The effects of thoracic load carriage on maximal ambulatory work tolerance and acceptable work durations

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.

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 physiological demands of helicopter winch rescue in water and over land

Physically demanding water and over land winch rescues are critical tasks for helicopter paramedics. To assess the physiological demands of winch rescue, 14 intensive care flight paramedics (12 male, 2 female, mean (±SD) age 44.3 (±5.4) years, experience 7.1 (±5.2) years) completed land and water-based task simulations. For the land task, VO₂ was 41.7 (±4.5) mL kg^-1 min^-1, or 86 (±11) % of VO_2peak. Task duration was 7.0 (±3.6) min, or 53 (±27) % of maximal acceptable work duration (MAWD) (13.2 (±9.0) min). For the water task, VO₂ was 36.7 (±4.4) mL kg^-1 min^-1, (81 (±12) % of VO_2peak). Water task duration was 10.2 (±1.1) min, or 47.6 (±4.8) % of calculated MAWD (21.0 (±15.6) min). These results demonstrate that helicopter rescue paramedics work at very high physiological workloads for moderate durations, and these demands should be considered when developing selection tests and when deploying to rescues, to ensure staff are capable of task performance. Practitioner summary: Paramedics performed helicopter winch rescue task simulations in water and over land. Paramedics worked at 81% of VO_2peak for 10.2 min and 86% of VO_2peak for 7 min for swim and land tasks respectively. Rescue organisations should consider these demands when selecting and credentialing staff and when deploying to incidents. Abbreviations: HEMS: helicopter emergency medical service; ICFP: intensive care flight paramedic; MAWD: maximal acceptable work duration; PES: physical employment standards; SAR: search and rescue.

Cardiorespiratory responses of load carriage in female and male soldiers

PURPOSE

To investigate the effect of sex and load carriage on cardiorespiratory responses to high intensity exercise in male and female soldiers.

METHODS

Soldiers (9 women, 9 men) performed a graded treadmill test until exhaustion with no load (NL) and combat-gear with body armor (CG). Cohen's d effect sizes, paired t-tests and ANOVA were used to study differences between conditions. A mixed linear regression model analyzed the relationship between heart rate (HR) and oxygen uptake (V̇O₂) with load and between sexes.

RESULTS

Wearing CG resulted in, for both sexes, a decreased time to exhaustion (-11 min), lower V̇O_2peak (L/min) ES = 0.56; VO_2peak (mL//kg/min) ES = 2.44, both p < 0.001, a net decrease in minute ventilation (ES = 3.53) and no change in HR_max. No sex-difference were present except for absolute V̇O_2peak. The VO₂ and HR relationship showed a cardiorespiratory reduction wearing CG vs. NL. Added load was equal between sexes, although female soldiers' CG relative to body mass was higher (25%) than male soldiers' (20%), p < 0.01.

CONCLUSION

Wearing CG reduces soldiers' cardiorespiratory capacity and exercise performance level, although the reduction cannot be explained solely based on the added load of CG, instead CG seems to restrict the capacity to fully ventilate. No sex differences were found in relative cardiorespiratory responses to wearing CG compared to NL.

Scaling the peak and steady-state aerobic power of running and walking humans

PURPOSE

The first aim of this experiment was to evaluate the appropriateness of linear and non-linear (allometric) models to scale peak aerobic power (oxygen consumption) against body mass. The possibilities that oxygen consumption would scale allometrically across the complete metabolic range, and that the scaling exponents would differ significantly between basal and maximal-exercise states, were then evaluated. It was further hypothesised that the scaling exponent would increase in a stepwise manner with elevations in exercise intensity. Finally, the utility of applying the scaling exponent derived for peak aerobic power to another population sample was evaluated.

METHODS

Basal, steady-state walking and peak (treadmill) oxygen-consumption data were measured using 60 relatively homogeneous men (18-40 year; 56.0-117.1 kg), recruited across five mass classes. Linear and allometric regressions were applied, with the utility of each scaling method evaluated.

RESULTS

Oxygen consumption scaled allometrically with body mass across the complete metabolic range, and was always superior to both ratiometric analysis and linear regression. The scaling exponent increased significantly from rest (mass^0.57) to maximal exercise (mass^0.75; P < 0.05), but not between steady-state walking (mass^0.87) and maximal exercise (P > 0.05). When used with an historical database, the maximal-exercise exponent successfully removed the mass bias.

CONCLUSION

It has been demonstrated that the oxygen consumption of healthy humans scales allometrically with body mass across the entire metabolic range. Moreover, only two scaling exponents (rest and exercise) were required to produce mass-independent outcomes from those data. Accordingly, ratiometric and linear regression analyses are not recommended as scaling methods.

Effects of modern military backpack loads on walking speed and cardiometabolic responses of US Army Soldiers

INTRODUCTION

Military leaders must understand how modern military equipment loads affect trade-offs between movement speed and physiological strain to optimize pacing strategies.

PURPOSE

To evaluate the effects of load carried in a recently developed military backpack on the walking speed and cardiometabolic responses of dismounted warfighters.

METHODS

Fifteen soldiers (1 woman, 14 men; age, 22 ± 2 years; height, 173 ± 7 cm; body mass (BM), 73 ± 10 kg) completed incremental walking tests with four external load conditions (0, 22, 44, or 66% BM) using the US Army's newest backpack: the Modular Lightweight Load-Carrying Equipment 4000 (MOLLE 4000). Oxygen uptake (V̇O₂) and heart rate (HR) were evaluated relative to maximal values (V̇O_2max and HR_max respectively). Testing ceased when participants completed the highest tested speed (1.97 m s^-1), exceeded a respiratory exchange ratio (RER) of 1.00, or reached volitional exhaustion.

RESULTS

Peak speed significantly decreased (p < 0.03) with successively heavier loads (0% BM, 1.95 ± 0.06 m s^-1; 22% BM, 1.87 ± 0.10 m s^-1; 44% BM, 1.69 ± 0.13 m s^-1; 66% BM, 1.48 ± 0.13 m s^-1). Peak V̇O₂ was significantly lower (p < 0.01) with 0% BM (47 ± 5% V̇O_2max) than each load (22% BM, 58 ± 8% V̇O_2max; 44% BM, 63 ± 10% V̇O_2max; 66% BM, 61 ± 11% V̇O_2max). Peak HR was significantly lower (p < 0.01) with 0% BM (71 ± 5% HR_max) versus each load (22% BM, 83 ± 6% HR_max; 44% BM, 87 ± 6% HR_max; 66% BM, 88 ± 6% HR_max).

CONCLUSION

Overburdened warfighters suffer severe impairments in walking speed even when carrying recently developed military load carriage equipment. Our results suggest that the relative work intensity of heavy load carriage may be better described when expressed relative to HR_max versus V̇O_2max.

Physiological interactions with personal-protective clothing, physically demanding work and global warming: An Asia-Pacific perspective

The Asia-Pacific contains over half of the world's population, 21 countries have a Gross Domestic Product <25% of the world's largest economy, many countries have tropical climates and all suffer the impact of global warming. That 'perfect storm' exacerbates the risk of occupational heat illness, yet first responders must perform physically demanding work wearing personal-protective clothing and equipment. Unfortunately, the Eurocentric emphasis of past research has sometimes reduced its applicability to other ethnic groups. To redress that imbalance, relevant contemporary research has been reviewed, to which has been added information applicable to people of Asian, Melanesian and Polynesian ancestry. An epidemiological triad is used to identify the causal agents and host factors of work intolerance within hot-humid climates, commencing with the size dependency of resting metabolism and heat production accompanying load carriage, followed by a progression from the impact of single-layered clothing through to encapsulating ensembles. A morphological hypothesis is presented to account for inter-individual differences in heat production and heat loss, which seems to explain apparent ethnic- and gender-related differences in thermoregulation, at least within thermally compensable states. The mechanisms underlying work intolerance, cardiovascular insufficiency and heat illness are reviewed, along with epidemiological data from the Asia-Pacific. Finally, evidence-based preventative and treatment strategies are presented and updated concerning moisture-management fabrics and barriers, dehydration, pre- and post-exercise cooling, and heat adaptation. An extensive reference list is provided, with >25 recommendations enabling physiologists, occupational health specialists, policy makers, purchasing officers and manufacturers to rapidly extract interpretative outcomes pertinent to the Asia-Pacific.

Factors Affecting Performance on an Army Urban Operation Casualty Evacuation for Male and Female Soldiers

INTRODUCTION

This study was conducted to determine what physical and physiological characteristics contribute to the performance of an urban operation casualty evacuation (UO) and its predictive test, FORCE combat (FC) and describe the metabolic demand of the UO in female soldiers.

METHODS

Seventeen military members (9 M and 8 F) completed a loaded walking maximal aerobic test, the UO and FC. Heart rate reserve (HRR) and completion time were used as efficiency/performance measures. Oxygen consumption (VO2) was directly measured for UO on five female participants with a portable indirect calorimetry system, and analyzed using descriptive statistics. Stepwise multiple regression analysis was used to determine the contribution of the non-modifiable (age, sex, height) and modifiable characteristics (lean body mass to dead mass ratio (LBM:DM), VO2max corrected for load (L.VO2max), peak force (PF) measured on an isometric mid-thigh pull (IMTP) and medicine ball chest throw distance (Dist) on to the performance of each exercise.

RESULTS

LBM:DM and PF were the only factors included in the stepwise regression model for UO, predicting 70% of UO performance (p < 0.01). For FC, L.VO2max only was included in the stepwise regression model predicting 54% of FC performance (p < 0.01). Sex, age and height were not included in the regression model. The average metabolic cost of UO was 21.4 mL of O2*kg-1*min-1 in female soldiers while wearing PPE.

CONCLUSION

This study showed that modifiable factors such as body composition, PF on IMTP and L.VO2max are key contributors to performance on UO and FC performance.

Wearing body armour and backpack loads increase the likelihood of expiratory flow limitation and respiratory muscle fatigue during marching

The effect of load carriage on pulmonary function was investigated during a treadmill march of increasing intensity. 24 male infantry soldiers marched on six occasions wearing either: no load, 15 kg, 30 kg, 40 kg or 50 kg. Each loaded configuration included body armour which was worn as battle-fit or loose-fit (40 kg only). FVC and FEV₁ were reduced by 6 to 15% with load. Maximal mouth pressures were reduced post load carriage by up to 11% (inspiratory) and 17% (expiratory). Increased ventilatory demands associated with carrying increased mass were met by increases in breathing frequency (from 3 to 26 breaths·min^-1) with minimal changes to tidal volume. 72% of participants experienced expiratory flow limitation whilst wearing the heaviest load. Loosening the armour had minimal effects on pulmonary function. It was concluded that as mass and exercise intensity are increased, the degree of expiratory flow limitation also increases. Practitioner Summary: This study investigated the effect of soldier load carriage on pulmonary function, to inform the trade-off between protection and burden. Load carriage caused an inefficient breathing pattern, respiratory muscle fatigue and expiratory flow limitation during marching. These effects were exacerbated by increases in mass carried and march intensity.

A database of predictor test sex bias for development of military physical employment standards

BACKGROUND

Hesitation to employ females for physically demanding jobs is often due to sex related physical abilities. A physical employment standard (PES) identifies individuals who are physically capable for work.

OBJECTIVE

A database containing 300 + sources of physical performance tests (PFTs) will inform potential sex bias for PES development.

METHODS

Weighted means and probability density curves illustrate the percentage overlap between male and female performance on PFT data from the armed forces of 11 countries and the open literature. Where female training data were available, the change in percentage overlap illustrates the potential for reduction in sex-related differences.

RESULTS

PFTs demonstrating the extremes of sex disparity were bench press (11 sources) and sit-ups (14 sources) with 9% and 93% overlap in performance, respectively. Training for bench press; pull ups; VO2max; and upright pull improved female performance by 12%, 22%, 35%, and 23% respectively. This translated into narrowing the gap between male and female mean performance by 1%, 4%, 5%, and 10% respectively.

CONCLUSIONS

The ability of PFT to predict performance is essential; however, PFTs with more overlap will facilitate development of PES with reduced sex bias. PFTs with the greatest potential for improvement in females are identified here.

Effects of Personal Protective Equipment on the Performance of Federal Highway Policemen in Physical Fitness Tests

Marins, EF, Cabistany, L, Bartel, C, Dawes, J, and Del Vecchio, FB. Effects of personal protective equipment on the performance of Federal Highway Policemen in physical fitness tests. J Strength Cond Res 34(1): 11-19, 2020-Personal protective equipment (PPE), worn by police officers, provides protection and can modify physiological and performance responses during physical efforts. Physiological, perceptual, and physical responses were compared with PPE (WPPE) and without PPE (NPPE) among the Brazilian Federal Highway Policemen (FHP). Nineteen (n = 19) FHP completed 2 experimental trials: NPPE and WPPE (load = 8.3 kg). Tests of aerobic and anaerobic power, muscular strength and endurance, as well as change of direction speed (CODS) were performed under both conditions. Heart rate reductions were shown at the second ventilatory threshold (1.4%) and maximal exercise (1.5%) WPPE. In comparison with NPPE, physical performance in the WPPE condition showed decreases in the following: treadmill time at maximal exercise (21%, p < 0.001); time in the isometric trunk test (28.9%, p < 0.001); vertical jump height (11.6 and 10.5%, p < 0.001); standing long jump distance (7.3%, p < 0.001); bar hang time (14.8%, p < 0.05); and CODS (2.6%, p < 0.05). Results indicate that PPE use reduces physical performance of FHP in cardiorespiratory, strength, power, and CODS tests.

Influence of work clothing on physiological responses and performance during treadmill exercise and the Wildland Firefighter Pack Test

This study investigated physiological responses and performance during three separate exercise challenges (Parts I, II, and III) with wildland firefighting work clothing ensemble (boots and coveralls) and a 20.4 kg backpack in four conditions: U-EX (no pack, exercise clothing); L-EX (pack, exercise clothing); U-W (no pack, work clothing); and, L-W (pack and work clothing). Part I consisted of randomly-ordered graded exercise tests, on separate days, in U-EX, L-EX and L-W conditions. Part II consisted of randomly-ordered bouts of sub-maximal treadmill exercise in the four conditions. In Part III, subjects completed, in random-order on separate days, 4.83 km Pack Tests in L-EX or L-W conditions. In Part I, peak oxygen uptake was reduced (p < .05) in L-W. In Part II, mass-specific oxygen uptake was significantly higher in both work clothing conditions. In Part III, Pack Test time was slower (p < .05) in L-W. These results demonstrate the negative impact of work clothing and load carriage on physiological responses to exercise and performance.

Respiratory Effects of Thoracic Load Carriage Exercise and Inspiratory Muscle Training as a Strategy to Optimize Respiratory Muscle Performance with Load Carriage

Many occupational and recreational settings require the use of protective and/or load-bearing apparatuses worn over the thoracic cavity, known as thoracic load carriage (LC). Compared to normal, unloaded exercise, thoracic LC exercise places an additional demand on the respiratory and limb locomotor systems by altering ventilatory mechanics as well as circulatory responses to exercise, thus accelerating the development of fatigue in the diaphragm and accessory respiratory muscles compared to unloaded exercise. This may be a consequence of the unique demands of thoracic LC, which places an additional mass load on the thoracic cavity and can restrict chest wall expansion. Therefore it is important to find effective strategies to ameliorate the detrimental effects of thoracic LC. Inspiratory muscle training is an intervention that aims to increase the strength and endurance of the diaphragm and accessory inspiratory muscle and may therefore be a useful strategy to optimize performance with thoracic LC.

The Influence of Body Mass on Physical Fitness Test Performance in Male Firefighter Applicants

OBJECTIVE

The influence of body mass on test performance was investigated in 414 male firefighter applicants who completed a maximal treadmill test and five task-simulation tests while dressed in fire protective ensemble.

METHODS

Subjects were assigned to six mass categories from less than 70 kg to more than 110 kg, in 10 kg increments (n = 69 in each).

RESULTS

Treadmill performance was lower (P < 0.05) in the two heaviest groups. Charged hose advance time was slower in the two lightest groups. The lightest group had slower times for weighted sled pull, forcible entry, and victim rescue tests. The heaviest group was slower on the ladder climb test.

CONCLUSION

Lighter subjects had a small advantage in endurance-oriented tests while higher mass appeared to improve performance slightly in strength-oriented tests. However, mass explained only 4% to 19% of the variance in performance.

Load carriage, human performance, and employment standards

The focus of this review is on the physiological considerations necessary for developing employment standards within occupations that have a heavy reliance on load carriage. Employees within military, fire fighting, law enforcement, and search and rescue occupations regularly work with heavy loads. For example, soldiers often carry loads >50 kg, whilst structural firefighters wear 20–25 kg of protective clothing and equipment, in addition to carrying external loads. It has long been known that heavy loads modify gait, mobility, metabolic rate, and efficiency, while concurrently elevating the risk of muscle fatigue and injury. In addition, load carriage often occurs within environmentally stressful conditions, with protective ensembles adding to the thermal burden of the workplace. Indeed, physiological strain relates not just to the mass and dimensions of carried objects, but to how those loads are positioned on and around the body. Yet heavy loads must be borne by men and women of varying body size, and with the expectation that operational capability will not be impinged. This presents a recruitment conundrum. How do employers identify capable and injury-resistant individuals while simultaneously avoiding discriminatory selection practices? In this communication, the relevant metabolic, cardiopulmonary, and thermoregulatory consequences of loaded work are reviewed, along with concomitant impediments to physical endurance and mobility. Also emphasised is the importance of including occupation-specific clothing, protective equipment, and loads during work-performance testing. Finally, recommendations are presented for how to address these issues when evaluating readiness for duty.