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Original Research

Evaluation of Circuit-Training Intensity for Firefighters

Abel, Mark G1; Mortara, Anthony J1; Pettitt, Robert W2

Author Information
Journal of Strength and Conditioning Research: October 2011 - Volume 25 - Issue 10 - p 2895-2901
doi: 10.1519/JSC.0b013e31820da00c
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Abstract

Introduction

Tactical strength and conditioning is a relatively new, yet rapidly growing field. As such, little is known in the scientific community regarding the most effective training strategy to prepare firefighters. Firefighting is a strenuous occupation that requires optimal levels of physical fitness. Inadequate fitness levels may reduce occupational performance and increase the risk of injury to the firefighter. Given the strenuous demands of firefighting, it is obvious that firefighters must engage in a regular exercise program to enhance physical fitness and job preparedness.

Firefighting presents a unique challenge from a physical training perspective. Unlike some sports or occupations that focus on a single training goal (e.g., power, strength, or endurance), firefighting requires the operator to optimize multiple training goals simultaneously. For instance, firefighters must possess power to perform forcible entry maneuvers, strength to advance hose lines and perform salvage and overhaul tasks, and aerobic and muscular endurance to carry equipment up flights of stairs. In essence, firefighters must train in a manner that effectively stresses both the anaerobic and aerobic energy systems.

Unfortunately, there is limited research identifying the optimal training strategy to improve the occupational performance of firefighters. Peterson et al. (17) demonstrated that both traditional and undulating periodization models improve performance on a firefighter-specific physical ability test. However, conventional training models that require several minutes of rest between sets are more time consuming and may not optimize the stress placed on the anaerobic and aerobic energy systems. In contrast, a circuit-based strength and conditioning workout inclusive of short rest periods and relatively high intensities may enhance the stress placed on the anaerobic and aerobic energy systems as used when performing firefighting tasks. However, it is unknown whether a circuit workout can sufficiently stress the anaerobic and aerobic energy systems to simulate the intensities of tasks performed on the fire ground. Specifically, research has indicated that firefighters produce relative heart rates of approximately 79–88% of maximum heart rate (HRmax) and peak blood lactate values of 6–13 mmol·L−1 while performing tasks on the fire ground (10,13,22). If a circuit-based workout does place an appropriate stress on the anaerobic and aerobic energy systems, firefighters and tactical strength and conditioning professionals may consider using it as a training modality to enhance job preparedness. Therefore, the primary purpose of this study was to compare the aerobic and anaerobic intensities of a circuit-based workout to data reported on firefighters performing fire ground and rescue tasks (i.e., aerobic intensity: 79–88% of HRmax; peak blood lactate concentration: 13 ± 3 mmol·L−1). We hypothesized that a circuit-based workout would yield similar heart rate and blood lactate responses compared to performing fire ground and rescue tasks.

The advantage of using heart rate and blood lactate to assess exercise intensity is the objective nature of these measurements. However, these assessments require relatively expensive equipment and technical expertise. It would be beneficial for firefighters to have a less expensive and user friendly method of evaluating exercise intensity. Rating of perceived exertion (RPE) has been found to be a valid subjective measure of resistance training intensity (6). However, RPE has not been validated using a circuit-training workout that uses multiple rotations of a circuit and brief interexercise recovery periods. Therefore, the secondary purpose of this study was to validate an existing 10 point category–ratio (CR) RPE scale for circuit training with firefighters. We hypothesized that the CR scale would be a valid subjective measure of circuit-training intensity.

Methods

Experimental Approach to the Problem

Firefighting is a strenuous occupation that requires high levels of both aerobic and anaerobic fitness. Circuit training is a type of strength and conditioning program that stresses both the anaerobic and aerobic energy systems (2,11). Therefore, circuit training may be an effective training modality to prepare firefighters to perform their job more efficiently. This study descriptively compared the aerobic and anaerobic intensity of a circuit-training workout to reported physiological data on firefighters performing actual fire ground and rescue tasks. Heart rate measurements were taken during the exercise session to describe the aerobic intensity. Blood lactate measurements were taken before and after the exercise session to describe the anaerobic intensity. These heart rate and blood lactate values were then statistically compared to heart rate and blood lactate data reported in the literature on firefighters performing fire suppression and rescue tasks. In this study, the circuit-training workout served as the independent variable, whereas heart rate and blood lactate served as dependent variables. The dependent measures of heart rate and blood lactate were selected for use in this study because they are commonly used as objective measures of physical activity (10,12,21-24).

As a secondary purpose of this study, the firefighters' RPE for the exercise intensity was evaluated to determine if RPE is a valid measure of exercise intensity for circuit-training workouts. Validation of the RPE scale for circuit training may allow firefighters to use RPE as a proxy measure for objectively determined intensity (e.g., heart rate and blood lactate) without the use of sophisticated equipment. The firefighters used a previously validated CR rating of perceived exertion (RPE) scale (0–10 scale) to identify their exercise and global session intensity (6). In this case, each individual exercise, each rotation of the circuit (i.e., first rotation through the exercise circuit vs. second rotation through the exercise circuit), and the entire workout served as the independent variables, whereas the exercise RPE, circuit RPE, and global session RPE represented the dependent variables.

Subjects

Twenty male career firefighters participated in this study. The firefighters' physical characteristics are summarized in Table 1. This study used a convenience sample of firefighters who were already participating in a supervised on-duty physical training program. The fire department for which the subjects were recruited is located in the southeastern USA. All subjects provided written informed consent after a detailed explanation was provided about the aims, benefits, and risks involved with the investigation. Subjects were told that they were free to withdraw from the study at any time, without penalty. All procedures used in this study were approved by the University's Institutional Review Board before the initiation of the study.

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Table 1:
Physical characteristics of the firefighters.*

Testing Procedures

One testing session was completed at the Fire Department's Training Center. Resting blood lactate was measured via finger stick with the subject in a seated position, after a 5-minute resting period. Post–workout blood lactate was also measured with the subjects in a seated position within 5 minutes of completing the workout. After sterilizing the finger tip, a puncture was made with a sterile lancet. The first drop of blood was wiped away. The second drop of blood was applied to an assay strip and inserted into the lactate testing device (Lactate Plus, Lactate.com., Waltham, MA, USA). The calibration of the lactate testing device was checked with high and low control solutions before each testing session. This portable blood lactate analyzer has demonstrated high levels of validity (R2 = 0.99) and test–retest reliability (Intraclass correlation coefficient [ICC] = 0.99) at low, moderate, and high blood lactate levels (1). Standing height (to the nearest 0.1 cm) was measured without shoes using a stadiometer (Road Rod 214, Seca, Hanover, MD, USA). Body mass (nearest 0.1 kg) was measured without shoes using a digital scale (TBF-521, Tanita Corporation, Arlington Heights, IL, USA). Heart rate was measured via telemetry (Polar A1, Electro Oy, Finland) and was expressed as a percentage of age-predicted HRmax (220 − age in years). The training session heart rate data were reported in 2 ways. First, to describe the mean heart rate during circuit-1 and circuit-2, heart rate data were recorded manually (via watch transmitter) after the completion of each exercise. The average heart rate for each circuit was then calculated using each postexercise heart rate value during circuit-1 and separately for circuit-2. Second, to calculate the average heart rate throughout the entire exercise session, a device (ActiTrainer, Pensacola, FL, USA) was used to record the subjects' heart rate every 15 seconds for the duration of the exercise session. The average of these 15 second heart rate values was used to express the global exercise session heart rate for each subject. The heart rate recording device was placed in a neoprene sleeve on the subject's upper arm during the exercise session. The data from this device were later downloaded using ActiLife software and imported into a spreadsheet for analysis. The RPE was evaluated immediately after the completion of each exercise and after the workout using a validated 0–10 CR scale (6,16). Subjects were familiarized with the RPE scale by using this scale to report RPE in previous exercise sessions.

Workout Parameters

The workout was comprised of performing 2 rotations through a circuit of 12 stations composed of resistance training and cardiovascular exercises that used all major muscle groups (i.e., shoulders, chest, back, legs, and core musculature; Table 2). The subject started at a given exercise station and performed 12 repetitions within 30 seconds for each externally resisted exercise. The subjects self-selected a resistance for each of the externally resisted exercises. Specifically, the subjects had been training regularly for 6 months with the investigators and performed this particular workout at least twice the week before the testing session. During these previous workouts, the subjects were asked to find a weight that was challenging for them to perform 12 repetitions. Subjects performed as many repetitions as possible for the body weight–based exercises (e.g., push-ups). Then, a 30-second recovery period was provided while the firefighter moved to the next exercise station. Each subject also exercised for 3 minutes on a treadmill or stair climber machine as part of the circuit. A 5-minute general warm-up was performed before the workout and included walking or stationary leg ergometry, followed by 5 minutes of static and dynamic stretching for all major muscle groups. Finally, a cooldown and additional static stretching for major muscle groups were performed after the workout.

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Table 2:
Externally resisted exercises and resistance (kilograms) used in the circuit-based workout for firefighters.*†

Statistical Analyses

Basic descriptive statistics (mean ± SD) were used to describe the dependent variables. One sample t-tests were used to compare the sample's relative heart rate and post–workout blood lactate values to the values reported in the literature from actual firefighting tasks. Paired samples t-tests were used to evaluate differences in the mean circuit (i.e., circuit-1 vs. circuit-2) relative heart rate (i.e., % HRmax), mean circuit RPE, and between the relative heart rate and RPE for each exercise of circuit-1 vs. circuit-2. Because of the use of multiple paired sample t-tests, a Bonferroni adjustment was used to account for the inflation of type 1 error. Therefore, the level of significance was set at p < 0.004 (0.05/12 comparisons = 0.004) for those tests. Similarly, paired samples t-tests were used to evaluate differences between resting and post–workout blood lactate levels. Effect sizes for the change in blood lactate, relative heart rate, and RPE between circuit-1 vs. circuit-2 were calculated as ([meancircuit-1 − meancircuit-2]/pooled SD). Effect sizes were interpreted according to Cohen's d: small effect: ≤ 0.2, medium effect: 0.3–0.7, large effect: ≥0.8 (4). Data from 5 out of 6 of the primary outcome variables were normally distributed (i.e., Fisher's coefficient of skewness <1.96) according to Fisher's coefficient of skewness (skewness/standard error of skewness). One variable, resting blood lactate, was positively skewed because of the presence of 2 outliers. However, the results were unchanged when these subjects were removed from the analyses. Therefore, these subjects were retained for all statistical analyses. Finally, bivariate correlations were used to evaluate the relationship between global exercise session RPE (i.e., subjective measure) vs. postexercise blood lactate and mean exercise session relative heart rate (objective measures of exercise intensity).

Results

Objective and subjective measures of the firefighters' workout intensity are displayed in Table 3. The mean circuit-training heart rate was similar to previously reported heart rate responses from firefighters performing simulated smoke-diving tasks (79.4 ± 5.4 vs. 79 ± 6% HRmax, p = 0.741), but lower than previously reported heart rate responses from firefighters performing fire suppression tasks (79.4 ± 5.4 vs. 88 ± 6% HRmax, p < 0.001). During the workout, the firefighters' postexercise heart rate increased from 80.3 ± 6.5% of HRmax during the first rotation of the circuit to 86.1 ± 6.0% of HRmax during the second rotation of the circuit (p < 0.001; effect size: Cohen's d = 0.93; Power = 1.0) despite no changes in the mean external resistance lifted between circuit-1 vs. circuit-2 (p ≥ 0.089; Table 2). Post–workout blood lactate was similar to peak blood lactate values previously reported after performing simulated firefighter rescue tasks (11.8 ± 3.1 vs. 13 ± 3 mmol·L−1, p = 0.084). Post–workout blood lactate was significantly greater than pre–workout resting blood lactate (p < 0.001; effect size: Cohen's d = 4.6; Power = 1.0).

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Table 3:
Mean ± SD/SE of objective and subjective measures of circuit-training intensity in firefighters.*

The global exercise session RPE was not significantly related to postexercise blood lactate (r = 0.29, p = 0.231) or mean exercise session relative heart rate (r = 0.42, p = 0.071). The mean RPE of the individual exercises during the first rotation of the circuit increased from 5.3 ± 1.5 (“Hard”) to 6.5 ± 1.5 (“Hard”–“Very hard”) during the second rotation of the circuit (p < 0.001; effect size: Cohen's d = 0.82; Power = 1.0) despite no change in mean external resistance lifted between circuit-1 vs. circuit-2. Figure 1 demonstrates relative heart rate and RPE responses by exercise during the first and second rotations of the circuit. Regarding individual exercises, 5 exercises (leg press, dumbbell shoulder press, deadlift, step-up, and abdominal crunch) produced significant increases in relative heart rate and RPE from circuit-1 to circuit-2. Although not all exercises produced significant (i.e., p < 0.004) increases in both relative heart rate and RPE in circuit-1 vs. circuit-2, 10/11 exercises for relative heart rate and 11/11 exercises for RPE yielded p values <0.05, indicating a marked trend that heart rate and RPE increased the second time each exercise was performed. The global session RPE was 7.3 ± 1.2 (“Very hard”).

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Figure 1:
Mean postexercise relative heart rate and rating of perceived exertion (RPE) responses by exercise and circuit number (first rotation vs. second rotation) during a circuit-based workout in firefighters. %HRmax = percent of maximum heart rate. The descriptors for the 0–10 category–ratio scale are provided. aSignificant difference in the relative heart rate response for a given exercise between circuit-1 vs. circuit-2 (p < 0.004). bSignificant difference in the RPE response for a given exercise between circuit-1 vs. circuit-2 (p < 0.004). *Exercise was not included in the statistical comparison of circuit-1 vs. circuit-2 because the stair climber and treadmill workouts were only performed during 1 of the 2 circuits. The cable pulldown exercise was performed as an alternative to the lat pulldown when the lat pulldown machine was already being used by another participant.

Discussion

The findings from this study indicate that a circuit-based workout yielded a similar anaerobic response, but lower cardiovascular response compared to reports of firefighters performing occupational tasks. Several investigations have reported that the most demanding firefighting tasks produce peak blood lactate concentrations ranging from 6 to 13.2 mmol·L−1 (10,23). The circuit-based workout used in this study produced a mean post–workout blood lactate level that was within the range reported by Gledhill and Jamnik (10) and statistically similar to the group mean peak blood lactate value reported by Von Heimburg et al. (23). These results suggest that the circuit-based workout placed a similar degree of stress on the anaerobic system as compared to performing the most demanding tasks on the fire ground. It appears that using a workout composed of intense bouts of exercise combined with brief rest periods (e.g., 30 seconds) may provide a more effective stress on the glycolytic energy system compared to traditional resistance training programs. In support of this contention, Kelleher et al. (11) demonstrated that postexercise blood lactate levels were significantly higher after a superset resistance training workout (≈11 mmol·L−1) compared to a tradition resistance training workout (≈7 mmol·L−1).

In terms of aerobic fitness, numerous studies have reported the high demand that firefighting tasks place on the aerobic energy system (5,7,8,10,22,24). Specifically, Sothmann et al. (22) have reported that firefighters' perform fire ground tasks at 88 ± 6% of HRmax for 15 ± 7 minutes in response to actual fire-related emergencies. The circuit-based workout used in this study yielded heart rate values that averaged 79.4 ± 5.4% of HRmax for a duration of 28.5 ± 4.4 minutes. Overall, the average heart rate–derived intensity of this workout was lower than the heart rate reported by Sothmann et al. (22); however, the duration of the circuit-based workout was about twice as long compared to the emergency tasks performed in that study (29 vs. 15 minutes). Another investigation reported average heart rate values that were 79 ± 6% of HRmax when firefighter students performed simulated smoke-diving operations in a heated simulator for 17 ± 4 minutes (13). Thus, the heart rate–derived intensity of the circuit-based workout used in this study is lower than in some firefighter tasks, but at an appropriate level for other tasks. To increase the heart rate response, the current workout could be modified to include more multijoint functional exercises such as dragging a rescue mannequin, hitting a tire with a sledge hammer, or carrying 5 gallon buckets full of sand or water to simulate an equipment carry.

It is important to note that heart rate is influenced by a variety of factors. For instance, during the circuit-training session, heart rate was primarily influenced by the intensity of the exercise performed and duration of the interexercise recovery periods. However, on the fire ground, heart rate is not only a function of the intensity of the physical work performed, but it is also affected by the thermal environment (e.g., working in burning structure and wearing protective gear), hydration status, and emotional stress. Thus, it is difficult to identify the relative contribution of the physical work performed during firefighting on the heart rate response.

Although this study demonstrates that circuit training may serve as one important component of a comprehensive strength and conditioning program to prepare firefighters for occupational tasks, there are no longitudinal data to support its use to improve firefighter job performance. In contrast, there have been a few training interventions that have used traditional periodized strength and conditioning programs to improve firefighter performance. For instance, Roberts et al. (20) demonstrated that a 16-week exercise program (1 h·d−1, 3 d·wk−1) comprised of traditional periodized resistance training and aerobic training significantly improved firefighter recruits' O2max, muscle endurance, flexibility, and body composition. Unfortunately, a firefighter-specific physical ability test was not conducted to evaluate occupational performance. In addition, Peterson et al. (17) conducted a 9-week training intervention (1–1.5 h·d−1, 3 d·wk−1) comparing the effects of an undulated training program vs. a traditional linear periodized training program in firefighter trainees. Both training programs significantly improved upper and lower body muscular strength, peak power output, vertical jump height, and performance on a firefighter-specific physical ability test (i.e., “The Grinder”). However, the undulated training group demonstrated greater improvements than did the traditional training group in the firefighter-specific physical ability test (17).

The performance of firefighting tasks has been found to be correlated to measures of basic strength (15,19), among other physical fitness domains. One concern is that circuit-training programs composed of relatively low intensities and short recovery periods may not optimize strength and power as needed to perform some firefighting tasks. In support of this contention, numerous investigations have demonstrated that low intensity (i.e., ≤80% of 1 repetition maximum) resistance training is less effective at improving muscular strength and power compared to high-intensity resistance training programs (3,14). Therefore, given the specific muscular and performance adaptations that occur from circuit training, it is important that firefighters use a comprehensive periodized (linear or flexible nonlinear) resistance training program consisting of a range of training intensities, recovery periods, and volumes. To that end, future research is needed to evaluate the training adaptations produced by a periodized circuit-training program for firefighters.

The secondary purpose of this study was to validate the use of RPE as a subjective measure of intensity during a circuit-based workout. Although there was a positive trend, global exercise session RPE was not significantly related to mean exercise session relative heart rate (p = 0.07) or postexercise blood lactate (p = 0.23). Given the strong trend toward a significant correlation (i.e., p = 0.07), it is possible that a larger sample size may have provided a significant relationship between global session RPE and mean exercise session relative heart rate. The use of RPE to subjectively quantify resistance training intensity has been conducted by numerous investigators (6,9,12,18). However, this study is unique in that it evaluated the effect of exercise-induced fatigue on ratings of perceived exertion after 2 rotations of a circuit-based workout (Figure 1). The findings indicate that in 7 of the 8 externally resisted exercises, there was a significant increase in the RPE from the first to the second rotation of the circuit, despite no change in the external resistance. Thus, fatigue likely produced the increased ratings of perceived exertion. Likewise, there was a concurrent increase in relative heart rate that paralleled the changes in RPE during the second rotation of the circuit, demonstrating an increase in objective exercise intensity.

The RPE has also been used to assess the subjective intensity of firefighters while performing repeated live-fire drills (21). In a manner similar to that of the present study, RPE (using 6–20 scale) increased progressively (set 1: 13—“Somewhat hard”; set 2: 15—“Hard”; set 3: 18—Between “Very hard” to “Extremely hard”) while firefighter recruits performed 3 sets of standardized firefighter-specific tasks. Similar to the progressive trends in perceived exertion found in this study, Smith et al.'s (21) data indicated that performing multiple sets of live-fire drills also increased the firefighters' perception of physical exertion. Thus, using RPE during a circuit-based workout may be helpful for firefighters because (a) it may by similar to their perception of exertion while performing multiple or repeated fire ground tasks, (b) it assists in evaluating exercise and fire-task intensity, and (c) it may be used to evaluate a training program's effectiveness. That is, when repeatedly performing a standardized workout, reductions in exercise or session RPE may indicate improvements in workout tolerance.

Practical Applications

The field of tactical strength and conditioning has become increasingly popular over the past few years as evidenced by the development of special interest groups within the National Strength and Conditioning Association. One subgroup within this field includes firefighter-strength and conditioning professionals. These professionals are responsible for preparing individuals for the physical demands of firefighting. The literature provides some insight as to the anaerobic and aerobic demands of firefighting. However, there is a lack of scientific data regarding the identification of an appropriate physical training program that adequately prepares firefighters to meet these physiological demands. Data from this study indicate that a circuit-based workout may produce a similar anaerobic stress as compared to performing fire suppression and rescue tasks. However, it should be noted that this type of workout produced a lower cardiovascular stress compared to performing firefighting tasks. In addition, strength and conditioning programs composed exclusively of low-intensity circuit-training exercises may not optimize increases in strength and power as required for some firefighting tasks. Therefore, strength and conditioning professionals should include high-intensity cardiovascular exercises and periodize circuit-training programs by using a range of training intensities and volumes to adequately prepare firefighters for the aerobic and anaerobic strength/power demands of firefighting.

Although circuit-training exercise parameters are highly variable, we have provided some basic guidelines that tactical strength and conditioning professionals can use to develop effective circuit-training workouts for firefighters. For instance, the circuits should include approximately 5–15 functional multijoint exercises that simulate the movement patterns used for fire suppression and emergency tasks (e.g., deadlift, “woodchop” with cable machine, stair climb with hose bundles, drag a rescue mannequin, kneel while pulling a fire hose attached to a weighted sled). Firefighters may be instructed to perform a set number of repetitions during the exercise interval or perform as many repetitions as possible within a predetermined exercise time. Brief recovery periods (e.g., 20–60 seconds) should be used between exercises, such that the work-to-rest ratio is approximately 1:1. It is advisable to have a strength and conditioning professional supervise the training session and monitor the exercise and recovery intervals to ensure the firefighters rotate as a unit. Finally, it is important that the firefighters perform a dynamic warm-up, cooldown, and flexibility training during the workout session.

Beyond the physical training benefits, circuit training may be appropriate for firefighters for several reasons. First, because of the brief recovery periods and ability of multiple firefighters to exercise simultaneously, circuit training provides an efficient use of on-duty exercise time. Second, circuit training provides for an efficient use of space at the fire station or training center because each firefighter is typically performing a different exercise (thus multiple benches or squat racks are unnecessary). Finally, by exercising as a group, circuit training may create a motivational atmosphere and foster a team-based environment that is inherent to the function of emergency operations. In conclusion, there are many advantages to using a circuit-training program for firefighters. However, a circuit-training program should be supplemented with moderate-to-high intensity cardiovascular exercise and implemented in a periodized manner to adequately prepare firefighters for the variable physical demands of firefighting.

Acknowledgments

Funding for this project has been provided by the Kentucky Fire Commission. The authors would like to thank Chief Gerald Tatum and the firefighters for their assistance on this research project. In addition, the authors would like to thank the students who assisted in training the firefighters. The results of this study do not constitute endorsement of any product by the authors or the National Strength and Conditioning Association.

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Keywords:

tactical; strength; resistance; heart rate; RPE

© 2011 National Strength and Conditioning Association