetitions with submaximal loads [39, 40], and time-to-exhaustion during cardiovascular tasks [40]. Thus, HIFT/MM-HIIT can be used to increase muscular endurance for training academies and beyond, which may ultimately have a positive effect on firefighting performance [41].

Contrary to our hypothesis, subjects in the EG did not significantly increase their muscular strength during the 12-week MM-HIIT intervention. As shown in Table 4, percent changes did not differ between EG and CG, which suggests that recruits from the current fire department do not typically improve their strength during training academies. Alternatively, the biceps dynamometry test may only reflect changes in strength for the elbow flexors, and the subjects may not spend enough time performing isolated elbow flexion exercises [42]. The current results do not reflect previous studies, as the literature generally agrees that recruits increase their strength during training academies [12 , 16]. For instance, it has been reported that recruits increase their hand-grip strength [15], estimated 1-RM for bench press and back squat [12], and true 1-RM for bench press and back squat [16]. Although speculative, it is possible that differences between studies stem from inconsistent measurement techniques, as the firefighter academy literature includes several field- and laboratory-based assessments for strength. Because muscular strength tends to increase at a variety of repetition ranges and associated external loads [43, 44], it would be interesting for future researchers to compare different work-to-rest configurations with light, moderate, and heavy loads duringMM-HIIT.

Similar to the muscular strength outcomes, back flexibility did not improve for the EG or CG, which means that our hypothesis was not supported. However, this result is consistent with previous research, as recruits tend to not increase their sit-and-reach scores during training academies [11 , 16]. Collectively, these data imply that recruits may not perform enough isolated flexibility and mobility work during their physical preparation process. For example, there is evidence that 6 weeks of yoga training improved back flexibility and total-body movement competency (i.e., Functional Movement Screen; FMS) in shift-based professional firefighters [45]. In a similar manner, Cornell et al. [13] reported that FMS scores improved after 14 weeks of academy training, although back flexibility was not measured or reported in this study. Because the FMS includes total-body movements that require mobility and stability in multiple planes (e.g., squatting, stepping, and lunging) [13, 45], it may be a more appropriate tool for assessing mobility and flexibility for the firefighting population.

For practitioners, it is interesting that circuit training with tactical gear [3] and traditional periodization models [16] both improve firefighter performance because this presents two extremes of a spectrum, where exercising with minimal and maximal gym access both elicit positive effects. The current MM-HIIT program required only four forms of resistance that were easy to transport and use outside at the fire station: body weight, dumbbells (6.8–13.6 kg), Dynamax medicine balls (4.5–9.1 kg), and Versa Logs (9.1–18.2 kg). Considering that physical preparation during our traditional academies was completed with state-of-the-art equipment for cardiovascular and resistance training, the data in Table 4 are intriguing. In fact, the lack of difference between EG and CG for nearly every component of fitness suggests that MM-HIIT (i.e., like HIFT) [19 –21] could be a suitable substitute for traditional training techniques, which corroborates recent research in firefighters [11, 15].

Several limitations should be considered when interpreting the results from this research, and it is worth highlighting that these limitations are consistent with previous training academy studies [11 –17]. Most noteworthy, because subjects were not randomly assigned to different experimental conditions, it was impossible to isolate the effect of MM-HIIT from other competing factors (e.g., physical activity performed during occupational tasks). Also, because this was not a true, randomized-controlled trial, we selected a post-hoc CG from pre-existing data [11], which limits the veracity of the comparisons made between the EG and CG in the current study. Dietary information was not collected during this study, so the influence of caloric intake and macronutrient distribution are unknown. Furthermore, because we had to use measurement tools that were approved by the local fire department and the associated labor union, we were confined to field-based techniques for measuring components of fitness. Last, these data were collected in healthy, previously trained recruits and should be cautiously generalized to other populations.

5 Conclusions

The present study demonstrated that 12 weeks of MM-HIIT helped improve several measurements of body composition and physical fitness in a class of recruits who participated in a training academy amidst the COVID-19 pandemic. Despite training outdoors and with minimal equipment, MM-HIIT led to similar outcomes as previous academies that had unlimited gym access and were able to use a variety of traditional training modalities. These results are critical for fitness professionals who train occupational athletes, such as firefighters, who may have limited time, equipment, and/or access to a formal gym setting. The current training sessions involved 12 total-body exercises that were arranged in four clusters of three exercises, which generally followed a pattern of total-body calisthenics + lower-body movement + upper-body movement. We encourage practitioners to consider our MM-HIIT program as a malleable template that can be modified to accommodate the specific needs of their athletes and access to equipment.

Footnotes

Acknowledgments

The authors would like to thank the recruits who volunteered for this study and congratulate them for successfully completing the academy. In addition, they would like to thank Adam Martinez for his assistance with the conditioning sessions and fitness testing during the academy.

Conflict of interest

The authors report no conflicts of interest.

Funding

The authors did not receive funding for this project.

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