Intraspecific Correlations of Basal and Maximal Metabolic Rates in Birds and the Aerobic Capacity Model for the Evolution of Endothermy

The underlying assumption of the aerobic capacity model for the evolution of endothermy is that basal (BMR) and maximal aerobic metabolic rates are phenotypically linked. However, because BMR is largely a function of central organs whereas maximal metabolic output is largely a function of skeletal muscles, the mechanistic underpinnings for their linkage are not obvious. Interspecific studies in birds generally support a phenotypic correlation between BMR and maximal metabolic output. If the aerobic capacity model is valid, these phenotypic correlations should also extend to intraspecific comparisons. We measured BMR, Msum (maximum thermoregulatory metabolic rate) and MMR (maximum exercise metabolic rate in a hop-flutter chamber) in winter for dark-eyed juncos (Junco hyemalis), American goldfinches (Carduelis tristis; Msum and MMR only), and black-capped chickadees (Poecile atricapillus; BMR and Msum only) and examined correlations among these variables. We also measured BMR and Msum in individual house sparrows (Passer domesticus) in both summer, winter and spring. For both raw metabolic rates and residuals from allometric regressions, BMR was not significantly correlated with either Msum or MMR in juncos. Moreover, no significant correlation between Msum and MMR or their mass-independent residuals occurred for juncos or goldfinches. Raw BMR and Msum were significantly positively correlated for black-capped chickadees and house sparrows, but mass-independent residuals of BMR and Msum were not. These data suggest that central organ and exercise organ metabolic levels are not inextricably linked and that muscular capacities for exercise and shivering do not necessarily vary in tandem in individual birds. Why intraspecific and interspecific avian studies show differing results and the significance of these differences to the aerobic capacity model are unknown, and resolution of these questions will require additional studies of potential mechanistic links between minimal and maximal metabolic output

CARDIORESPIRATORY RESPONSES TO THE USFS WILDLAND FIREFIGHTER ARDUOUS PACK TEST

C.J. Alfiero,C.L. Dumke FACSM, B.C. Ruby FACSM, M.W. Bundle University of Montana, Missoula, MT US wildland firefighters administer over 30,000 physical tests per year to qualify candidates for the occupational demands of fire suppression. The primary assessment is the arduous pack test (APT) a 4.83 km hike that must be completed in 45 min while wearing a 20.45 kg pack. Delivery of individual feedback to guide the physical training of candidates is hampered by two factors; first, passing the pack test is widely considered the minimum performance level necessary needed for this occupation, and second, the binary nature of the assessment presents candidates with a task representing an unknown and self-selected exercise intensity. PURPOSE: To determine the cardiorespiratory response elicited by the APT within a subject population whose aerobic capacity and body masses vary. METHODS: 61 young (age = 22.8 ± 3.2 yrs) adults (36 males, Mb = 79.5 ± 8.8kg; 25 females, Mb = 67.5 ± 13.5kg; study range: 55.4 - 119.6 kg) performed the APT and subsequently underwent a hiking inclined-treadmill test to VO2peakwhile wearing a skin mounted heart rate (HR) monitor and 20.45-kg pack. RESULTS: 50 of the 63 subjects achieved the 45-min cutoff with a finishing time of 41.8 ± 2.1min, the non-passers had a mean time of 47.7 ± 2.7 min. Non-passers were 77% female and 23% male. The VO2peakvalues of the passing and non-passing groups were 49.4 ± 7.2 and 42.6 ± 9.6 mlO2kg-1 min-1, respectively; the study range was 62.1 to 30.8 mlO2kg-1 min-1. HR, whether expressed as a fraction of the subject’s maximum rate (passers = 81.2 ± 17.1 and non-passers = 79.9 ± 12.7% of HRmax), or as the fraction of the HR reserve (passers = 68.0 ± 7.9 and non-passers = 67.7 ± 15.3% of HR reserve) were not different between the groups. Regression of VO2peakon completion time yielded a negative relationship (R2 = 0.45). In contrast, the HR responses and completion time were consistent among the participants (R2\u3c 0.01 for both % of HRmaxand % of HR reserve). CONCLUSION: To successfully complete the APT candidates must achieve a HR reserve of 68% or less while maintaining a walking speed of 1.8 m s-1. These data suggest that monitoring HR during load carriage may be used to identify candidates with adequate and inadequate pre-fire season readiness

LOAD CARRIAGE AND ENERGY EXPENDITURE DURING TREADMILL VS FIELD TRIALS

K.E. Kamrud, R.J. Skoric, B.A. Ruple, J.T. Strang, C.J. Alfiero, M.W. Bundle University of Montana, Missoula, MT PURPOSE: Field trials evaluating the metabolic rates of individuals performing sustained bouts of overground load carriage are rare and difficult to compare to laboratory values. We evaluated whether the metabolic cost of load carriage was similar between 5min steady-speed treadmill trials administered at the average speed used during a 4.83 km outdoor field trial. METHODS: Subjects (n=10 [4 female], Mb = 74.4±12.4kg) performed a 4.83 km loaded (20.45kg) walking trial around a local athletic track (Missoula MT elev 975m). Subject speed was recorded every 400m throughout the trial, and expired gases were collected for 90 sec and stored in a Douglas bag for subsequent gas faction and volume analysis beginning at the 5, 20 and 35 min points. During a separate visit, we measured VO2from the subjects as they completed loaded, 5min, treadmill trials at speeds selected to match those used throughout the field trial. RESULTS: Average speed (1.88±0.09 m s-1) during the field trial fluctuated by an average maximum of 10.5±3.3% per subject throughout the trials. Accordingly, we compared the measured speed during the three periods of gas collection to similar speeds administered on the treadmill (i.e. within 0.1 m s-1). Measured rates of VO2throughout the overground trial were similar across time; t5= 29.0±4.5, t20= 30.1±4.5, t35= 31.1±4.3ml kg-1 min-1. During treadmill walks at speeds corresponding to those used overground, laboratory measures of VO2were t5= 27.8±5.7, t20= 39.4±5.4, t35= 30.4±4.8ml kg-1 min-1. As a result the greatest difference between laboratory and field measures was 2.0±6.2ml kg-1 min-1, obtained at the 20 min point of the trial. CONCLUSION: The strong numerical agreement and statistically indistinguishable values obtained during the overground vs treadmill loaded trials suggest that, speed dependent rather than environmental factors influence metabolic cost. We further conclude that laboratory based treadmill results can provide valid insight to the energy use patterns of overground, self-selected, locomotion patterns. This work was supported by a multi-investigator award, US Forest Service Agreement # 16-CR-11138200-005, to MB and colleagues

METABOLIC COST OF LOAD CARRIAGE: EVALUATING EXISTING MODELS WITH EMPERICAL DATA

J.T. Strang, C.J. Alfiero, C.L. Dumke FACSM, B.C. Ruby FACSM, M.W. Bundle University of Montana, Missoula, MT PURPOSE: Despite extensive and ongoing scientific study into the metabolic requirements of load carriage, an understanding quantifying the effect of speed, load, sex and body mass has yet to come forth and the extent to which established models predict these requirements is largely untested. Specifically, because existing experimental efforts have typically focused on relatively modest walking speeds using loads representing a fixed portion of the subject’s mass, extending the available predictions to applications where individuals complete a common task carrying an identical absoluteload provides estimates of unknown accuracy. Here, we measured the energy use in a large subject group walking at speeds surrounding the 1.8 m s-1necessary to successfully complete the 4.83 km USFS wildland firefighter arduous pack (20.5kg) test, and compared these results to estimates available from the prevailing models. METHODS: We measured VO2from 61 young (age = 22.8±3.2 yrs) adults (36 males, Mb = 79.5±8.8kg; 25 females, Mb = 67.5±13.5kg; study range: 55.4-119.6 kg) as they performed four, 5min trials, with a 20.5kg pack, on a level treadmill at 1.7, 1.8, 1.9 m s-1, and their individual average speed from a previously administered pack test. We used the methods of Pandolf et al. 1977 and Ludlow & Weyand 2017 to generate VO2estimates for the 217 individual trials we administered. RESULTS: Measured values of VO2increased from 22.5±3.3 and 24.2±4.1 ml kg-1 min-1at 1.7 m s-1,to 31.3±5.2 and 30.5±4.3 ml kg-1 min-1at the fastest speed administered for males and females respectively. In contrast, the accuracy of the predictive models decreased with speed and yielded prediction errors of -12.1 and -22.6% at 1.7 m s-1 for the Pandolf and Ludlow & Weyand methods respectively, these errors were -17.2 and -31.0% at the fastest speeds administered. When evaluated at the speed subjects used in the field trial the prediction models underestimated energy expenditure by 5.7±4.6 and 9.8±4.8 ml kg-1 min-1respectively. CONCLUSION: We conclude that existing predictive models do not retain their internal accuracy, and substantially underestimate measured values when applied to a group of male and female subjects undertaking relatively fast walking speeds with a heavy load. Supported by US Forest Service Agreement # 16-CR-11138200-005, to CD, BR and M

DOES THE METABOLIC COST OF LOAD CARRIAGE DIFFER BETWEEN MALES AND FEMALES?

R.L. Baker, K.M. Berglund, H.E. Habighorst, J.T. Strang, C.J. Alfiero, M.W. Bundle University of Montana, Missoula, MT PURPOSE: The scientific understanding of energy use during load carriage suggests that the additional metabolic increment necessary to support an external load is determined by the load’s percentage of the subject’s body weight. Accordingly, for comparison purposes experimental undertakings often adjust the mass of an external load to represent a constant fraction of each subject’s mass. However, in occupational and applied settings, individuals are frequently asked to support similar absolute loads irrespective of their body weight. Here, we evaluated whether the energy requirements in male and female subjects differed during treadmill walking across a range of speeds, while supporting a common 20.5kg external load. METHODS: We measured VO2during three, 5min trials, administered with a 20.5kg pack, on a level treadmill at 1.7, 1.8, 1.9 m s-1, from 20 young adults(age = 22.1±2.4 yrs),who had been assigned as sex-matched pairs on the basis of mass (10 males, Mb= 72.6±6.3kg; 10 females, Mb = 72.8±6.2kg; range 63.6-82.7kg, difference between pairs = 0.6±0.5kg, max 1.4kg). RESULTS: Measured values of VO2in females were 24.7±4.2, 28.9±3.7, and 30.8±3.3ml kg-1 min-1at 1.7, 1.8, and 1.9 m s-1, respectively, whereas these values in males, although lower, were statistically(min p-value = 0.08) indistinguishable and were 23.1±3.3, 25.8±3.7, and 30.1±4.6 ml kg-1 min-1at the same speeds. Nonetheless, we note our data provide 27 points of comparison, with identical loads, at similar speeds (3 of 10 female subjects were unable to complete the 1.9 m s-1trial), across 10 males and females who are very similar in mass; in 8 of these 27 points of comparison females were more economical than their matched pair. CONCLUSION: Our data lends support to the presence of a sex based difference in load carriage economy, warranting further study. We note also that the similar rates of energy expenditure between the sexes observed here, translate to higher relative intensities for females due to their likely lower mass-specific aerobic capacities (i.e. VO2max). This work was supported by a multi-investigator award, US Forest Service Agreement # 16-CR-11138200-005, to MB and colleagues

METABOLIC AND CARDIOVASCULAR ALTERATIONS DURING CRITICAL TRAINING IN WILDLAND FIREFIGHTERS

S.C. Gurney, K.S. Christison, C.M. Williamson-Reisdorph, K.G.S. Tiemessen, J.A. Sol, T.S. Quindry, M.W. Bundle, C.G. Palmer, J.C. Quindry, FACSM, C.L. Dumke, FACSM University of Montana, Missoula, MT Wildland firefighters (WLFF) are confronted with numerous physical and mental stressors. Pre-fire season includes an intense 2- week critical training (CT) period; a preparatory phase that can result in injury, illness, and rhabdomyolysis. PURPOSE: To identify physiologic changes in metabolic biomarkers that occur during 2 weeks of CT in WLFF. METHODS: Eighteen male (29.4±1.1 yr, 182.1±1.6 cm) and three female (26.7±2.6 yr, 169.5±4.2 cm) participants were recruited from a Type I interagency hotshot fire crew and monitored over their 2-week CT. Fitness was assessed via BLM Fitness Challenge (push-ups, pull- ups, sit-ups, 1.5 mi run). Subjects were asked to fast and abstain from caffeine for blood draws on days 1, 4, 8, and 11. Plasma was analyzed for changes in the metabolic profile and oxidative stress markers 3-Nitrotyrosine (3NT), 8-Isoprostane (8ISO), and Lipid Hydroperoxides (LOOH). A one-way repeated measures ANOVA was used to analyze 8ISO, 3NT, and LOOH. Paired samples t-tests were used to compare metabolic biomarkers. Data presented as mean±SEM. RESULTS: CT elicited decrease in total cholesterol (TC) (173.6±12.1 to 153.4±8.6 mg·dL-1, p=0.01), hemoglobin A1c (5.2±0.1 5.1±0.1 %, p=0.003), hemoglobin (15.5±0.4 to 14.3±0.3 g·dL- 1, p=0.003), and estimated plasma volume to (53.8±0.7 to 50.7±1.4 %, p=0.005) from day 1 to 11. No difference was observed in high- density lipoprotein cholesterol. A main effect for time was observed in 8ISO (p- 1). 3NT was significantly elevated from day 4 to day 8 (d4: 2.4±0.6, d8: 2.9±0.6 mg·mL-1). LOOH showed a significant increase across all days (d1: 2.2±0.4, d4: 2.8±0.5, d8: 3.4±0.5, d11: 4.0±0.6 mM). Fitness was significantly correlated with DTC (r=0.58, p=0.046) and D8ISO (r=0.60, p=0.050). CONCLUSION: These data suggest the exertion required of WLFF during CT results in positive alterations to the metabolic profile. The changes in oxidative stress markers may reflect rapid adaptation to the CT stressors. These data suggest that WLFF are able to adapt quickly to the physical stress of CT, where fitness may be a protector of metabolic perturbations. Funded by the US Forest Service 16-CR-11138200-005

SEASONAL CHANGES IN CARDIOVASCULAR FUNCTION, RISK FACTORS, AND OXIDATIVE STRESS OF WILDLAND FIREFIGHTERS

C.M. Williamson-Reisdorph, K.S. Christison, S.C. Gurney, K.G.S. Tiemessen, J.A. Sol, T.S. Quindry, C.G. Palmer, M.W. Bundle, C.L. Dumke, FACSM, J.C. Quindry, FACSM University of Montana, Missoula, MT Wildland firefighters (WLFF) experience extreme physiological strain throughout a typical season due to intense occupational demands and consistent woodsmoke exposure. There is a rationale to indicate that accumulated physiological strain, and oxidative stress, throughout a WLFF season has the potential to negatively alter cardiovascular function and risk factors. PURPOSE: The purpose of the study was to examine the effects of a season on cardiovascular function, risk factors, and markers of oxidative stress in WLFF. METHODS: Fourteen members of a Type I interagency hotshot crew participated in the study (males: n=13, females: n=1, age: 30.1 years ± 4.8). Pre- and post-seasonal resting measurements (May, October) were obtained for heart rate variability (lnRMSSD, lnHF, lnHF, LF:HF), pulse wave velocity (PWV), blood lipid panels (TC, TG, LDL, HDL), metabolic biomarkers (blood glucose, HbA1c), blood pressure (SBP, DBP) and blood oxidative stress (3-nitrotyrosine, 8-isoprostane, lipid hydroperoxides). Paired samples t-tests were used to identify differences among pre- and post- seasonal values. RESULTS: There were no seasonal effects observed on resting heart rate variability, PWV, 3-nitrotyrosine, 8-isoprostane, TC, TG, LDL, blood glucose, SBP, or DBP (p\u3e0.05). A significant reduction occurred in HDL (Pre: 53 mg/dL ± 14, Post: 45 mg/dL ± 18, p=0.043) and both HbA1c (Pre: 5.2% ± 0.2 , Post: 5.3% ± 0.2, p=0.034) and lipid hydroperoxides (Pre: 2.5 ± 0.5, Post: 4.9 ± 0.6, p = 0.003 ) increased from pre- to post-season. CONCLUSION: These data suggest a WLFF season did not impact resting markers of heart rate variability, pulse wave velocity, 3-nitrotyrosine, and 8-isoprostane. Alterations in metabolic biomarkers of cardiovascular risk factors (HDL and HbA1c) and lipid hydroperoxides demonstrate unfavorable seasonal changes, suggesting that the WLFF season may increase cardiovascular risk. Funded by the USFS 16-CR-11138200-005