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

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

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).

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.