Abstract
BACKGROUND:
Muscular strength and endurance are important attributes for structural firefighting. Matching resistance exercises to firefighter job demands is not well-established.
OBJECTIVE:
This study compared the electromyographic (EMG) activity of major muscles during the Candidate Physical Ability Test (CPAT) and weight lifting exercises in firefighters.
METHODS:
A repeated measures study was conducted in 13 full-duty career firefighters (1 F, 12 M; age 18–44 years). Participants completed seven weight lifting exercises at a university laboratory. They separately completed the CPAT at a firefighting training grounds. During each activity, surface EMG (% maximum voluntary isometric contraction - MVIC) of major muscle groups was recorded and compared between exercises and CPAT.
RESULTS:
No difference in EMG activity was observed between exercises and CPAT for the deltoid, trapezius, lumbar multifidus, gluteal, and biceps femoris muscles. EMG activity was significantly greater during the CPAT for the abdominal obliques (32.3% ±27.7% vs. 12.1% ±8.3%, p < 0.001) and for the latissimus dorsi (21.8% ±25.1% vs. 11.4% ±7.7%, p < 0.001).
CONCLUSIONS:
Standard weight lifting and abdominal oblique exercises should be incorporated into resistance training programs for firefighters.
Keywords
Introduction
Firefighting is demanding and requires the physical and mental strength to attend to a multitude of emergency scenes. Frequently, these emergency calls result in injuries and cardiovascular events that can persist for years and result in long-term morbidity [1]. While firefighter job activities are multi-faceted, the main component of their job is physical, requiring them to be strong enough to repeatedly perform these duties. Thus, optimal physical performance is imperative.
The International Association of Fire Fighters (IAFF) and International Association of Fire Chiefs (IAFC) understand these fitness requirements and have measures in place to aid fire departments in employing people to be well equipped for their career, with a long-term goal of injury reduction. The Firefighter Candidate Physical Ability Test (CPAT), a pre-employment functional capacity evaluation, was developed and approved by the IAFF-IAFC Joint Labor Management Wellness Fitness Initiative (WFI) to provide fire departments with a pool of individuals possessing the minimum physical requirements to perform training as an entry level firefighter [2]. The CPAT is designed with specific duties and goals reflecting those that would be performed during an emergency call.
Previous studies have examined simulated firefighting activities with respect to cardiovascular capacity and other factors [3–6], however specific muscle activation patterns during simulated firefighting activities and resistance exercise training remain unclear. Given the importance of resistance training to overall health, wellbeing, and physical fitness in firefighters [1], further research is needed to shape exercise programming.
The purpose of this study was to compare the electromyographic (EMG) activity of major muscles during the CPAT to weight lifting exercises in firefighters. This study will provide useful information for developing resistance exercise programs to match job demands.
Methods
Experimental approach to the problem
This study was an observational cohort design with repeated measures in which career firefighters reported to the study site to perform weight lifting exercises on one visit and CPAT events at another facility on a subsequent visit. The weight lifting exercises performed were low load and included the back squat, Romanian deadlift, overhead press, bent-over row, banded Romanian deadlift, glute hyperextension, and kneeling rotational throw. The CPAT events were completed using a 23-kg vest and included the hose drag, equipment carry, ladder raise and extension, forcible entry, search, rescue, ceiling breach and pull, and stair climb [2]. Surface EMG was recorded bilaterally during all activities for the deltoid (shoulders), trapezius (upper back), latissimus dorsi (mid back), abdominal oblique (abdominals), lumbar multifidus (low back), gluteus maximus (main glutes), gluteus medius (upper glutes), and biceps femoris (hamstring) muscles. Surface EMG was analyzed to compare muscle activity between the weight lifting exercises and CPAT events. The study was approved by the university’s human subjects institutional review board and each participant provided informed consent.
Subjects
A convenience sample of career firefighters (n = 13, 1 F, 12 M; age 31.8±5.7 years; body height 1.80±0.07 m, body mass 90.3±13.4 kg, BMI 27.2±3.9 kg/m2) from the Tampa Bay region of Florida was recruited by word-of-mouth or posted flyer to participate in this study. Approval by the university’s Institutional Review Board for the Protection of Human Subjects was obtained prior to initiating the study procedures. Inclusion criteria were: full, active duty career firefighter with ≥1 year of continuous service; previous experience with successfully completing the CPAT or other simulated firefighting activity assessment without adverse events; and age 18–44 years. Exclusion criteria were: cardiovascular contraindications to exercise as identified by history and physical examination suggesting that medical clearance is necessary as defined by American College of Sports Medicine’s (ACSM’s) criteria [7]; resting blood pressure outside of normal limits (normal limits: systolic 90–139 mm Hg; diastolic 60–89 mm Hg); Resting heart rate outside of normal limits at screening (normal limits: 60–100 beats per minute for sedentary adults); orthopedic contraindications to resistance exercise as identified by history and physical examination; current systemic inflammatory disease; contraindications to placement of electrodes on surface of skin (e.g. skin allergy, skin rash, open wound); currently receiving care for musculoskeletal injury, disabled due to a physical condition, or diagnosed with or receiving treatment for a psychological or psychiatric disorder; active workers’ compensation or personal injury case; pregnant; simultaneously enrolled in another biomedical clinical trial; drug or alcohol abuse within the past year; and unable or unwilling to complete the study procedures.
Sample size calculation
We hypothesized that the difference in surface EMG activity of the posterior chain muscles between CPAT and weight lifting activities would be less than 20%, which was deemed clinically-meaningful [8, 9]. Based on previous work assessing surface EMG activity of the posterior chain muscles during resistance exercise [8, 9], a sample size of n = 13 was found to be adequate to detect a 20% difference in posterior chain muscle surface EMG activity between CPAT and weight lifting exercises at a power = 0.80 and alpha = 0.05. While many major muscle groups were assessed, we focused on the posterior chain muscles (e.g. lumbar multifidus, gluteals, biceps femoris) due to previous research on movement patterns, the recruitment of these muscles in firefighting activities, and the association of firefighter back pain to posterior chain dysfunction [10–12].
Procedures
Screening
After providing informed consent, participants completed screening questionnaires, underwent assessment of heart rate, blood pressure, body height, and body mass, completed a urine pregnancy test (females only), and were interviewed by study personnel to determine eligibility.
Familiarization
Immediately following screening procedures, eligible participants were familiarized with the weight lifting exercises on Visit 1 and CPAT events on Visit 2). Participants were familiarized with the exercises using loads lighter than those used during data collection. For barbell exercises, participants used the unloaded barbell, for the kneeling rotational throw they used the lowest weighted medicine ball, for the banded Romanian deadlift, they used the lightest resistance band and only bodyweight was used to familiarize for the glute extension, to gain competency and learn proper form without causing any fatigue effects. Competency was determined based upon the exhibit of proper form and ability to follow the directions of the expert research team which was comprised of two exercise specialists with advanced education and training, a chiropractor, and a physical therapist. Participants were asked to perform the exercises until the research team was able to determine proper function.
EMG data collection and processing
During the manual muscle tests, weight lifting exercises, and CPAT events, surface EMG activity was recorded bilaterally from the deltoid, trapezius, latissimus dorsi, oblique abdominal, lower level of the lumbar multifidus, gluteus medius and maximus, and biceps femoris muscles. These muscles were selected due to the study’s emphasis on the posterior chain and pertinent upper extremity groups, along with technical limitation of the data capture system. EMG sensors were placed on the skin surface at the mid-point of the muscle bellies, according to the recommendations of the manufacturer (Noraxon USA, Scottsdale, AZ). The EMG sensors worn by the participant were small, lightweight, and did not disrupt study procedures. The sensors wirelessly transmitted their signal to the respective receivers which then transferred data to a computer for analysis. The Noraxon TeleMyo Direct Transmission System (Noraxon USA, Scottsdale, AZ) using Ag/AgCl snap electrodes (Noraxon USA, Scottsdale, AZ) with a 20-mm inter-electrode distance and a 1500-Hz sampling rate was used to collect electromyographic data. High frequency noise were removed using a 500-Hz low pass filter and movement artifacts were removed with a Butterworth high pass filter with a corner frequency of 20 Hz [13]. The signal was then full-wave rectified and normalized to a percentage of MVIC [14]. The entire span of EMG data was used for analysis for each muscle and exercise. The site selection, skin preparation procedures, and EMG data collection procedures were standardized and based on the manufacturer’s recommendations and previous work [8, 9].
Manual muscle tests
Immediately following site preparation and electrode placement, which took at least 60 minutes, participants underwent manual muscle tests for the selected muscles. The purpose of the manual muscle tests was to determine the maximum voluntary isometric contraction (MVIC) of the muscles under investigation to obtain peak EMG readings to normalize the EMG to % MVIC for further analyses and comparisons. Procedures for manual muscle tests were standardized following those described by Kendall [15] and the guidelines of the manufacturer (Noraxon USA, Scottsdale, AZ).
To obtain MVIC, the investigator positioned the specified muscle for isolation of maximum strength output in the applicable direction. The investigator instructed the participant to apply maximum force using the specified muscle pushing against the investigator (usually the investigator’s hands) for approximately three seconds, gradually building up force until maximum force was achieved. The investigator applied counterforce so that no movement occurred (i.e. isometric muscle test). The participant completed one trial for each muscle, with a 30-second rest between each tested muscle. All manual muscle tests were performed under the direct supervision of a chiropractor or physical therapist.
Weight lifting exercises
Immediately following MVIC determination and a five-minute rest, participants performed the following seven weight lifting exercises: back squat, Romanian deadlift, overhead press, bent-over row, banded Romanian deadlift, glute hyperextension, and kneeling rotational throw. These exercises are commonly used in strength, conditioning, fitness, work-site, and clinical settings for athletes, patients, and the general population, and have been shown to be safe [12, 16–27]. These exercises were selected since we desired to: 1) activate the largest number of major muscles groups with the fewest movements as possible, considering the time constraints of firefighters while on shift; and 2) activate the posterior chain and core muscles, considering our hypothesis that firefighter job activities place great demands on these muscles.
Participants performed 1 set of 10 repetitions for each exercise with approximately a three-minute rest between each exercise. We selected 10 repetitions because this is the mid-range of repetitions recommended for strength and hypertrophy training [7, 28]. During each exercise repetition, except for the rotational throw, the participant moved their body in a slow, controlled, and standardized manner until a certain position was achieved, then returned to the starting position, taking approximately four seconds for the entire repetition including the concentric and eccentric components. The order of exercises was randomly assigned.
The back squat, Romanian deadlift, and bent-over row were performed using a barbell according to National Strength and Conditioning Association (NSCA) exercise descriptions [29]. The squat and Romanian deadlift were completed at a load of 75% bodyweight (BWT) and the bent-over row was completed at 45% BWT. The overhead press was performed using a barbell at a load of 35% BWT in a manner similar to the standing barbell overhead press as previously described [30].
The kneeling rotational ball throw was performed bilaterally in a manner similar to the half-kneeling side-twist throw as previously described [26], using a 5.5-kg medicine ball for women or 6.4-kg medicine ball for men. In this study, participants threw the ball to the exercise specialist instead of against a wall. The throw and receive actions were performed at the speed and force based on the participant’s ability. The banded Romanian deadlift was performed with an elastic resistance band with a capacity of 22.7–54.5 kg in a manner adapted from a previously described movement [31]. In the study, participants only used the elastic band to encourage proper glute activation and hip movement patterns. The glute hyperextension was performed on a variable angle roman chair without external load with hands placed on the sternum [8]. The pelvic pad height was lowered and the participant’s feet were positioned in neutral or slight external rotation to emphasize glute activation. The movement reinforced activation of the glutes by focusing on glute contraction throughout the entire range of motion and limiting movement of the lumbar spine [32].
Load determination
The loads selected for the weight lifting exercises were determined by referencing NSCA’s recommendation for assigning load based on % BWT [33]. This is an alternative to the one repetition maximum (1RM) method for determining training load. It is a more pragmatic approach to determining training load for fire service implementation and is safer than performing a 1 RM test for on-shift firefighters. The selected loads, based on % BWT, were consistent with NSCA recommendations [33].
CPAT Events
Approximately 48 hours following completion of the weight lifting exercises, participants reported to the firefighter training grounds to complete the CPAT. The CPAT consists of 8 events (listed in the order performed in the current study): hose drag, equipment carry, ladder raise and extension, forcible entry, search, rescue, ceiling breach and pull, and stair climb. The events are traditionally completed in circuit fashion in a standardized sequence without rest between events [2]. In the current study, an approximately 3-minute rest was given between each event to match the rest period provided during the weight lifting portion of data collection. In addition, the stair climb was performed last, since it resulted in perspiration build up at the electrode sites, which impacted the EMG signal. For each event except the stair climb, the participant wore a 22.7-kg vest. For the stair climb, the participants wore a 34.1-kg vest.
For the hose drag, participants grasped the nozzle of a 4.5-cm hose, which is 61 m in length. They placed the hose over their shoulder and dragged it 23 m into a pre-marked area. They immediately assumed a kneeling position and pulled it 15 m until they reached a pre-marked line on the hose. For the equipment carry, participants removed two power saws (14.5 kg and 12.7 kg) from a tool cabinet, and placed them on the ground. Then, they picked up the saws, one in each hand, and walked a total of 23 m, returning to the tool cabinet where they placed both saws on the ground, and lifted them into the cabinet. For the ladder raise and extension, participants raised a 7-m ladder, hand over hand, until it was upright against a wall. Then, they grasped a rope attached to the extension portion of a second erect ladder and pulled the rope, hand over hand, until the ladder was fully extended and then immediately lowered the extension in a controlled fashion. For forcible entry, participants used a 4.5-kg sledge hammer to repeatedly strike a force-measuring target, until the target was moved to a predetermined position. During the search, participants crawled in a quadruped position through a 20-m dark tunnel, which contains changes in height and width. For the rescue, participants grasped a harness affixed to the torso of a 75-kg mannequin and dragged it around a drum for 23 m. For the ceiling breach and pull, participants used a 2-m pike pole to raise a 27-kg overhead load, 3 times. Then, they hooked the pole to an overhead 36-kg load, and pulled it down five times. This activity was repeated four times. For the stair climb, participants stepped onto a step mill, beginning with a 20-seconds warm up, ascending the stairs at a rate of 50 steps per minute, and progressing to 60 steps per minute for 3 minutes.
Statistical analysis
Means, standard deviations, and 95% confidence intervals were calculated for EMG values (as EMG values (as % MVIC)) for each muscle (8 muscles), CPAT event (8 events), and weight lifting exercise (7 exercises). Outliers (value of greater than 4 z score) were removed. After removing outliers, 99.7% of data remained and were normally distributed. The primary analysis was to compare EMG values (as % MVIC) of the CPAT Overall to Lift Overall for each muscle. The overall EMG value (as % MVIC) was calculated as the mean across the 8 CPAT events (CPAT Overall) and across the 7 weight lifting exercises (Lift Overall) for the 8 muscles, averaging the value of the right and left sides. T-tests and linear regression (adjusted for body height and body mass) were used to test for differences between CPAT Overall and Lift Overall for the 8 muscles. Within CPAT, analysis of variance (ANOVA) was used to compare EMG values (as % MVIC) among the 8 muscles for the 8 CPAT events. Similarly, within the weight lifting exercises, ANOVA was used to compare EMG values (as % MVIC) among the 8 muscles for the 8 CPAT events. On an exploratory basis, pairwise comparison of EMG values (as % MVIC) measures can be assessed using the relative 95% confidence intervals. Statistical significance was defined as p≤0.05.
Results
Comparison of EMG during the CPAT to the weight lifting exercises is shown in Table 1. EMG for each CPAT event and muscle group is shown in Table 2. EMG for each weight lifting exercise and muscle group is shown in Table 3. No difference (p > 0.05) in EMG activity was observed between weight lifting exercises (Lift Overall) and CPAT (CPAT Overall) for the deltoid, trapezius, lumbar multifidus, gluteus maximus and medius, and biceps femoris muscles. EMG activity was significantly greater (p < 0.001) during the CPAT compared to the weight lifting exercises for the abdominal obliques. EMG activity was significantly greater (p < 0.001) during the weight lifting exercises compared to CPAT for the latissimus dorsi. Within the CPAT events, EMG activity for abdominal obliques and lumbar multifidus was generally greater than other muscle groups. Within the weight lifting exercises, EMG activity for the lumbar multifidus was generally greater than other muscle groups.
Surface EMG data (shown as % MVIC) for the CPAT events (CPAT Overall) and weight lifting exercises (Lift Overall) for each muscle group
Surface EMG data (shown as % MVIC) for the CPAT events (CPAT Overall) and weight lifting exercises (Lift Overall) for each muscle group
Key: Values are mean (standard deviation).
Surface EMG data (% MVIC) for each CPAT event and muscle group
Key: Values are mean (95% confidence interval).
Surface EMG data (% MVIC) for each weight lifting exercise and muscle group
Key: Values are mean (95% confidence interval).
Adverse events and side effects. Three adverse events were reported related to skin irritation at the electrode sites, all of which were reported the morning following the participants’ involvement in suppression of a large structure fire. Nearly half (46.2%, 6/13) of participants experienced minor and self-limiting delayed onset muscle soreness following the weight lifting exercises.
This study accomplished its primary aim of comparing the EMG activity of relevant muscle groups during the CPAT to weight lifting exercises in firefighters. The study provides useful information for strength and conditioning specialists to utilize when designing exercise programs to match job demands in tactical athletes such as firefighters. The findings of this study support the primary hypothesis of less than 20% difference in EMG activity between CPAT and weight lifting exercises for the studied muscles groups, except for the abdominal obliques.
In the CPAT events, the abdominal oblique muscles (32.3% MVIC) and the lumbar multifidus muscles (32.5% MVIC) showed consistently greater EMG activity than any of the other assessed muscle groups. This finding supports previous literature that suggests activation of the core muscles is vital to meet the demands of the on-shift firefighter [12]. This finding is also consistent with the observation from a previous study that the cross-sectional area of the L4 and L5 multifidus muscles in firefighters is significantly larger than the general population [34]. The relatively larger lumbar multifidus in firefighters may be a specific adaptation to imposed demands [28]. Furthermore, for many of the CPAT events, the relatively high activation suggests that the abdominal obliques and the multifidus functioned as prime movers of the body in addition to their stabilizing and motor control functions.
The weight lifting exercises were sufficient in activating the lumbar multifidus compared to the CPAT (CPAT: 32.5%, Lifts: 38.8%), but were insufficient for the abdominal obliques (CPAT: 32.3%, Lifts 12.1%). The selected exercises, excluding the ball throw, were primarily sagittal plane movements. Thus, the selected exercises sufficiently challenged the multifidus in its primary role as a spinal extensor [35], but did not sufficiently challenge the abdominal obliques, which primarily stabilizes, rotates, and flexes the vertebral column [35]. While the rotational ball throw was included to primarily target the abdominal obliques, its activation (20.2% MVIC) was below the mean of the CPAT events (32.3% MVIC). The rotational ball throw is a ballistic movement with a brief activation period and long interval between repetitions as the ball is returned the participant. Because we averaged the activation across the entire set, this may partially explain why the rotational throw activation was lower. Similarly, many CPAT events activated the abdominal obliques as a stabilizer and a prime mover throughout the event, while the rotational ball throw only activated the abdominal as a prime mover. Another possible explanation for the discrepancy is the load of the ball (4.5–9.1 kg) was substantially less than the load during many of the CPAT events.
In the present study, the gluteal muscles were activated at a low level (15% MVIC) during the CPAT compared to other posterior chain muscles (i.e. abdominal obliques, lumbar multifidus), which may suggest sub-optimal glute activation. This finding possibly indicates that the glutes are unable to counteract forces of firefighting job demands and is another possible reason for an increased risk of low back injuries in firefighters [1]. Another plausible reason for the relatively low activation of the glutes is that firefighting job demands do not require considerable activation from this large muscle group. While future research is needed, exercises to improve strength, firing patterns, and motor control of the gluteal muscles should be considered in firefighters.
Limitations
Several limitations should be considered when interpreting the results of this study. The EMG data were analyzed as a mean of the entire set of weight lifting exercises and each CPAT event, introducing variability due to the between repetition interval and speed at which CPAT events were performed. This method was chosen because CPAT activities are continuous with no discernable discrete parts to break out and compare to each discrete repetition of the lifts, thus making the analysis a more pragmatic comparison, despite the potential variability induced. We chose a limited selection of muscles to analyze. Thus, activation of other major muscle groups that may contribute to force production during the CPAT is unknown. The limitations of the wireless EMG system and ability to keep multiple sensors attached without disrupting CPAT performance made inclusion of all muscle groups impractical. Notable excluded muscles were the quadriceps, gastrocnemius, biceps brachii, triceps brachii, and forearms. The weight lifting exercises were only performed with one load. The effect of other loads on muscle activity is unknown. The exercise loads were determined by a percentage of the participants’ bodyweight for pragmatic reasons previously mentioned. Participant body composition and level of resistance training may have altered the relative intensity of the loads. The order of the CPAT events was altered from the prescribed order and was not randomized. Pilot testing revealed that the stair climb caused an excessive amount of perspiration in the non-climate controlled fire training facility and resulted in electrodes falling off or failing. Thus, we chose to move the stair climb from the first event to the final event. For similar pragmatic reasons, we chose not to randomize the order of the CPAT events, which may have introduced variability due to fatigue. The participant sample was relatively small and from a specific population of firefighters within a single geographic area, therefore generalizability is unknown.
Future research is needed to fully characterize job demands to develop resistance exercise training programs in firefighters. For example, the impact of load carriage and unbalanced loads (e.g. single arm, rotational movements) that challenge obliques should be assessed. Alternative exercises (e.g. functional) that can be pragmatically implemented while firefighter are on-shift should be assessed, as well. Different exercise loads and muscle groups should be compared. The influence of on-shift resistance exercise on fatigue and job performance should also be examined.
Conclusion
This study’s results suggest evidence that the selected weight lifting exercises are adequate for activation of muscles utilized during firefighting activities, except for the obliques. This study also indicates that the obliques and multifidus are significantly activated during every firefighting activity in the CPAT. Thus, firefighter exercise programs should emphasize training the low back and core using the weight lifting exercises assessed in this study, along with other exercises to activate the obliques.
Occupational health specialists can use the results of this study to develop work-site exercise programs matching firefighter job demands. Based on the findings of the present study in combination with findings of other studies [3, 36], recommendations for matching exercise programs to firefighter job demands are as follows: Firefighters are tactical athletes and should train accordingly. Weight lifting exercises, along with body weight and functional training, should be incorporated into firefighter resistance training programs. In addition to resistance training, High Intensity Interval Training (HIIT) cardiovascular exercises should be performed to optimize agility and endurance levels [37]. Firefighters should focus on training the core and posterior chain, particularly the low back, obliques, glutes, and hamstrings, since core and posterior chain strength, endurance, and motor control are requirements for firefighters’ job demands [12]. To properly train the posterior chain muscles, the weight lifting exercises assessed in this study, along with additional exercises for the obliques, should be implemented. A variety of open and closed chain exercises, such as the Romanian deadlift and glute hyperextension, should be implemented to ensure adequate posterior chain muscle activation, accommodate individual preferences, and foster adherence.
Conflict of interest
None to report.
Footnotes
Acknowledgments
This study was funded by the Lincoln Endowed Chair Chiropractic Research Program, the Center for Neuromusculoskeletal Research, Morsani College of Medicine, University of South Florida. We thank Pinellas Technical Community College and the participating firefighters from St Petersburg Fire Rescue and Hillsborough Country Fire Rescue for their assistance with data collection.