“Tactical athletes” is a term for military and emergency personnel whose occupational performance is highly dependent on physical abilities and thus may benefit from job-specific physical training (16). Most of these athletes have minimum standards for physical performance that they must regularly meet, for either selection to a specific group or for continued membership in it. Because of the physical demands of both these occupations and their performance tests, tactical athletes have recently become a topic of increased interest in the broad field of exercise and sport science. However, as the needs of their occupations vary from those of sport athletes, specific tests and protocols are likely needed for them to more fully apply to these individuals. In addition, as different tactical athletes have different specific job tasks, the current study focuses on personnel that must carry torso-borne personal protective equipment.
Military physical performance research has previously evaluated a variety of variables including aerobic capacity, anaerobic capacity, and muscular strength. In particular, aerobic capacity is of interest, as many military task demands, such as long-duration foot patrols, are aerobic-dominant activities. In addition, every branch of the military uses an aerobic-dominant running task in their physical fitness tests. To understand the aerobic component of these tasks, research has used various ergometer protocols (e.g., treadmill). Literature involving treadmill protocols typically report V̇o2, respiratory exchange ratio (RER), and blood lactate concentration, and use V̇o2peak values obtained during these protocols synonymously with “aerobic capacity.” This term is somewhat misleading, as a V̇o2max value is the theoretical maximum V̇o2peak value an individual can achieve. V̇o2peak is sensitive to a variety of variables, including protocol duration (1,22) and exercise mode (17–19,21), whereas V̇o2max would represent the greatest V̇o2peak, regardless of these variables.
Several studies place the optimal V̇o2 protocol duration between 8 and 12 minutes (1,22), and most standardized protocols aim for participants to reach volitional fatigue within this time span. However, because of individual differences, not all participants on a standardized protocol will have the same time to exhaustion. To eliminate this issue, some self-paced, individualized protocols have been developed with target protocol durations within this range (6,20). However, these protocols have been primarily tested on participants with exceptional aerobic capacity, with the participants in both of these studies having a mean V̇o2peak over 60 ml·kg−1·min−1. Regarding mode specificity, work rate may be used to compare protocols (15), although further investigation into this method involving different modes is needed to fully understand the utility of this variable. Until this issue is resolved, it might be prudent for performance, training, and testing modalities to coincide with each other, and many mode-specific protocols have been developed to that end.
Military personnel commonly wear body armor, which represents a fixed, torso-borne load that must be carried during a variety of tasks. Load carriage in general has been investigated, including the effects of added weight to the extremities and the torso (8). Torso-borne load carriage has been studied in the context of metabolic efficiency and task performance (14), and a variable-load treadmill protocol has been developed to that end (13). Although this approach produced a V̇o2peak value similar to that of a nonweighted protocol, tactical athletes carry a relatively fixed load. In order for the treadmill protocol to better reflect task demands for tactical athletes, a fixed-weight protocol is needed. To assess the ability of a fixed-load, torso-borne weighted-walking treadmill protocol to produce a similar V̇o2peak as a nonweighted treadmill protocol, the current study was conducted. The weighted-walking protocol herein has been reportedly used in military populations, but this study is the first to evaluate the protocol in detail.
Experimental Approach to the Problem
A cross-over design was incorporated, where participants performed 2 sessions using 1 test protocol and then 2 additional sessions using another protocol. The sequence of the 2 protocols was counterbalanced to control for order effects. The 2 treadmill protocols involved either running without any additional load or walking with an additional, torso-borne load. Dependent variables were V̇o2peak, RER, work rate for the stage at which participants reached volitional fatigue, and work rate for the previous stage.
A total of 16 recreationally active men (training a minimum of 3 times per week, age range 18–26 years old) were recruited for this investigation. A minimum sample size of 15 was determined a priori using G*Power (University of Dusseldorf, Dusseldorf, Germany) (5) based on correlational p H1 = 0.9, α = 0.05, and power = 0.9. Participants were recruited from the university population, the university's Reserve Officer Training Corps, social media, and word-of-mouth. Institutional review board approval was obtained from the University of Memphis before the beginning of subject recruitment, and participants were informed of the benefits and risks of participation before the start of the study. All subjects gave written informed consent.
Participants came to the laboratory for 5 separate sessions. Each session lasted approximately 1 hour, and sessions were separated by a minimum of 48 hours to allow for recovery. Participants were asked to abstain from any additional strenuous lower-body activity for 48 hours before each data collection session, and each session was scheduled at the same time of the day (±1 hour). We attempted to mimic the conditions for testing as much as possible. The first session was for initial orientation, during which time consent paperwork was competed, demographic information was recorded (age, military experience, height, weight, and body composition), and all protocols were practiced. Participants practiced several stages from both protocols on the treadmill while wearing the mouthpiece and regulator. To practice stages involved in the weighted-walking protocol, participants also wore the weighted vest.
The remaining 4 sessions began with a 5-minute general warm-up on a stationary bike at a self-selected pace (7). Participants were then allowed to self-select any additional specific warm-up that they wished to perform. Although individual warm-ups varied, participants were asked to keep the warm-up consistent between each of their sessions. After the warm-up, participants performed either the weighted-walking treadmill protocol or the nonweighted treadmill protocol.
Body composition was estimated using Dual-energy X-Ray Absorptiometry (DXA) (Hologic Discovery-W; Hologic, Inc., Bedford, MA, USA). Participants were asked to lie supine with the hips internally rotated. A licensed DXA operator then placed participants into the appropriate position for a DXA scan and performed the whole-body scan.
During each incremental treadmill protocol, gas exchange was collected through a Parvo Medics TrueOne 2400 metabolic system (Parvo Medics, Inc., Sandy, UT, USA). Respiratory exchange ratio and V̇o2 were sampled breath-by-breath, recorded as an average every 15 seconds, and the greatest 15-second average relative V̇o2 was reported as V̇o2peak. Similarly, the greatest 15-second average RER was reported as the peak. Throughout the test, a heart rate monitor (Polar Electro, Lake Success, NY, USA) sampled participants' heart rates. All tests were performed using a mouthpiece and nose clip.
The nonweighted graded exercise test originated from Bloomer et al. (4) and involved 2-minute stages with incremental increases in either incline or speed on a treadmill. The speed started at 3.0 mph at a 0.0% grade. The first 4 workload increases consisted of 0.5 mph increases in speed. After an initial addition of a 5% grade, the protocol alternated between 0.5 mph increases in speed or 2.5% point increases in grade. Participants could choose to walk or run at any given stage during this nonweighted protocol, and the protocol ended once the participants reached volitional fatigue.
Each subject wore a 15.88 kg (35.00 lb) evenly weighted vest (The XVEST USA, LLC, TX, USA) for the duration of the weighted-walking graded exercise test and progressed as shown in Table 1. Participants adjusted the vest to ensure it had minimal motion during the treadmill protocol while allowing for maximal comfort. This test was terminated at either volitional fatigue or once both feet were simultaneously airborne (as during jogging or running). Each stage lasted 3 minutes. As with the nonweighted protocol, participants performed this protocol on 2 occasions to establish reliability and precision of the protocol.
Reliability of the protocols was assessed through Hopkins' calculations for intraclass correlation coefficient (ICC) (10), with a minimum acceptable value of 0.7 for reliability (2). Precision was assessed using both SEM and coefficient of variation. After assessing reliability and precision, Pearson's r was used to assess the correlation between the second trial of the protocol involving incremental running, and the second trial of the novel weighted-walking protocol involving incremental walking with a fixed weight. A Bland-Altman plot was created to visually represent and assess the criterion validity of the new protocol. Cohen's d was used to calculate effect size, and was interpreted based on Hopkins' interpretations (11). A paired-samples t-test assessed the differences between the mean of the 2 protocols. For all statistics, α was set a priori to 0.05.
One participant did not complete the entire study protocol because of an unrelated surgery after the orientation session. Anthropometric and demographic data from this participant were not included in the analysis (Table 2). Fifteen participants successfully completed all sessions within approximately 2 weeks from the first testing session.
Both protocols were reliable; V̇o2peak values for the first nonweighted trial (4.04 ± 0.53 L·min−1) and the second nonweighted trial (4.10 ± 0.55 L·min−1) yielded an ICC of 0.87. The V̇o2peak values for the first weighted trial (3.64 ± 0.48 L·min−1) and the second weighted trial (3.62 ± 0.58 L·min−1) yielded an ICC of 0.79 (Figure 1). The Bland-Altman plot for V̇o2peak for the second trial of each protocol does not show evidence of heteroscedasticity (Figure 2). Precision based on SEM for the nonweighted protocol and the weighted protocol was 0.13 and 0.17, respectively. Although somewhat redundant, the coefficient of variation values corroborates these results for the nonweighted protocol (3.1%) and for the weighted protocol (5.1%). As the 2 protocols were reliable, remaining analyses were performed on the second testing session for each protocol. The V̇o2peak values from the second trial of both protocols had a correlation of r = 0.90, p < 0.01, classified as “nearly perfect” (11). Although the protocols were reliable, the V̇o2peak values from both protocols differed significantly, t = 7.547, d = 2.47, p < 0.01, with the mean of the nonweighted protocol 13.3% greater than the mean of the weighted protocol (Figure 2).
Because the protocols yielded different V̇o2peak values, the protocols themselves were compared using work rate in an attempt to explain the difference in V̇o2peak (12). Work rate from the final stage of the nonweighted protocol (188.6 ± 53.5 W) was less than for the weighted protocol (257.2 ± 57.5 W), t = −5.85, d = 3.60, p < 0.01. In the nonweighted protocol, the mean work rate during the final stage successfully completed (155.3 ± 52.8 W) and the final stage attempted (188.6 ± 53.5 W) represented an average increase of 33.3 W or 21.4%. During the weighted protocol, the mean work rate during the final stage successfully completed (170.6 ± 54.8 W) and the final stage attempted (257.2 ± 57.5 W) represented an average increase of 86.6 W or 51%. The peak work rates for the last stage successfully completed were not significantly different, although the magnitude of the difference was large, t = −1.44, d = 0.83, p = 0.17. Peak work rate means for the penultimate and final stages are shown in Figure 3 and estimated mechanical work rates are show in Figure 4. (Table 3 and Figure 4).
The unloaded and loaded protocols performed herein for V̇o2peak seemed to be reliable (ICC ≥ 0.70). Therefore, further analysis of both protocols is warranted for the current investigation and for future studies. Coefficient of variation values for nonweighted protocols have been reported as being below 5% in endurance-trained athletes (3,9). This study's coefficient of variation values reflect this in both the nonweighted protocol (3.1%) and the weighted protocol (5.1%), despite this sample's lower training status. A lack of studies reporting coefficient of variation in recreationally trained individuals may be due to the difficulty recruiting less-trained participants to perform multiple tests to exhaustion. In addition, although other protocols have measured V̇o2peak with a torso-borne load (13), this protocol seems to be the first fixed-weight torso-borne treadmill protocol. Participants' lack of familiarity with this type of load carriage may also have further implications for intraparticipant weighted-walking performance variation. The Bland-Altman plot of these 2 treadmill protocols (Figure 2) visually represents the agreement, and consequently the bias, between the protocols.
The most important findings of this study are that the protocols were highly correlated, yet substantially different in V̇o2peak. In the interest of better understanding this novel protocol, work rate was examined. V̇o2max protocols using different modalities and yielding different V̇o2peak values have been shown to have similar peak work rates (15). For this reason, work rates were analyzed for the stages in the protocol. The work rate at which participants failed differed significantly between the protocols, with the weighted protocol's final-stage work rate an average of 139.2 W greater than the nonweighted protocol's final-stage work rate. Interestingly, the last successfully completed stage did not differ between the 2 protocols, suggesting that the increase in work rate between stages in the weighted-walking protocol may have been excessively large. This magnitude of increase could result in greater peripheral fatigue in the weighted protocol, partially explaining the lower V̇o2peak. This may reflect the large jump in incline and work rate being handled anaerobically rather than aerobically. The increase in work rate during the weighted protocol may have caused participants to reach volitional fatigue because of factors other than aerobic capacity, such as peripheral fatigue or limitations of an anaerobic energy system. If this protocol instead involved smaller increases in work rate, the aerobic system might have been stressed to a greater degree before the involvement of anaerobic energy systems, thereby yielding a greater V̇o2peak value. A proposed modification to the weighted protocol that involves smaller increases in work rate, a shorter first stage, and overall shorter stage duration but with similar protocol duration is shown in Table 3, whereas the projected accompanying changes in work rate are modeled in Figure 4.
This study was limited by several factors. V̇o2max is mode specific, and individuals who train a specific modality tend to attain the greatest V̇o2 values during their mode of choice. The target population of this study was tactical athletes, but the sample consisted of university students. Of these individuals, only 2 reported weighted-walking during training. No other participants reported any experience with weighted-walking, indicating that a mismatch may have existed between the sample in the current population and the intended target.
In conclusion, this novel weighted-walking protocol is both reliable and precise and can be used to determine a V̇o2peak with a nearly perfect correlation with a nonweighted V̇o2peak. However, to better understand the utility of this protocol, future studies need to include subjects that routinely perform loaded walking and involve smaller increases in work rate in an attempt to attain a V̇o2peak that is closer to V̇o2max. In addition, studies may want to examine a potential correction factor to obtain a V̇o2peak similar to a participant's nonweighted V̇o2peak. At this point, the protocol is promising but requires further investigation.
These data suggest that the current weighted treadmill walking protocol is both reliable and precise and produces V̇o2peak values with a near-perfect correlation with a nonweighted protocol. This suggests that testing individuals' aerobic capacity using a torso-borne, fixed-weight walking task may have utility but does not represent a truly unique piece of information. However, the current protocol needs to be modified to ensure more appropriate increases in work rate.
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