Firefighting is a very dangerous and physically strenuous occupation evidenced by the 64 on-duty deaths in 2011 and 81 on-duty firefighter deaths in 2012 (22). Heart attacks account for approximately 50% of these fatalities, and for every fatal cardiac event experienced by a firefighter, it is estimated that 17 additional nonfatal cardiac events occur among members of the fire service (20). Geibe et al. (8) showed that according to autopsy, an on-duty cardiac event is approximately 4 times more likely to be fatal, if a firefighter has established coronary heart disease (CHD), is a current smoker, or is hypertensive. However, these authors were not able to include low aerobic fitness in their analysis, although according to the American Heart Association, low aerobic fitness is among the risk factors for developing CHD and experiencing a heart attack (2). In fact, for the general population, some evidence suggests that low aerobic fitness has a stronger influence on CHD-related death than other risk factors (smoking, hypertension, hypercholesterolemia, obesity, and diabetes mellitus) (24). For firefighters, it has been hypothesized that low aerobic fitness may significantly increase risk for heart attack (4,6,20). Data illustrate that cardiovascular events are more likely to be precipitated by strenuous tasks than nonemergency tasks (14,20). For example, during fire suppression, the relative risk of heart attack is between 12 and 136 times higher than during nonemergencies (14). Several researchers have demonstrated that these job tasks can require high aerobic capacity (7,11,23,25). During simulated emergency scenarios, including walking, crawling, climbing stairs, and pushing, pulling, and carrying objects, all while wearing heavy gear, firefighters have exhibited a mean oxygen uptake of 31.5 ± 10.6 ml·kg·−1·min−1 (7), 38.5 ± 5.3 ml·kg·−1·min−1 (25), 43.8 +/− 5.5 ml/kg/min (11), and 44.0 ± 5.0 ml·kg·−1·min−1 (23). The unpredictable nature of these bouts of exertion can place even greater strain on the cardiovascular system. These situations are more strenuous for less fit firefighters, which likely increases their risk of experiencing a cardiac event.
Furthermore, evidence supports that for firefighters, possessing high aerobic fitness is associated with better outcomes on other CHD risk factors (4,6,8). In 968 male firefighters who underwent maximal graded exercise tests (GXT), higher maximal oxygen uptake (V[Combining Dot Above]O2max) was associated with lower resting diastolic blood pressure (p < 0.01), total cholesterol/HDL ratio (p < 0.01) and fasting blood glucose (p = 0.03), and higher HDL-cholesterol (p < 0.01), independent of age and body mass index (BMI) (4). With the same cohort of firefighters, Durand et al. (6) found that frequent (self-reported) participation in exercise, which was associated with higher aerobic fitness, had similar favorable effects on CHD risk factors, and these effects were consistent across all BMI categories.
In light of the proposed protection from heart attack afforded to firefighters by being aerobically fit, the International Association of Firefighters (IAFF) and the International Association of Fire Chiefs (IAFC) have advocated that all firefighters undergo regular V[Combining Dot Above]O2max testing (administered by qualified exercise professionals), and that they strive to possess a V[Combining Dot Above]O2max of at least 42 ml·kg·−1·min−1 (13). This threshold was identified by reviewing the studies in which oxygen consumption (V[Combining Dot Above]O2) was measured while firefighters participated in simulated emergency scenarios (7,11,23,25). Two investigations revealed that V[Combining Dot Above]O2 levels reached 42 ml·kg·−1·min−1 during these tasks (11,23). To improve adherence to the recommendation that firefighters undergo regular V[Combining Dot Above]O2max testing, the IAFF has endorsed the “Gerkin” treadmill test, a GXT in which participants run up a progressively increasing incline. This test is practical because it can be conducted in-house, requires only a standard treadmill, and has an acceptable margin of error associated with the prediction of V[Combining Dot Above]O2max (standard error of the estimate [SEE] = 3.7 ml·kg·−1·min−1) (21). Although the Gerkin protocol is of tremendous value to the fire service, a limitation is that it requires running. This is not ideal for 2 reasons: (a) Because the incidence of musculoskeletal injury is very high in the fire service (16), many firefighters cannot run for the length of time required to complete the test without exacerbating symptoms of knee, hip, and back pain, and (b) firefighters do not typically run as a function of their duties. Job task analyses reveal that firefighters and fire chiefs do not identify running as a critical and frequently performed job duty (19), and widely used firefighter physical ability tests derived from these job task analyses involve little to no running (The Firefighter Combat Challenge, and the Candidate Physical Ability Test [CPAT]) (26). The CPAT is used by a majority of fire departments today as an entry level test for recruits and is endorsed by the IAFF, while the Firefighter Combat Challenge has become a popular competition that firefighters from different departments compete in. Firefighters undergo both tests in full gear.
Therefore, another test is needed to predict the aerobic capacity of firefighters, which does not require running and ideally represents the physical demands of the job. Because firefighters regularly perform job tasks that require walking while wearing protective gear weighing up to 37 kg (7,11,23,25), a test that involves walking while wearing a weighted vest would better replicate their day-to-day activity and would be challenging enough for even very-fit firefighters. In fact, there are several firefighter job entry tests (which do not provide the ability to predict V[Combining Dot Above]O2max) that use a weighted vest. One such test, used to screen applicants for wildland firefighting, is called the “Pack Test” in which individuals walk a 3-mile course outside wearing a backpack weighing 20.4 kg. A weighted vest is also worn during the CPAT (19). Hence, a V[Combining Dot Above]O2max test with a weighted vest may be ideal in that it does not require running, is more job specific than other testing modalities, and would still pose a challenge to even fit firefighters.
Given the high incidence of cardiac events in the fire service and the tight relationship between aerobic fitness and risk for heart attack, it is imperative that screening tests be available to help firefighters understand their risk. However, these assessments must be feasible for fire departments to implement with respect to financial and personnel constraints; they must be able to be conducted in-house, be minimally invasive, and take little time. Another goal is for the test to be job specific and one that all firefighters can complete. The purpose of this study was to develop a treadmill walking test using a weighted vest, which can be performed on a standard treadmill, to predict V[Combining Dot Above]O2max in structural firefighters.
Experimental Approach to the Problem
To develop a walking test to measure V[Combining Dot Above]O2max of firefighters, researchers determined the following parameters for a proposed testing protocol: (a) a walking speed that would ensure a brisk pace without having to run, as a function of leg length, (b) a suitable amount of weight to add to a weighted vest (ZFO Sports, San Jose, CA, USA), as a percentage of body weight, and (c) the appropriate grade increases on the treadmill. Firefighters were then recruited to undergo the test. If the study participants elicited the criteria for reaching their V[Combining Dot Above]O2max during this protocol, an equation could be derived to predict V[Combining Dot Above]O2max based on the following independent variables: time achieved on the test, height, weight, walking speed, and age.
To develop the walking treadmill protocol, the research team recruited volunteers and experimented with stage length (1 minute, 2 minutes, etc.), grade increase, and vest weight (as a percentage of body weight) before recruiting study participants. The vest weight was standardized by body weight to better equalize the conditions for lighter vs. heavier participants. (Having all subjects carry the same amount of weight would have resulted in lighter individuals achieving fatigue sooner than heavier subjects.) Furthermore, the vest needed to be heavy enough so that subjects would be challenged and could reach their V[Combining Dot Above]O2max, but not so heavy that they would fatigue early. Ultimately, the goal was to establish a protocol that would allow the participants to achieve a steady-state heart rate with each stage (to avoid underestimation of V[Combining Dot Above]O2max) and ensure that firefighters across a range of fitness levels would achieve the criteria for reaching their V[Combining Dot Above]O2max.
The following protocol was piloted and ultimately chosen for use in this study by the researchers: before beginning the test, the participants completed a warm-up by walking on the treadmill for 5 minutes and then stretching the hamstrings and back muscles for 5 minutes. After this warm-up, they put on a vest weighing 20% of their body weight (within 0.5 kg). Vests are donned over the head (much like a safety vest) and secured by Velcro straps around the midsection. Weight is easily loaded and unloaded in secure pockets, distributed equally around the torso to minimize injury risk. The weight increments allowed us to get within 0.5 kg of the calculated weight for each firefighter.
The test began with the participant walking on the treadmill at 3.0 mph at 0% grade for the first 3 minutes. At the end of the third minute, the treadmill speed was increased to the predetermined walking speed that was based on leg length. They walked at this speed at 0% grade for 1 minute. At the end of the fourth minute, the grade on the treadmill increased to 1% and continued to increase by 1% each minute after that. The participant continued to walk at the predetermined speed while the grade increased each minute until exhaustion.
One outcome of the pretesting protocol development phase was the finding that a pre warm-up without the weighted vest was necessary. This adjustment was made after 2 of our initial participants complained of back pain and shin splints during the test. Thus, we implemented the warm-up described above before donning the vest and beginning the test (which includes another warm-up of 3 minutes of walking at 3.0 mph at 0% grade). This protocol change was well received and resulted in no further complaints from the remainder of the study participants.
A total of 38 paid and volunteer male firefighters from 2 departments in Oregon volunteered to participate in the study. All firefighters were structural firefighters who typically work in urban and suburban areas and fight building fires. The average age was 31.2 ± 7.7 years and ranged from 20 to 44 years (Table 1). Participants were free from chronic disease, including asthma and diabetes, which would deem them “high risk,” and they possessed no more than 1 risk factor for CHD as determined by the ACSM risk stratification guidelines (9). Participants were also free from injury to the back or lower extremity, which would affect their performance on a GXT. This study was approved by the Oregon State University Institutional Review Board, and all participants gave written informed consent before participation.
Firefighters completed a standard health history questionnaire before the assessment. Height, weight, and resting heart rate and blood pressure were recorded before each test. To determine walking speed, we used a Froude number (a dimensionless number that defines a speed/length ratio) that allowed us to standardize the walking speed of each participant by their leg length. First, leg length was measured from the greater trochanter to the lateral malleolus on the participant's right leg using a cloth measuring tape. Two separate measurements were taken by different researchers. If the 2 measurements were within 2 cm of one another, they were averaged; if the measurements were not within 2 cm, leg length was measured again until they were. Then, we used the principle: Froude number = U2·g·L−1, where U is traveling speed (m·s−1), g is the acceleration of gravity (9.8 m·s−1), and L is leg length (cm) to derive our formula for walking speed. A Froude number of 0.5 corresponds to the speed at which most people transition from walking to running; a higher Froude number indicates a faster pace (1). Thus, to ensure that the participants' pace did not encourage them to run, we decreased the Froude number to 0.4 in the calculation used to estimate the walking speed. Each participant's measured leg length was entered into the following equation:
The result of this equation was multiplied by 2.236 to obtain mph because our treadmill had units of mph. The calculated walking speeds ranged from 3.6 to 4.3 mph.
Throughout the V[Combining Dot Above]O2max tests, gas exchange was measured and recorded every 15 seconds by a metabolic measuring system (ParvoMedics, Sandy, UT, USA). The metabolic cart was calibrated using a 3-L syringe and air and gas mixtures of known composition before each testing session (once per day). Heart rate and rhythm were measured by a 12-lead electrocardiogram (ECG; Schiller, Baar, Switzerland). Blood pressure was monitored manually by the researcher during the tests. The criteria to determine whether firefighters reached their V[Combining Dot Above]O2max were either: (a) a rise in oxygen consumption of <2.0 ml·kg·−1·min−1 after a stage increase, or at least 2 of the following, (b) a respiratory exchange ratio of ≥1.15, (c) a heart rate of <10 b·min−1 of the participant's age-predicted maximum heart rate, or (d) a rating of perceived exertion (RPE) of ≥17 on the Borg's RPE scale ranging from 6 to 20 (10,12).
Maximal oxygen uptake was recorded as the highest measured value achieved during the last four 15-second intervals of the test, which reflects the procedures by other authors who have developed V[Combining Dot Above]O2max prediction equations (21).
After participating in the walking test, 13 participants volunteered to participate in the Gerkin running treadmill test to exhaustion (21). These tests took place between 1 and 4 weeks after their walking tests. Before the running tests, participants confirmed that their responses on the health history questionnaire had not changed since their first V[Combining Dot Above]O2max test.
A stepwise linear regression was used to create the prediction equation with measured relative V[Combining Dot Above]O2max as the dependent variable and time achieved on the test, height, weight, walking speed, and age as possible independent variables. The alpha level for significance was 0.05. For entry into the prediction equation, the time achieved on the test was converted to whole minutes. For example, if a participant achieved a time of 14:30, then 14.5 minutes was used in the analysis.
For cross-validation, a Bland-Altman plot was used to assess agreement between the measured relative V[Combining Dot Above]O2max during the walking test and that of the running test (5).
Performance indicators during the V[Combining Dot Above]O2max test are shown in Table 1. All of the firefighters met the criteria for achieving V[Combining Dot Above]O2max. Influential diagnostics revealed no outliers. Relative V[Combining Dot Above]O2max was normally distributed, although the average was slightly high (48.4 ± 6.5 ml·kg·−1·min−1), indicating that the sample was relatively fit. The average time achieved on the test was 16.95 ± 2.57 minutes and ranged between 8 and 22 minutes. The stepwise linear regression included time achieved on the test as the only significant variable predicting V[Combining Dot Above]O2max (p < 0.001). Time explained 76% of the variation in V[Combining Dot Above]O2max (R2 = 0.76), while the SEE for the linear prediction was 3.2 ml·kg·−1·min−1. The results of the regression are shown in Table 2. The equation derived is as follows:
Figure 1 shows a plot of the actual V[Combining Dot Above]O2max of the participants during the walking test and the predicted V[Combining Dot Above]O2max derived from the equation.
On average, participants' measured relative V[Combining Dot Above]O2max during the walking test was 4.62 ± 5.86 ml·kg·−1·min−1 lower than their V[Combining Dot Above]O2max during the running test. The difference ranged from −11.1 to 9 ml·kg·−1·min−1. The Bland-Altman plot in Figure 2 depicts the limits of agreement (2 SDs above and below the mean difference); the lower limit was −16.34 (95% confidence interval [CI], 10.2−22.47), and the upper limit was 7.11 (95% CI, 0.97–13.24).
We sought to develop a treadmill walking protocol and corresponding equation for predicting V[Combining Dot Above]O2max in firefighters, which would take minimal time and resources. The weighted vest protocol we developed resulted in all firefighters reaching their V[Combining Dot Above]O2max. The protocol accurately predicted V[Combining Dot Above]O2max in our sample. As evidence to this, the prediction equation derived from our test has a smaller SEE (3.2 ml·kg·−1·min−1) than the Gerkin protocol (3.7 ml·kg·−1·min−1) (21) and the Bruce protocol (3.35 ml·kg·−1·min−1), which is the most widely used treadmill test in existence (10).
The challenge, however when aiming to create a test that does not require an exercise testing treadmill, is that a standard treadmill is not capable of extremely high-grade increases. This is why the Bruce or Balke protocol, for example, cannot be conducted on a standard treadmill. For the making of this (Moore) protocol, we were able to use an exercise testing treadmill, and as such, we were able to increase the grade to 19% for the longest test in our study (22minutes). However, if this test was to be conducted in the field, some participants would reach the maximal grade on a standard treadmill before reaching exhaustion. Most standard commercially available treadmills are capable of 15% grade. Implementing our protocol, a participant must achieve a time of 19 minutes to reach a 15% grade. The predicted V[Combining Dot Above]O2max of a person achieving a time of 19 minutes on this test is 52.9 ml·kg·−1·min−1. Thus, we acknowledge that this is a limitation of the protocol for those very-fit individuals who want to learn their max capacity. However, we contend that the most important objective of this test is to screen firefighters for low aerobic capacity. Furthermore, a V[Combining Dot Above]O2max of 52.9 ml·kg·−1·min−1 is very high for any adult and well above the identified risk threshold for firefighters of 42 ml·kg·−1·min−1. A participant need to only achieve a 10% grade (a time of 14 minutes) to reach a predicted V[Combining Dot Above]O2max of 42 ml·kg·−1·min−1. In our relatively fit sample of firefighters, 6 of the 38 participants walked for longer than 19 minutes. The mean V[Combining Dot Above]O2max of these individuals was 57.9 ml·kg·−1·min−1, while all had measured V[Combining Dot Above]O2max values well above 42 ml·kg·−1·min−1. Thus, we believe this protocol to be a useful and valuable screening tool for the fire service, although we acknowledge it may not be suitable for predicting V[Combining Dot Above]O2max of highly fit firefighters.
Cross-validation confirmed that it is possible for firefighters' V[Combining Dot Above]O2max to be underestimated by this protocol, although the 2 observations that yielded the greatest difference (−11 ml·kg·−1·min−1) were associated with a walking time of ≥20 minutes, which as mentioned could not be accomplished on a standard treadmill. Furthermore, the greatest differences between V[Combining Dot Above]O2max values measured during the running test vs. the walking test tended to be among those with higher fitness levels. It is well known that people's V[Combining Dot Above]O2max can differ from one mode to another (e.g., bike vs. treadmill), and this is primarily dependent on the individual's proficiency in the exercise mode being used to measure aerobic capacity (17). Although V[Combining Dot Above]O2max was underestimated by more than 5 ml·kg·−1·min−1 for 7 of the 13 individuals in the cross-validation group (54%), 3 participants (23%) yielded the same or higher V[Combining Dot Above]O2max during walking compared with running. It is possible that leg fatigue induced by wearing a weighted vest contributed to the trend toward achieving a lower V[Combining Dot Above]O2max during walking compared with running, but we do not have data to investigate whether this is true. Further testing across a broader range of fitness levels is needed to better understand the true utility of the Moore protocol.
Because of the high rate of back injury among the firefighter population, we recommended that the firefighters undergoing the test should be coached to avoid bending forward at their waist when fatigue begins to set in. At higher intensities, we often had to encourage participants to “stand up straight” as a precaution to prevent increased spinal loads. Data support that spinal loads, and subsequently, stress may increase with forward bending that promotes greater spinal flexion (15).
Aside from the low SEE and the fact that this test is more job specific than other testing modalities, strength of our protocol is that it will be very feasible for fire departments to implement. First, it requires little training to administer. The tester needs only to weigh the participant, calculate 20% of their body weight, and ensure the weighted vest carries that amount of weight (being careful to account for the weight of the vest alone). Second, it uses the most typical piece of exercise equipment owned by most fire departments. One limitation of our study is that the prediction equation developed is only applicable to male firefighters because we were only able to recruit 2 female firefighters, too few to include in our analysis. However, it would certainly be of interest to investigate the utility of this protocol among a sample of female firefighters. A second limitation is that we were not able to derive a submaximal test because the weighted vest tended to cause artifact with our 12-lead ECG, so we believed that the measurement of heart rate was not precise enough for those purposes. However, many authors have stated that it is preferable for firefighters to undergo maximal exercise testing as opposed to submaximal because the prediction of V[Combining Dot Above]O2max is much more accurate using a maximal protocol (21), and because it is important to screen firefighters for cardiac complications before they enter physical situations on duty that require near maximal exertion (3,18). One of the major advantages of using a V[Combining Dot Above]O2max test is that it can also serve as a cardiac “stress test.” Although not necessary for the determination of V[Combining Dot Above]O2max, a qualified exercise physiologist or other health professionals can perform the test while the participant is connected to an ECG and interpret the data. Many exercise physiologists work with fire departments across the country and could use this (Moore) protocol to perform these dual services (measuring V[Combining Dot Above]O2max and conducting a stress test). Firefighters who possess abnormalities on the ECG can then be encouraged to see their cardiologist. In the American Journal of Cardiology, physicians Raymond and Barringer claim that performing submaximal tests does not properly simulate the working conditions of firefighters, and that relying on these tests for screening would result in failure to identify those at risk for experiencing myocardial ischemia (poor perfusion) or other life-threatening conditions during the high-intensity activities that they constantly contend with as a function of their jobs (18).
Although there is a fairly large body of literature surrounding firefighter health, the incidence of heart attacks has remained devastatingly high over the past 10 years (14,20). As such, there is clearly a need for adequate testing and screening for low aerobic capacity in this population. This new protocol will allow practitioners to screen firefighters for low aerobic capacity outside the laboratory and ideally onsite at the fire station. Furthermore, because this test was designed to screen firefighters whose aerobic capacity may put them at increased risk for a cardiac event, the test could be truncated once a firefighter achieves the minimum threshold (42 ml·kg·−1·min−1), particularly for individuals with musculoskeletal problems. Alternatively, it may permit firefighters, who were previously unable to undergo the Gerkin test, the opportunity to participate in the V[Combining Dot Above]O2max testing and gain increased knowledge of their fitness.
Compared with other protocols, this (Moore) protocol allows for excellent prediction of V[Combining Dot Above]O2max and is more representative of the job demands of firefighting. Furthermore, because our test requires only a standard treadmill, a weighted vest, and a qualified professional to administer it, the financial investment for a fire department is likely meager. Moreover, if a cardiac event is avoided as a result of undergoing the test, the return on investment would be astronomical.
The authors would like to thank the firefighters of the Corvallis and Albany, Oregon fire departments, for their willingness to perform tests of maximal exertion for the purpose of our research. A potential conflict of interest exists for Dr. Karlie Moore, an investigator on this study who acts as a health and fitness consultant for firefighters and fire departments. However, the development of this test does not lead to greater compensation for Dr. Moore. The results of the present study do not constitute endorsement of the product by the authors or the NSCA.
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