Accuracy of Metabolic Cost Predictive Equations During Military Load Carriage

Metabolic Costs of Walking with Weighted Vests

INTRODUCTION

The US Army Load Carriage Decision Aid (LCDA) metabolic model is used by militaries across the globe and is intended to predict physiological responses, specifically metabolic costs, in a wide range of dismounted warfighter operations. However, the LCDA has yet to be adapted for vest-borne load carriage, which is commonplace in tactical populations, and differs in energetic costs to backpacking and other forms of load carriage.

PURPOSE

The purpose of this study is to develop and validate a metabolic model term that accurately estimates the effect of weighted vest loads on standing and walking metabolic rate for military mission-planning and general applications.

METHODS

Twenty healthy, physically active military-age adults (4 women, 16 men; age, 26 ± 8 yr old; height, 1.74 ± 0.09 m; body mass, 81 ± 16 kg) walked for 6 to 21 min with four levels of weighted vest loading (0 to 66% body mass) at up to 11 treadmill speeds (0.45 to 1.97 m·s -1 ). Using indirect calorimetry measurements, we derived a new model term for estimating metabolic rate when carrying vest-borne loads. Model estimates were evaluated internally by k -fold cross-validation and externally against 12 reference datasets (264 total participants). We tested if the 90% confidence interval of the mean paired difference was within equivalence limits equal to 10% of the measured walking metabolic rate. Estimation accuracy, precision, and level of agreement were also evaluated by the bias, standard deviation of paired differences, and concordance correlation coefficient (CCC), respectively.

RESULTS

Metabolic rate estimates using the new weighted vest term were statistically equivalent ( P < 0.01) to measured values in the current study (bias, -0.01 ± 0.54 W·kg -1 ; CCC, 0.973) as well as from the 12 reference datasets (bias, -0.16 ± 0.59 W·kg -1 ; CCC, 0.963).

CONCLUSIONS

The updated LCDA metabolic model calculates accurate predictions of metabolic rate when carrying heavy backpack and vest-borne loads. Tactical populations and recreational athletes that train with weighted vests can confidently use the simplified LCDA metabolic calculator provided as Supplemental Digital Content to estimate metabolic rates for work/rest guidance, training periodization, and nutritional interventions.

The test-retest reliability of physiological and perceptual responses during treadmill load carriage

PURPOSE

Understanding the test-retest reliability of physiological responses to load carriage influences the interpretation of those results. The aim of this study was to determine the test-retest reliability of physiological measures during loaded treadmill walking at 5.5 km h-1 using the MetaMax 3B.

METHODS

Fifteen Australian Army soldiers (9 male, 6 female) repeated two 12-min bouts of treadmill walking at 5.5 km h-1 in both a 7.2 kg Control condition (MetaMax 3B, replica rifle) and a 23.2 kg Patrol condition (Control condition plus vest) across three sessions, separated by one week. Expired respiratory gases and heart rate were continuously collected, with the final 3 min of data analysed. Ratings of Perceived Exertion and Omnibus-Resistance Exercise Scale were taken following each trial. Reliability was quantified by coefficient of variation (CV), intra-class correlation coefficients (ICC), smallest worthwhile change (SWC), and standard error of the measurement.

RESULTS

Metabolic and cardiovascular variables were highly reliable (≤ 5% CV; excellent-moderate ICC), while the respiratory variables demonstrated moderate reliability (< 8% CV; good-moderate ICC) across both conditions. Perceptual ratings had poorer reliability during the Control condition (12-45% CV; poor ICC) than the Patrol condition (7-16% CV; good ICC).

CONCLUSIONS

The test-retest reliability of metabolic and cardiovascular variables was high and relatively consistent during load carriage. Respiratory responses demonstrated moderate test-retest reliability; however, as the SWC differed with load carriage tasks, such data should be interpreted independently across loads. Perceptual measures demonstrated poor to moderate reliability during load carriage, and it is recommended that they only be employed as secondary measures.

A comparison of physical performance during one- and two-person simulated casualty drags

The ability to drag a casualty to safety is critical for numerous physically demanding occupations. This study aimed to establish whether the pulling forces during a one-person 55 kg simulated casualty drag is representative of a two-person 110 kg drag. Twenty men completed up to 12 × 20m simulated casualty drags using a drag bag (55/110 kg) on a grassed sports pitch, with completion times and forces exerted measured. Completion time for the one-person 55 and 110 kg drags were 9.56 ± 1.18s and 27.08 ± 7.71s. Completion time for the 110 kg two-person drags for forwards and backwards iterations were 8.36 ± 1.23s and 11.04 ± 1.11s. The average individual force exerted during the one-person 55 kg drag was equivalent to the average individual contribution during the two-person 110 kg drag (t₍₁₆₎ = 3.3780, p < 0.001); suggesting a one-person 55 kg simulated casualty drag is representative of the individual contribution to a two-person 110 kg simulated casualty drag. Individual contributions can however vary during two-person simulated casualty drags.

Challenging traditional carbohydrate intake recommendations for optimizing performance at high altitude

PURPOSE OF REVIEW

To highlight emerging evidence challenging traditional recommendations to increase carbohydrate intake to optimize performance at high altitude.

RECENT FINDINGS

Several studies have now clearly demonstrated that, compared with sea level, exogenous carbohydrate oxidation during aerobic exercise is blunted in lowlanders during initial exposure to high altitude. There is also no apparent ergogenic effect of ingesting carbohydrate during aerobic exercise on subsequent performance at high altitude, either initially after arriving or even after up to 22 days of acclimatization. The inability to oxidize and functionally benefit from exogenous carbohydrate intake during exercise after arriving at high altitude coincides with hyperinsulinemia, accelerated glycogenolysis, and reduced peripheral glucose uptake. Collectively, these responses are consistent with a hypoxia-mediated metabolic dysregulation reflective of insulin resistance. Parallel lines of evidence have also recently demonstrated roles for the gut microbiome in host metabolism, bioenergetics, and physiologic responses to high altitude, implicating the gut microbiome as one potential mediator of hypoxia-mediated metabolic dysregulation.

SUMMARY

Identification of novel and well tolerated nutrition and/or pharmacological approaches for alleviating hypoxia-mediated metabolic dysregulation and enhancing exogenous carbohydrate oxidation may be more effective for optimizing performance of lowlanders newly arrived at high altitude than traditional carbohydrate recommendations.

Real-world walking economy: can laboratory equations predict field energy expenditure?

We addressed a practical question that remains largely unanswered after more than a century of active investigation: can equations developed in the laboratory accurately predict the energy expended under free-walking conditions in the field? Seven subjects walked a field course of 6,415 m that varied in gradient (-3.0 to +5.0%) and terrain (asphalt, grass) under unloaded (body weight only, W_b) and balanced, torso-loaded (1.30 × W_b) conditions at self-selected speeds while wearing portable calorimeter and GPS units. Portable calorimeter measures were corrected for a consistent measurement-range offset (+13.8 ± 1.8%, means ± SD) versus a well-validated laboratory system (Parvomedics TrueOne). Predicted energy expenditure totals (mL O₂/kg) from four literature equations: ACSM, Looney, Minimum Mechanics, and Pandolf, were generated using the speeds and gradients measured throughout each trial in conjunction with empirically determined terrain/treadmill factors (asphalt = 1.0, grass = 1.08). The mean energy expenditure total measured for the unloaded field trials (981 ± 91 mL O₂/kg) was overpredicted by +4%, +13%, +17%, and +20% by the Minimum Mechanics, ACSM, Pandolf, and Looney equations, respectively (corresponding predicted totals: 1,018 ± 19, 1,108 ± 26, 1,145 ± 37, and 1,176 ± 24 mL O₂/kg). The measured loaded-trial total (1,310 ± 153 mL O₂/kg) was slightly underpredicted by the Minimum Mechanics equation (-2%, 1,289 ± 22 mL O₂/kg) and overpredicted by the Pandolf equation (+13%, 1,463 ± 32 mL O₂/kg). Computational comparisons for hypothetical trials at different constant speeds (range: 0.6-1.8 m/s) on variable-gradient loop courses revealed between-equation prediction differences from 0% to 37%. We conclude that treadmill-based predictions of free-walking field energy expenditure are equation-dependent but can be highly accurate with rigorous implementation.NEW & NOTEWORTHY Here, we investigated the accuracy with which four laboratory-based equations can predict field-walking energy expenditure at freely selected speeds across varying gradients and terrain. Empirical tests involving 6,415-m trials under two load conditions indicated that predictions are significantly equation dependent but can be highly accurate (i.e., ±4%). Computations inputting identical weight, speed, and gradient values for different theoretical constant-speed trials (0.6-1.8 m/s) identified between-equation prediction differences as large as 37%.