Influence of carrying heavy loads on soldiers' posture, movements and gait

No physiological or biomechanical sex-by-load interactions during treadmill-based load carriage

This study investigated whether physiological demand or gait mechanics differ between sexes during treadmill load carriage. Female (n = 15) and male (n = 15) military recruit-type participants with no load carriage experience completed three 10-minute walking trials at a self-selected speed with increasing relative body-borne loads (0%, 20%, and 40% body weight). A range of cardiorespiratory, perceptual and biomechanical variables were measured. Self-selected walking speed was similar between sexes (4.6-4.8 km·h^-1, p > .05) and there were no significant sex-by-load interactions for any variables. Absolute VO₂ and VCO₂ were greater in males (difference 175-178 mL·min^-1, p < .001), however, when relative to body mass, VO₂ was similar between sexes (p > .05). Across all loads, cadence was 7 ± 2 steps·min^-1 faster (p = .004) and stance time was 0.06 ± 0.02 s shorter (p = .013) in females. Increasing load resulted in greater physiological demand, cadence, % stance time, and step length (p < .05). Practitioner summary: Literature comparing physiological and biomechanical variables between sexes during load carriage is scarce. Physiological and biomechanical sex differences were limited to relative measures associated with physical size (height and mass). Future research may pool male and female participants when conducting trials up to ten minutes in length. Abbreviations: BW: body weight; COM: centre of mass; HR: heart rate; HR_max: maximum heart rate; RER: respiratory exchange ratio; RPE: rating of perceived exertion; VCO₂: volume of carbon dioxide; VE: ventilation; VO₂: volume of oxygen; VO_2max: maximum volume of oxygen.

Validation of an IMU Suit for Military-Based Tasks

Investigating the effects of load carriage on military soldiers using optical motion capture is challenging. However, inertial measurement units (IMUs) provide a promising alternative. Our purpose was to compare optical motion capture with an Xsens IMU system in terms of movement reconstruction using principal component analysis (PCA) using correlation coefficients and joint kinematics using root mean squared error (RMSE). Eighteen civilians performed military-type movements while their motion was recorded using both optical and IMU-based systems. Tasks included walking, running, and transitioning between running, kneeling, and prone positions. PCA was applied to both the optical and virtual IMU markers, and the correlations between the principal component (PC) scores were assessed. Full-body joint angles were calculated and compared using RMSE between optical markers, IMU data, and virtual markers generated from IMU data with and without coordinate system alignment. There was good agreement in movement reconstruction using PCA; the average correlation coefficient was 0.81 ± 0.14. RMSE values between the optical markers and IMU data for flexion-extension were less than 9°, and 15° for the lower and upper limbs, respectively, across all tasks. The underlying biomechanical model and associated coordinate systems appear to influence RMSE values the most. The IMU system appears appropriate for capturing and reconstructing full-body motion variability for military-based movements.

Physiological and biomechanical effects on the human musculoskeletal system while carrying a suspended-load backpack

Many people need to carry heavy loads in a backpack to perform occupational, military, or recreational tasks. Suspended-load backpacks have been shown to reduce dynamic peak forces acting on the body and lower an individual's metabolic cost during walking. However, little is known about the physiological and biomechanical effects of a suspended-load backpack on the human musculoskeletal system. The goal of this study was to determine the impact of different types of backpacks on metabolic cost, joint kinetics, gait kinematics, and muscle activity while individuals carried the same load of 15 kg at a walking speed of 5 km/h and running speed of 7 km/h on an instrumented treadmill. A group of six healthy participants participated in experiments in which two different backpacks were worn under three different conditions: suspended-load backpack working condition (SLB_ON), suspended-load backpack locking condition (SLB_OFF), and ordinary backpack condition (ORB). The results showed that carrying the backpack in the SLB_ON condition can reduce lower limb muscle activities and biological joint work while decreasing the metabolic cost by 15.25 ± 4.21% and 8.81 ± 2.46% during walking and 12.53 ± 2.39% and 6.99 ± 2.37% during running compared to carrying the backpack in the SLB_OFF and the ORB conditions, respectively. However, the SLB_ON condition may cause increased shoulder strain and dynamic stability and balance problems. These results suggest that the control of load movement in a suspended-load backpack should be considered when locomotion performance is optimized in future studies.

Effects of Backpack Loads on Leg Muscle Activation during Slope Walking

Hikers and soldiers usually walk up and down slopes with a load carriage, causing injuries of the musculoskeletal system, especially during a prolonged load journey. The slope walking has been reported to lead to higher leg extensor muscle activities and joint moments. However, most of the studies investigated muscle activities or joint moments during slope walking without load carriage or only investigated the joint moment changes and muscle activities with load carriages during level walking. Whether the muscle activation such as the signal amplitude is influenced by the mixed factor of loads and grades and whether the influence of the degrees of loads and grades on different muscles are equal have not yet been investigated. To explore the effects of backpack loads on leg muscle activation during slope walking, ten young male participants walked at 1.11 m/s on a treadmill with different backpack loads (load masses: 0, 10, 20, and 30 kg) during slope walking (grade: 0, 3, 5, and 10°). Leg muscles, including the gluteus maximus (GM), rectus femoris (RF), hamstrings (HA), anterior tibialis (AT), and medial gastrocnemius (GA), were recorded during walking. The hip, knee, and ankle extensor muscle activations increased during the slope walking, and the hip muscles increased most among hip, knee, and ankle muscles (GM and HA increased by 46% to 207% and 110% to 226%, respectively, during walking steeper than 10° across all load masses (GM: p = 1.32 × 10−8 and HA: p = 2.33 × 10−16)). Muscle activation increased pronouncedly with loads, and the knee extensor muscles increased greater than the hip and ankle muscles (RF increased by 104% to 172% with a load mass greater than 30 kg across all grades (RF: p = 8.86 × 10−7)). The results in our study imply that the hip and knee muscles play an important role during slope walking with loads. The hip and knee extension movements during slope walking should be considerably assisted to lower the muscle activations, which will be useful for designing assistant devices, such as exoskeleton robots, to enhance hikers’ and soldiers’ walking abilities.

Muscle Activity during Postural Stability Tasks: Role of Military Footwear and Load Carriage

Decrements to postural control manifest as an increase in muscle activity, indicating continuous attempts to maintain body equilibrium and postural stability. Extrinsic factors such as footwear, and intrinsic factors such as muscle fatigue, can affect postural stability. The purpose of this study was to analyze the impact of two types of military footwear and a military-type load-carrying task on lower extremity muscle activity during various postural stability tasks. Sixteen males’ (age: 26.63 ± 3.93 years; mass: 87 ± 12.4 kg; height: 178.04 ± 6.2 cm) muscle activity from knee flexors, extensors, ankle dorsiflexors, and plantar flexors were measured using electromyography in standard (STD) and minimalist (MIN) military footwear, before (PRE) and after (POST) a simulated workload during sensory organization and motor control tests on the Neurocom EquitestTM. Mean muscle activity was analyzed using 2 (footwear) × 2 (time) repeated measures ANOVA with an alpha level of 0.05. Results revealed a requirement of significantly greater muscle activity in POST and STD. MIN demonstrated lesser balance decrements POST workload, which could be attributed to its design characteristics. Results will help in suggesting footwear design characteristics to minimize muscular exertion while eliciting better postural control, and to prevent postural instability due to overexertion in military personnel.

Effect of a load distribution system on mobility and performance during simulated and field hiking while under load

This study was conducted to test a modular scalable vest-load distribution system (MSV-LDS) against the plate carrier system (PC) currently used by the United States Marine Corps. Ten Marines engaged in 1.6 km load carriage trials in seven experimental conditions in a laboratory study. Kinematic, kinetic, and spatiotemporal gait parameters, muscle activity (electromyography), heart rate, caloric expenditure, shooting reaction times, and subjective responses were recorded. There was lower mean trapezius recruitment for the PC compared with the MSV-LDS for all conditions, and muscle activity was similar to baseline for the MSV-LDS. Twenty-seven Marines carrying the highest load were evaluated in the field, which measured an increase in energy expenditure with MSV-LDS; however, back discomfort was reduced. The field evaluation showed significantly reduced estimated ground reaction force on flat-ground segments with the MSV-LDS, and the data suggest both systems were comparable with respect to mobility and energy cost. Practitioner summary: This study found that a novel load distribution system appears to redistribute load for improved comfort as well as reduce estimated ground reaction force when engaged in hiking activities. Further, hiking with a load distribution system enables more neutral walking posture. Implications of load differences in loads carried are examined. Abbreviations: AGRF: anterior-posterior ground reaction forces; CAREN: Computer Assisted Rehabilitation Environment; GRF: ground reaction forces; HR: heart rate; ML-GRF: mediolateral ground reaction forces; MOLLE: Modular Lightweight Load-carrying Equipment; MSV-LDS: modular scalable vest-load distribution system; NHRC: Naval Health Research Center; PC: plate carrier; PPE: personal protective equipment; RPE: rating of perceived exertion; SAPI: small arms protective insert; sEMG: surface electromyography; USMC: United States Marine Corps; VGRF: Ground reaction forces in the vertical.

The effects of a loaded rucksack and weighted vest on metabolic cost and stride frequency in female adults

This study evaluated the metabolic cost and stride frequency during exercise while wearing an evenly distributed weight vest in recreationally fit women. Nine healthy women performed a modified Balke treadmill test until volitional fatigue in one of three conditions; (1) unloaded (2) rucksack and (3) weighted vest. Wearing a weighted vest did not show improvement of V̇_O2peak, HR_peak, peak stride frequency or average stride frequency (all p ≥ 0.05). However, total time of exercise was significantly longer in the evenly distributed weight vest condition compared to the rucksack condition (p = 0.024) despite similar V̇_O2peak and HR_peak. These results may provide practical information when females in tactical populations are preparing for missions in which heavy loads must be carried. Practitioner summary: This study compared the effects of load carriage mode on metabolic cost and stride frequency during graded exercise in females. The weighted vest showed significantly longer exercise time despite similar V̇_O2peak and HR_peak. Development of standardised weight distribution mode may serve as an advantageous strategy for females in tactical settings. Abbreviations: HR: heart rate; V̇O2: oxygen uptake; RPE: rating of perceived exertion; V̇O2peak: peak oxygen uptake; HRpeak: peak heart rate.

The metabolic demand of external load carriage in golfers: a comparison of a single versus double-strap golf bag

BACKGROUND

A golf bag filled with a set of clubs provides a substantial load. When carried over distance this can increase the demands placed upon the golfer, leading to discomfort, fatigue and injuries. This study aimed to compare the metabolic demands of 2 methods of golf bag carriage.

METHODS

A total of 16 healthy male recreational golfers participated in the study. Participants were given an initial familiarization session in which their self-selected walking speed was determined. This was utilized as the treadmill speed for all subsequent trials. The testing protocol consisted of 3 randomized trials of treadmill walking for 5 minutes in each of three conditions: unloaded, single-strap bag and double-strap bag. Equipment consisted of a double-strap golf bag with a standard set of clubs weighing 12.5kg. For all trials oxygen consumption (L·min-1), VO2 (mL·kg·min-1) respiratory minute volume (VE) (L·min-1), and heart rate (HR) were measured.

RESULTS

Results showed that the double-strap bag required significantly less oxygen consumption (1.19±0.19 vs. 1.31±0.16 L·min-1, P<0.01) relative oxygen consumption (14.49±2.06 vs. 15.93±2.25 mL·kg·min-1, P<0.01), reduced respiratory minute volume (29.95±4.19 vs. 32.47±4.26 L·min-1, P<0.05), and lower heart rates (100.14±11.05 vs. 106.96±9.33 BPM, P<0.001) than the single-strap bag. Both methods of carriage showed significantly greater metabolic demands than the unloaded condition (P<0.05).

CONCLUSIONS

The decreased metabolic cost of carrying a double-strap golf bag may facilitate a reduction in fatigue and reduced mechanical stress. Golf bag transportation must therefore be recognized as a factor in reducing the risk of injury and improving playing performance.

Dynamic Postural Stability in Active, Adolescent Males Following Repeated Bouts of Aerobic Exercise in Hot and Temperate Environments: A Pilot Study

Introduction

Proper jump-landing neuromuscular control is crucial in mitigating lower-extremity musculoskeletal injuries. The presence of fatigue, especially in extreme environments, may degrade dynamic postural stability (DPS) and result in lower-extremity injuries. This study aimed to evaluate the influence of moderate intensity exercise in hot (HOT) and temperate (TEMP) ambient temperatures and residual effects of a previous bout on DPS during a single-legged jump-landing. It was hypothesized that the participants would display worse DPS after HOT compared to TEMP.

Methods

Six recreationally active young males (16.8 ± 0.7 year, 1.88 ± 0.12 m, 83.8 ± 19.8 kg) completed two, 60-minute bouts of exercise with 60 minutes of rest between bouts in both HOT (35°C) and TEMP (22.2°C). Heart rate and core body temperature (T_c) were monitored continuously, and DPS was assessed before and after each bout.

Results

The DPS time and condition effects were not identified (p > 0.05), but HOT elicited some notable (d > 0.20) increases in heart rate, T_c, and DPS compared to TEMP.

Conclusions

The DPS decrements varied between subjects suggesting individual-specific etiology. Repeated bouts of exercise in HOT may place an individual at a greater risk for injury than TEMP if proper prevention strategies are not used.

A more compliant prosthetic foot better accommodates added load while walking among Servicemembers with transtibial limb loss

Selecting an optimal prosthetic foot is particularly challenging for highly active individuals with limb loss, such as military personnel, who need to seamlessly perform a variety of demanding activities/tasks (often with and without external loads) while minimizing risk of musculoskeletal injuries over the longer term. Here, we expand on prior work by comparing biomechanical and functional outcomes in two prosthetic feet with the largest differences in mechanical response to added load (i.e., consistently "Compliant" and "Stiff" forefoot properties). In each foot, fourteen male Servicemembers with unilateral transtibial limb loss (from trauma) completed instrumented gait analyses in all combinations of two loading conditions (with and without 22 kg weighted vest) and two walking speeds (1.34 and 1.52 m/s), as well as the Prosthesis Evaluation Questionnaire. With the Stiff foot, sound limb peak loading was 2% smaller (p = 0.043) in the loaded versus unloaded condition, but similar between loading conditions in the Compliant foot (note, the Stiff foot was associated with larger loads, overall). Independent of load or walking speed, the Compliant (versus Stiff) foot provided 67.9% larger (p < 0.001) prosthetic push-off, 17.7% larger (p = 0.01) roll-over shape radii, and was subjectively favored by 10 participants. A more Compliant versus Stiff prosthetic foot therefore appears to better accommodate walking with and without added load, and reinforce the notion that mechanical properties of prosthetic feet should be considered for near-term performance and longer-term (joint) health.

Effects of carrying school bags on cervical and shoulder posture in static and dynamic conditions in adolescent students

Background Neck and Back Pain in Students is closely related to the school bags that they carry. There is a dearth of literature in terms of postural changes due to school bags in adolescents, especially those discussing postural changes in static and dynamic loading conditions. Objective To assess the effects of weight, position, and time of school bag carriage in static and dynamic conditions on cervical and shoulder posture in adolescent students. Subjects Seventy adolescents (35 Females, 35 Males) between 10 and 15 years of age. Methods: Craniohorizontal Angle, Craniovertebral Angle, Sagittal Shoulder Posture, Anterior Head Alignment were assessed in 7 different conditions using Photographic Method and MB Ruler. Results and Conclusion Significant changes in Sagittal Shoulder Posture and Craniohorizontal Angle were found with unilateral school bag carriage as compared to minimal significant postural changes with bilateral carriage. Maximal significant postural changes, in terms of all outcome measures were noted during loaded dynamic activity. Craniovertebral Angle kept significantly reducing with time. Immediate assessments taken with a school bag weighing 13.5% of the body weight showed a significant decrement in the Craniohorizontal Angle. Even a bag weighing less than 10% of the body weight proved to bring about biomechanical changes, when assessments were taken during the activities that a child has to perform with a school bag (Post Standing, Post Walking, and During Walking).

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.

Joint range of motion entropy changes in response to load carriage in military personnel

BACKGROUND

Overuse accounts for 82% of injuries in military personnel, and these occur predominantly in the spine and lower limbs. While non-linear analyses have shown changes in overall stability of the movement during load carriage, individual joint contributions have not been studied. The concept of entropy compensation between task, organism and environmental constraints is studied at a joint level.

RESEARCH QUESTION

The aim of this study was to investigate whether using different methods of loading by military personnel would have an effect on the sample entropy of the joint ranges of motion.

METHODS

Eleven male reserve infantry army soldiers (age: 22 ± 2 years; height: 1.80 ± 0.06 m; mass: 89.3 ± 14.4 kg) walked an outdoor, 800 m course under 5 load conditions: unloaded, 15 kg backpack, 25 kg backpack, 15 kg webbing and backpack and 25 kg webbing and backpack. Kinematic data was recorded at 240 Hz using the Xsens motion capture system. The ranges of motion (ROM) of the spine, hips and knee were calculated for each gait cycle. Mean ROM, coefficient of variation (CV) of the ROM and the sample entropy of the ROM were compared between conditions.

RESULTS

Spine side flexion ROM decreased significantly from the control condition in all loaded conditions, while sample entropy of the spine side flexion ROM increased in some conditions with no significant change in CV. Conversely, the hip flexion ROM increased significantly from the control, while sample entropy of the hip flexion ROM decreased.

SIGNIFICANCE

These results suggest that entropy compensation may propagate at a joint level. Understanding that a decrease in certainty with which a joint angle is selected, may be accompanied by an increase at a neighbouring joint. This could be significant in monitoring injuries as a result of environmental or task constraints.

Effect of locomotor demands on cognitive processing

Understanding how brain dynamics change with dual cognitive and motor tasks can improve our knowledge of human neurophysiology. The primary goals of this study were to: (1) assess the feasibility of extracting electrocortical signals from scalp EEG while performing sustained, physically demanding dual-task walking and (2) test hypotheses about how the P300 event-related potential is affected by walking physical exertion. Participants walked on a treadmill for an hour either carrying an empty rucksack or one filled with 40% of their body weight. During the walking conditions and during a seated control condition, subjects periodically performed a visual oddball task. We recorded scalp EEG and examined electrocortical dynamics time-locked to the target stimulus. Channel-level event-related potential analysis demonstrated that it is feasible to extract reliable signals during long duration loaded walking. P300 amplitude was reduced during loaded walking versus seated, but there was no effect of time on task. Source level activity and frequency analysis revealed that sensorimotor, parietal, and cingulate brain areas all contributed to the reduced P300 amplitude during dual-task walking. We interpret the results as supporting a prioritization of cortical resources for walking, leading to fewer resources being directed toward the oddball task during dual-task locomotion.

Predicting Ground Reaction Force from a Hip-Borne Accelerometer during Load Carriage

INTRODUCTION

Ground reaction forces (GRF) during load carriage differ from unloaded walking. Methods to quantify peak vertical GRF (pGRFvert) of Soldiers walking with loads outside of a laboratory are needed to study GRF during operationally relevant tasks.

PURPOSE

Develop a statistically based model to predict pGRFvert during loaded walking from ActiGraph GT3X+ activity monitor (AM) vertical acceleration.

METHODS

Fifteen male Soldiers (25.4 ± 5.3 yr, 85.8 ± 9.2 kg, 1.79 ± 9.3 m) wore an ActiGraph GT3X+ AM over their right hip. Six walking trials (0.67-1.58 m·s) with four loads (no load, 15, 27, 46 kg) and two types of footwear (athletic shoes and combat boots) were completed on an instrumented force plate treadmill. Average peak vertical AM acceleration (pACCvert) and pGRFvert were used to develop a regression equation to predict pGRFvert. The model was validated using a leave-one-subject-out approach. Root mean square error (RMSE) and average absolute percent difference (AAPD) between actual and predicted pGRFvert were determined. pGRFvert was also predicted for two novel data sets and AAPD and RMSE calculated.

RESULTS

The final equation to predict pGRFvert included pACCvert, body mass, carried load mass, and pACCvert-carried load mass interaction. Cross-validation resulted in an AAPD of 4.0% ± 2.7% and an RMSE of 69.5 N for leave-one-subject-out and an AAPD of 5.5% ± 3.9% and an RMSE of 78.7 N for the two novel data sets.

CONCLUSION

A statistically based equation developed to predict pGRFvert from ActiGraph GT3X+ AM acceleration proved to be accurate to within 4% for Soldiers carrying loads while walking. This equation provides a means to predict GRF without a force plate.

Effects of backpack weight on the performance of basic short-term/working memory tasks during flat-surface standing

This study empirically investigated the effects of backpack weight on the performance of three basic short-term/working memory (STM/WM) tasks during flat-surface standing. Four levels of backpack weight were considered: 0, 15, 25 and 40% of the body weight. The three STM/WM tasks were the Corsi block, digit span and 3-back tasks, corresponding to the visuo-spatial sketchpad, phonological loop and central executive of WM, respectively. Thirty participants conducted the STM/WM tasks while standing with loaded backpack. Major study findings were that (1) increased backpack weight adversely affected the scores of all three STM/WM tasks; and, (2) the adverse effect of backpack weight was less pronounced for the phonological loop STM task than the other STM/WM tasks. The study findings may help understand and predict the impacts of body-worn equipment weight on the worker's mental task performance for various work activities requiring simultaneous performance of mental and physical tasks. Practitioner summary: The current study empirically examined the effects of backpack weight on the performance of three basic STM/WM tasks. The study findings entail that reduces the weight of body-worn equipment can positively impact the worker's mental task performance in addition to reducing the worker's bodily stresses. Abbreviations: ACC: anterior cingulate cortex; AP: anterior-posterior; BW: body weight; CoP: centre of pressure; C-S: central executive working memory task and standing; DLPFC: dorsolateral prefrontal cortex; HIP: human information processing; ML: medio-lateral; PMC: premotor cortex; P-S: phonological loop short-term memory task and standing; SMA: supplementary motor area; STM: short-term memory; VLPFC: ventrolateral prefrontal cortex; V-S: visuo-spatial short-term memory task and standing; WM: working memory.

The effects of backpack carriage on gait kinematics and kinetics of schoolchildren

There has been a growing concern among clinical and educational practitioners, as well as, policy makers on the use of backpacks by schoolchildren. On a daily basis, pupils spend a significant amount of time carrying stuffed and heavy backpacks. The aim of this study was to investigate the effects of backpack carriage with different loads on spatiotemporal parameters of gait, plantar pressure and force distribution under different foot regions in schoolchildren. We have assessed fifty-seven primary school students (7–9 years-old) performing four walks of 10 m (carrying 0%, 10%, 15% of body mass in the backpack and the load they brought to school). A floor-based photocell system was used to collect the gait kinematics and insoles capacitive pressure sensors the kinetics. Children walked slower and at lower cadence with the load brought to school than in the other three conditions. There was no significant main effect on stride length. Backpack carriage with different loads did have a significant effect on plantar pressure and force distribution. We noted that heavier the load, higher the pressure and force under different foot regions. Our findings highlight that gait biomechanics of children (such as stride kinematics and pressure under the feet) is affected by carrying loads in the backpacks.

Mechanical and dynamic characterization of prosthetic feet for high activity users during weighted and unweighted walking

Many Service members and Veterans with lower-limb amputations have the potential for high function and the desire to resume physically demanding occupations that require them to carry heavy loads (e.g., military service, firefighters, farmers, ranchers, construction workers). However, it is currently unclear which prosthetic feet best accommodate heavy load carriage while also providing good overall function and mobility during unweighted activities. The main objective of this study was to investigate the ability of currently available prosthetic ankle-foot systems to accommodate weighted walking by examining the mechanical characteristics (i.e., forefoot stiffness) and dynamic function (i.e., rocker radius, effective foot length ratio, and late-stance energy return) of prosthetic feet designed for high activity users. Load versus deflection curves were obtained for nine prosthetic ankle-foot systems using a servohydraulic test frame and load cell. Effective roll-over shape characteristics and late-stance energy return measures were then obtained using quantitative gait analysis for three users with unilateral, transtibial amputation. Results from mechanical and dynamic testing showed that although forefoot stiffness varied across the nine feet investigated in this study, changes measured in roll-over shape radius and effective foot length ratio were relatively small in response to weighted walking. At the same time, prosthetic feet with more compliant forefoot keel structures appeared to provide more late-stance energy return compared to feet with stiffer forefoot keel structures. These results suggest that prosthetic ankle-foot systems with compliant forefoot keel structures may better accommodate weighted walking by reducing the metabolic cost of physically demanding activities. However, to more fully understand the biomechanical and functional implications of these results, other factors, such as the residual-limb strength of the user and the overall stiffness profile of the prosthetic foot, should also be considered.

Impact of SCBA size and fatigue from different firefighting work cycles on firefighter gait

Risk of slips, trips and falls in firefighters maybe influenced by the firefighter's equipment and duration of firefighting. This study examined the impact of a four self-contained breathing apparatus (SCBA) three SCBA of increasing size and a prototype design and three work cycles one bout (1B), two bouts with a five-minute break (2B) and two bouts back-to-back (BB) on gait in 30 firefighters. Five gait parameters (double support time, single support time, stride length, step width and stride velocity) were examined pre- and post-firefighting activity. The two largest SCBA resulted in longer double support times relative to the smallest SCBA. Multiple bouts of firefighting activity resulted in increased single and double support time and decreased stride length, step width and stride velocity. These results suggest that with larger SCBA or longer durations of activity, firefighters may adopt more conservative gait patterns to minimise fall risk. Practitioner Summary: The effects of four self-contained breathing apparatus (SCBA) and three work cycles on five gait parameters were examined pre- and post-firefighting activity. Both SCBA size and work cycle affected gait. The two largest SCBA resulted in longer double support times. Multiple bouts of activity resulted in more conservative gait patterns.

A comparison of economy and sagittal plane trunk movements among back-, back/front- and head-loading

It has been suggested that freedom of movement in the trunk could influence load carriage economy. This study aimed to compare the economy and sagittal plane trunk movements associated with three load carriage methods that constrain posture differently. Eighteen females walked at 3 km^.h^-1 with loads of 0, 3, 6, 9, 12, 15 and 20 kg carried on the back, back/front and head. Load carriage economy was assessed using the Extra Load Index (ELI). Change in sagittal plane trunk forward lean and trunk angle excursion from unloaded to loaded walking were assessed. Results show no difference in economy between methods (p = .483), despite differences in the change in trunk forward lean (p = .001) and trunk angle excursion (p = .021) from unloaded to loaded walking. We conclude that economy is not different among the three methods of load carriage, despite significant differences in sagittal plane trunk movements. Practitioner summary: This article shows, based on mean data, that there is no difference in economy among back, back/front and head-loading, despite differences in trunk movement. It is possible a combination of factors align to influence individual economy, rather than a single set of factors, applicable to all individuals for each method.

The Influence of Backpack Weight and Hip Belt Tension on Movement and Loading in the Pelvis and Lower Limbs during Walking

The introduction of hip belts to backpacks has caused a shift of loading from the spine to the hips with reported improvements in musculoskeletal comfort. Yet the effects of different hip belt tensions on gait biomechanics remain largely unknown. The goal of this study was to assess the influence of backpack weight and hip belt tension on gait biomechanics. Data from optical motion capture and ground reaction forces (GRF) during walking were acquired in nine healthy male subjects (age 28.0 ± 3.9 years). Six configurations of a commercial backpack were analyzed, that is, 15 kg, 20 kg, and 25 kg loading with 30 N and 120 N hip belt tension. Joint ranges of motion (ROM), peak GRF, and joint moments during gait were analyzed for significant differences by repeated measures of ANOVA with Bonferroni post hoc comparison. Increased loading led to a significant reduction of knee flexion-extension ROM as well as pelvis rotational ROM. No statistically significant effect of hip belt tension magnitudes on gait dynamics was found at any backpack weight, yet there was a trend of increased pelvis ROM in the transverse plane with higher hip belt tension at 25 kg loading. Further research is needed to elucidate the optimum hip belt tension magnitudes for different loading weights to reduce the risks of injury especially with higher loading.

Spatiotemporal and kinematic changes in gait while carrying an energy harvesting assault pack system

Soldiers are fielded with a variety of equipment including battery powered electronic devices. An energy harvesting assault pack (EHAP) was developed to provide a power source to recharge batteries and reduce the quantity and load of extra batteries carried into the field. Little is known about the biomechanical implications of carrying a suspended-load energy harvesting system compared to the military standard assault pack (AP). Therefore, the goal of this study was to determine the impact of pack type and load magnitude on spatiotemporal and kinematic parameters while walking at 1.34 m/s on an instrumented treadmill at decline, level, and incline grades. There was greater forward trunk lean while carrying the EHAP and the heavy load (decline: p < 0.001; level: p = 0.009; incline: p = 0.003). As load increased from light to heavy, double support stance time was longer (decline: p = 0.012; level: p < 0.001; incline: p < 0.001), strides were shorter (incline: p = 0.013), and knee flexion angle at heel strike was greater (decline: p = 0.033; level: p = 0.035; incline: p = 0.005). When carrying the EHAP, strides (decline: p = 0.007) and double support stance time (incline: p = 0.006) was longer, the knee was more flexed at heel strike (level: p = 0.014; incline: p < 0.001) and there was a smaller change in knee flexion during weight acceptance (decline: p = 0.0013; level: p = 0.007; incline: p = 0.0014). Carrying the EHAP elicits changes to gait biomechanics compared to carrying the standard AP. Understanding how load-suspension systems influence loaded gait biomechanics are warranted before transitioning these systems into military or recreational environments.

Postural balance and neck angle changes in school children while carrying a traditional backpack versus a double-sided bag

Study aim: This study compared the body balance and neck angle differences in children when carrying a traditional backpack versus a double-sided bag. Material and methods: By using the Biodex balance system and a three dimensional motion analysis system, the postural bal­ance and neck angles were assessed in 33 school children while carrying a traditional backpack or a double-sided bag, weigh­ing 15% of body weight. Results: The overall and anteroposterior stability indices were significantly higher when carrying a traditional backpack com­pared to no load and a double-sided bag (p < 0.05). The mediolateral stability index was significantly higher when carrying the traditional backpack and the double-sided bag compared to no load (p < 0.05). The craniohorizontal angle was significantly greater, and the craniovertebral angle and sagittal shoulder posture were significantly lesser when carrying the traditional back­pack compared to no load and the double-sided bag conditions (p < 0.05). Conclusion: Carrying the double-sided bag restores the body balance and head posture to a condition that is similar to the no load condition.

Assessing Infant Carriage Systems: Ground Reaction Force Implications for Gait of the Caregiver

Objective To assess the acute alterations of anterior infant carriage systems on the ground reaction force experienced during over-ground walking. Background Previous research has identified the alterations in posture and gait associated with an increased anterior load (external or internal); however, the forces applied to the system due to the altered posture during over-ground walking have not been established. Method Thirteen mixed gender participants completed 45 over-ground walking trials at a self-selected pace under three loaded conditions (unloaded, semi-structured carrier 9.9 kg, and structured carrier 9.9 kg). Each trial consisted of a 15-m walkway, centered around a piezoelectric force platform sampling at 1,200 Hz. Differences were assessed between loaded and unloaded conditions and across carriers using paired samples t tests and repeated measures ANOVA. Results Additional load increased all ground reaction force parameters; however, the magnitude of force changes was influenced by carrier structure. The structured carrier displayed increased force magnitudes, a reduction in the time to vertical maximum heel contact, and an increased duration of the flat foot phase in walking gait. Conclusion Evidence suggests that the acute application of anterior infant carriers alters both kinetic and temporal measures of walking gait. Importantly, these changes appear to be governed not solely by the additional mass but also by the structure of the carrier. Application These findings indicate carrier structure should be considered by the wearer and may be used to inform policy in the recommendation of anterior infant carriage systems use by caregivers.

An evaluation of measurement systems estimating gait speed during a loaded military march over graded terrain

This study aimed to evaluate the accuracy of three measurement systems estimating gait speed during a loaded military march over graded terrain. Systems developed by the Swiss and Netherlands Armed Forces and a commercial wrist-based device were evaluated in comparison to a Global Positioning System. The first part of the paper focuses on the development of the Dutch system, where speed is estimated from a chest worn accelerometer and body measurements. For this validation study 36 subjects were walking or running 13 laps of 200 m at different speeds. Results showed that walking and running speed can be estimated with a R²_adj of 0.968 and 0.740, respectively. In the second part of this paper, data from 64 soldiers performing a 35 km march were used to evaluate the accuracy of three measurement systems in estimating speed. Data showed that estimating gait speed with a single accelerometer can be accurate for military activity, even without prior individual calibration measurements. However, predictions should be corrected for confounders such as body size and shoe type to be accurate. Both, downhill and uphill walking led to changes in gait characteristics and to an overestimation of speed by up to 10%. Correcting for slope or gradient using altimetry in future algorithms/experiments could improve the estimation of gait speed.

Wheeled assistive device for load carriage - the effects on human gait and biomechanics

Soldiers are often required to carry loads which impose biomechanical strain on the human body. This can adversely affect physical performances. Recently, wheel-based devices (WBD) were designed to reduce the load on the soldier. In the present study, a prototype of this newly developed WBD was evaluated. Thirteen volunteers performed three exercise protocols on a treadmill as follows: (1) no load; (2) carrying 40% of their bodyweight with a backpack or; (3) with the WBD. Data acquisition included: gait parameters, vertical ground reaction forces (VGRF) and contact pressure acting on the shoulder. Biomechanical analysis showed that the WBD decreased the contact pressure on the shoulder and the VGRF. However, greater gait variability, in terms of cycle-to-cycle gait line generation, was observed, which might point to a difficulty in maintaining stability while walking. The study suggests that WBD has a potential to reduce the biomechanical strain on the soldier while carrying heavy loads. Future potential adjustments for the development of a better WBD-based solution are suggested. Practitioner Summary: The present research observed the potential biomechanical advantages of using a wheel-based device designed to reduce the load on the soldier. It contributed to a lower mechanical force on the soldier's body, yet causing modulations in gait control. Future design adjustments should be made to optimise the platform.

A quantitative evaluation of physical and digital approaches to centre of mass estimation

Centre of mass is a fundamental anatomical and biomechanical parameter. Knowledge of centre of mass is essential to inform studies investigating locomotion and other behaviours, through its implications for segment movements, and on whole body factors such as posture. Previous studies have estimated centre of mass position for a range of organisms, using various methodologies. However, few studies assess the accuracy of the methods that they employ, and often provide only brief details on their methodologies. As such, no rigorous, detailed comparisons of accuracy and repeatability within and between methods currently exist. This paper therefore seeks to apply three methods common in the literature (suspension, scales and digital modelling) to three 'calibration objects' in the form of bricks, as well as three birds to determine centre of mass position. Application to bricks enables conclusions to be drawn on the absolute accuracy of each method, in addition to comparing these results to assess the relative value of these methodologies. Application to birds provided insights into the logistical challenges of applying these methods to biological specimens. For bricks, we found that, provided appropriate repeats were conducted, the scales method yielded the most accurate predictions of centre of mass (within 1.49 mm), closely followed by digital modelling (within 2.39 mm), with results from suspension being the most distant (within 38.5 mm). Scales and digital methods both also displayed low variability between centre of mass estimates, suggesting they can accurately and consistently predict centre of mass position. Our suspension method resulted not only in high margins of error, but also substantial variability, highlighting problems with this method.

Physiological and Biomechanical Responses to Prolonged Heavy Load Carriage During Level Treadmill Walking in Females

Heavy load carriage has been identified as a main contributing factor to the high incidence of overuse injuries in soldiers. Peak vertical ground reaction force (VGRFMAX) and maximal vertical loading rates (VLRMAX) may increase during heavy prolonged load carriage with the development of muscular fatigue and reduced shock attenuation capabilities. The objectives of the current study were (1) to examine physiological and biomechanical changes that occur during a prolonged heavy load carriage task, and (2) to examine if this task induces neuromuscular fatigue and changes in muscle architecture. Eight inexperienced female participants walked on an instrumented treadmill carrying operational loads for 60 minutes at 5.4 km·h–1. Oxygen consumption (), heart rate, rating of perceived exertion (RPE), trunk lean angle, and ground reaction forces were recorded continuously during task. Maximal force and in-vivo muscle architecture were assessed pre- and posttask. Significant increases were observed for VGRFMAX, VLRMAX, trunk lean angle,, heart rate, and RPE during the task. Increased vastus lateralis fascicle length and decreased maximal force production were also observed posttask. Prolonged heavy load carriage, in an inexperienced population carrying operational loads, results in progressive increases in ground reaction force parameters that have been associated with overuse injury.

The effect of training on lumbar spine posture and intervertebral disc degeneration in active-duty Marines

Military training aims to improve load carriage performance and reduce risk of injuries. Data describing the lumbar spine (LS) postural response to load carriage throughout training are limited. We hypothesised that training would reduce the LS postural response to load. The LS posture of 27 Marines was measured from upright MR images: with and without load (22.6 kg) at the beginning, middle, and end of School of Infantry (SOI) training. Disc degeneration was graded at L5-S1. No changes in posture and disc degeneration were found throughout training. During load carriage the LS became less lordotic and the sacrum more horizontal. Marines with disc degeneration had larger sacral postural perturbations in response to load. Our findings suggest that the postural response to load is defined more by the task needs than by the physical condition of the Marine. Practitioner Summary: The effect of military training on lumbar spine posture is unknown. The lumbar posture of 27 Marines was measured from upright MR images, with and without load throughout infantry training. No changes in posture or IVD degeneration were found across training. Marines with degeneration at the L5-S1 level had larger sacral postural perturbations in response to load.

Validation of an instrumented dummy to assess mechanical aspects of discomfort during load carriage

Due to the increasing load in backpacks and other load carriage systems over the last decades, load carriage system designs have to be adapted accordingly to minimize discomfort and to reduce the risk of injury. As subject studies are labor-intensive and include further challenges such as intra-subject and inter-subject variability, we aimed to validate an instrumented dummy as an objective laboratory tool to assess the mechanical aspects of discomfort. The validation of the instrumented dummy was conducted by comparison with a recent subject study. The mechanical parameters that characterize the static and dynamic interaction between backpack and body during different backpack settings were compared. The second aim was to investigate whether high predictive power (coefficient of determination R²>0.5) in assessing the discomfort of load carriage systems could be reached using the instrumented dummy. Measurements were conducted under static conditions, simulating upright standing, and dynamic conditions, simulating level walking. Twelve different configurations of a typical load carriage system, a commercially available backpack with a hip belt, were assessed. The mechanical parameters were measured in the shoulder and the hip region of the dummy and consisted of average pressure, peak pressure, strap force and relative motion between the system and the body. The twelve configurations consisted of three different weights (15kg, 20kg, and 25kg), combined with four different hip belt tensions (30N, 60N, 90N, and 120N). Through the significant (p<0.05) correlation of the mechanical parameters measured on the dummy with the corresponding values of the subject study, the dummy was validated for all static measurements and for dynamic measurements in the hip region to accurately simulate the interaction between the human body and the load carriage system. Multiple linear regressions with the mechanical parameters measured on the dummy as independent variables and the corresponding subjective discomfort scores from the subject study as the dependent variable revealed a high predictive power of the instrumented dummy. The dummy can explain 75% or more of the variance in discomfort using average pressures as predictors and even 79% or more of the variance in discomfort using strap forces as predictors. Use of the dummy enables objective, fast, and iterative assessments of load carriage systems and therefore reduces the need for labor-intensive subject studies in order to decrease the mechanical aspects of discomfort during load carriage.

Determination of Orientation and Practice Requirements When Using an Obstacle Course for Mobility Performance Assessment

OBJECTIVE

Determine effect of orientation (introduction and familiarization) and practice (repeated performance) on human performance under various load conditions as assessed by an obstacle course.

BACKGROUND

Obstacle courses are commonly used as screening tools by military, police, and firefighters or to assess human capabilities and the effect of wearing personal protective equipment (PPE) and other occupationally necessary equipment on mobility task performance. Unfortunately, little is formally documented about the effect of orientation and practice on performance outcomes of obstacle or mobility courses being used.

METHOD

Forty-eight participants were recruited from the Canadian Army Infantry and Combat Engineer population. Participants either received regular or extensive orientation of the course before completing it. Following orientation, participants completed the course five consecutive times while wearing their PPE with full fighting order (FFO) and five consecutive times while wearing no PPE and non-FFO across a five-day period (maximum two runs per day), with ensemble presentation order counterbalanced. Total course completion time and individual obstacle completion times were measured for each run of the course.

RESULTS

While wearing FFO, participants continued to decrease the time required for completing the course; however, while wearing non-FFO, time to course completion did not significantly change over the five runs. There were no differences in course completion times for the regular and extensive course orientation groups.

CONCLUSIONS

Considerations required to mitigate orientation and practicing effects can differ depending on type or complexity of load condition. While wearing FFO, practicing effects can introduce undesired confounding factors into data collection.

APPLICATION

Any practice runs on an obstacle course prior to its use as an assessment tool should focus on the loaded (e.g., FFO) condition because improvement on loaded runs is likely transferred to unloaded, but this does not apply in the reverse.

Additional helmet and pack loading reduce situational awareness during the establishment of marksmanship posture

The pickup of visual information is critical for controlling movement and maintaining situational awareness in dangerous situations. Altered coordination while wearing protective equipment may impact the likelihood of injury or death. This investigation examined the consequences of load magnitude and distribution on situational awareness, segmental coordination and head gaze in several protective equipment ensembles. Twelve soldiers stepped down onto force plates and were instructed to quickly and accurately identify visual information while establishing marksmanship posture in protective equipment. Time to discriminate visual information was extended when additional pack and helmet loads were added, with the small increase in helmet load having the largest effect. Greater head-leading and in-phase trunk-head coordination were found with lighter pack loads, while trunk-leading coordination increased and head gaze dynamics were more disrupted in heavier pack loads. Additional armour load in the vest had no consequences for Time to discriminate, coordination or head dynamics. This suggests that the addition of head borne load be carefully considered when integrating new technology and that up-armouring does not necessarily have negative consequences for marksmanship performance. Practitioner Summary: Understanding the trade-space between protection and reductions in task performance continue to challenge those developing personal protective equipment. These methods provide an approach that can help optimise equipment design and loading techniques by quantifying changes in task performance and the emergent coordination dynamics that underlie that performance.

An alternative whole-body marker set to accurately and reliably quantify joint kinematics during load carriage

Body armor covers anatomical landmarks that would otherwise be used to track trunk and pelvis movement in motion analysis. This study developed and evaluated a new marker set, and compared it to placing markers on the skin and over-top of body armor. In our method, pelvis and trunk motions were measured using a custom-built sacral and upper-back marker cluster, respectively. Joint angles and ranges of motion were determined while participants walked without and with body armor. Angles were obtained from the new marker set and compared against conventional marker sets placed on the skin or over-top the body armor. Bland-Altman analyses compared the agreement of kinematic parameters between marker sets, while joint angle waveforms were compared using inter-protocol coefficient of multiple correlations (CMCs). The intra- and inter-session similarities of joint angle waveforms from each marker set were also assessed using CMCs. There was a strong agreement between joint angles from the new marker set and markers placed directly on the skin at key anatomical landmarks. The agreement worsened with markers placed on top of body armor. Inter-protocol CMCs comparing markers on body armor to the new marker set were poor compared to CMCs between skin-mounted markers and the new marker set. Intra- and inter-session repeatability were higher for the new marker set compared to placing markers over-top of body armor. The new marker set provides a viable alternative for researchers to reliably measure trunk and pelvis motion when equipment, such as body armor, obscures marker placement.

Effect of Load Magnitude and Distribution on Lumbar Spine Posture in Active-duty Marines

STUDY DESIGN

Repeated measures.

OBJECTIVE

The purpose of this study was to quantify the effect of operationally relevant loads and distributions on lumbar spine (LS) in a group of active-duty Marines.

SUMMARY OF BACKGROUND DATA

Low-back pain has been associated with heavy load carriage among military personnel. Although there are data describing the LS posture in response to load, the effect of varying load characteristics on LS posture remains unknown.

METHODS

Magnetic resonance images of Marines (n = 12) were acquired when standing unloaded and when carrying 22, 33, and 45 kg of load distributed both 50% to 50% and 20% to 80% anteriorly and posteriorly. Images were used to measure LS and pelvic postures. Two-way repeated-measures ANOVA and posthoc tests were used to compare LS posture across load magnitudes and distributions (α = 0.05). This project was founded by the US Army Medical Research Acquisition Activity, Award No. W81XWH-13-2-0043, under Work Unit No. 1310.

RESULTS

No changes in LS posture were induced when load was evenly distributed. When load was carried in the 20% to 80% distribution lumbosacral flexion increased as a result of sacral anterior rotation and overall reduced lumbar lordosis. This pattern was greater as load was increased between 22 and 33 kg, but did not increase further between 33 and 45 kg. We observed that the inferior LS became uniformly less lordotic, independently of load magnitude. However, the superior LS became progressively more lordotic with increasing load magnitude CONCLUSION.: Postural adaptations were found only when load was carried with a posterior bias, suggesting that load-carriage limits based on postural changes are relevant when loads are nonuniformly distributed. Although the tendency would be to interpret that loads should be carried symmetrically to protect the spine, the relationship between postural changes and injury are not clear. Finally, the operational efficiency of carrying load in this distribution needs to be tested.

LEVEL OF EVIDENCE

3.

Loading of the lumbar spine during backpack carriage

Backpack carriage is significantly associated with a higher prevalence of low back pain. Elevated compression and shear forces in the lumbar intervertebral discs are known risk factors. A novel method of calculating the loads in the lumbar spine during backpack carriage is presented by combining physical and numerical modelling. The results revealed that to predict realistic lumbar compression forces, subject-specific lumbar curvature data were not necessary for loads up to 40 kg. In contrast, regarding shear forces, using subject-specific lumbar curvature data from upright MRI measurements as input for the rigid body model significantly altered lumbar joint force estimates.

Walking With aBackpack Using Load Distribution and Dynamic Load Compensation Reduces Metabolic Cost and Adaptations to Loads

In this study, we showed a way of reducing the metabolic cost of walking with a backpack using load distribution and dynamic load compensation, provided by a wearable upper body device. This device distributes the backpackload between the shouldersand the pelvis, senses the vertical motion of the pelvis, and provides gait synchronized compensatory forces to reduce the dynamic loads from a backpack. It was hypothesized that by reducing dynamic loads from a backpack during load carriage, the user's gait and postural adaptation, muscular effort and metabolic cost would be reduced. This hypothesis was supported by biomechanical and physiological measurements on a group of young healthy subjects, as they walked on a treadmill under four different conditions: unloaded; with a backpack, loaded with 25% of their body weight, supported on the shoulders; with the same load distributed between the shoulders and the pelvis; and with dynamic load compensation in addition to load distribution. The results showed reductions in gait and postural adaptations, muscle activity, vertical and braking ground reaction forces, and metabolic cost while carrying the same backpack load with the device. We conclude that the device can potentially reduce the risk of musculoskeletal injuries and muscle fatigue associated with carrying heavy backpack loads while reducing the metabolic cost of loaded walking.

Multi-objective analysis for assessing simultaneous changes in regional spinal curvatures under backpack carriage in young adults

Change in sagittal spinal curvature from the neutral upright stance is an important measure of the heaviness and correctness of backpack use. As current recommendations, with respect to spinal profile, of backpack load thresholds were based on the significant curvature change in individual spinal region only, this study investigated the most critical backpack load by assessing simultaneously the spinal curvature changes along the whole spine. A motion analysis system was used to measure the curvature changes in cervical, upper thoracic, lower thoracic and lumbar regions with backpack load at 0, 5, 10, 15 and 20% of body weight. A multi-objective goal programming model was adopted to determine the global critical load of maximum curvature change of the whole spine in accordance with the maximum curvature changes of the four spinal regions. Results suggested that the most critical backpack load was 13% of body weight for healthy male college students. Practitioner Summary: As current recommendations of backpack load thresholds were based on the significant curvature change in individual spinal region only, this study investigated the backpack load by considering simultaneously the spinal curvature changes along the whole spine. The recommendation, in terms of the global critical load, was 13% of body weight for healthy male college students.

A Preliminary Investigation of the Effect of Protective Clothing Weight, Bulk and Stiffness on Combat Mobility Course Performance

Soldier loads continue to rise in response to new technological capabilities and emerging threats. However, literature addressing the extent to which load mass properties affects operational task performance and mission outcome is sparse. The objective of this preliminary study was to quantify the effect of PPE mass properties (weight, bulk and stiffness) on combat mobility, as measured using the standardized Load Effects Assessment Program (LEAP) course. Twenty-four soldiers completed the LEAP course in three clothing and individual equipment (CIE) configurations (UE: unencumbered; FFO: full fighting order (FFO) without body armour; and FFO+: FFO with body armour). Significant differences between clothing conditions were revealed for LEAP performance metrics (overall course time). Regression analysis revealed significant relationships between overall mobility performance and condition mass properties of weight, bulk, and stiffness. Outcomes will influence the design of future CIE and future research in this area.

Biomechanics of Load Carriage--Historical Perspectives and Recent Insights

Loads carried by the warfighter have increased substantially throughout recorded history, with the typical U.S. ground soldier carrying external loads averaging 45 kg during operations in Afghanistan. Incidence of disability in the U.S. Army has also increased sixfold since the 1980s, predominantly driven by increases in musculoskeletal injuries, with load carriage implicated as a possible mechanism. This article will provide a brief overview of the biomechanics of load carriage and will provide some recent insights into how the stress of the loads carried by military personnel can affect the musculoskeletal system. Studies into the biomechanics of load carriage have documented motion-related differences such as increased step rate, decreased stride length, and more trunk lean with increases in pack-borne loads. However, there is a paucity of literature on the relationship between load carriage and biomechanical mechanisms of overuse injury. Findings of recent studies will be presented, which add mechanistic information to increased stresses on the lower extremity. This was particularly true at the knee, where in one study, peak knee extension moment increased 115% when carrying a 55 kg load (0.87 ± 0.16 Nm·kg⁻¹) vs. no external load (0.40 ± 0.13 Nm·kg⁻¹). Efforts to model injury mechanisms require continued biomechanical measurements in humans while carrying occupationally relevant loads to be validated. Specifically, imaging technologies (e.g., bone geometry scans) should be incorporated to produce higher fidelity model of the stresses and strains experienced by the load carrier. In addition to laboratory-based biomechanics, data are needed to further explore the mechanistic relationship between load magnitude and injury; to this end, wearable sensors should continue to be exploited to accurately quantify biomechanical stresses related to load carriage in the field.

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.

The effects of military body armour on trunk and hip kinematics during performance of manual handling tasks (.)

Musculoskeletal injuries are reported as burdening the military. An identified risk factor for injury is carrying heavy loads; however, soldiers are also required to wear their load as body armour. To investigate the effects of body armour on trunk and hip kinematics during military-specific manual handling tasks, 16 males completed 3 tasks while wearing each of 4 body armour conditions plus a control. Three-dimensional motion analysis captured and quantified all kinematic data. Average trunk flexion for the weightiest armour type was higher compared with control during the carry component of the ammunition box lift (p < 0.001) and sandbag lift tasks (p < 0.001). Trunk rotation ROM was lower for all armour types compared with control during the ammunition box place component (p < 0.001). The altered kinematics with body armour occurred independent of armour design. In order to optimise armour design, manufacturers need to work with end-users to explore how armour configurations interact with range of personal and situational factors in operationally relevant environments. Practitioner Summary: Musculoskeletal injuries are reported as burdening the military and may relate to body armour wear. Body armour increased trunk flexion and reduced trunk rotation during military-specific lifting and carrying tasks. The altered kinematics may contribute to injury risk, but more research is required.

Effect of uphill walking with varying grade and speed during load carriage on muscle activity

Indian soldiers, while guarding the mountainous border areas, often carry loads in steep uphill gradients. This activity may predispose the risk of muscle injury. The present study aimed to examine the effects of an increasing load, speed and gradient during incremental uphill treadmill walking on different muscles. Twelve infantry soldiers walked on a treadmill at two speeds (2.5 and 4 km/h) with no load, and carrying 10.7, 17 and 21.4 kg loads at 0, 5, 10, 15, 20, 25% gradients. Electromyographic responses of erector spinae (>240%) and vastus medialis (>240%) were mostly affected, followed by soleus (>125%) and gastrocnemius medialis (>100%) at maximum speed, load and gradient combination compared to 0% gradient. Carrying 10.7 kg at 15% gradient and above was found to be highly strenuous and fatiguing with the risk of muscle injury. Uphill load carriage in slower speed is recommended for the maintenance of combat fitness of the individual at higher gradients. Practitioner Summary: The present article has evaluated the stress encountered by soldiers during load carriage at incremental uphill gradients while walking at different speeds by recording the muscular activities. Load carriage in steep uphill gradients is highly strenuous and may lead to muscle injury thus compromising the combat fitness.

Load distribution and postural changes in young adults when wearing a traditional backpack versus the BackTpack

Backpacks lead to poor posture due to the posterior placement of the load, which overtime may contribute to low back pain and musculoskeletal complications. This study examined postural and load distribution differences between a traditional backpack (BP) and a nontraditional backpack (BTP) in a young adult population. Using a 3D motion analysis system, 24 healthy young adults (22.5 ± 2.5 years, 12 male) completed both static stance and walking trials on a treadmill with No Load and with 15% and 25% of their body weight using the two different backpacks. There was a significant difference in trunk angle, head angle, and lower extremity joint mechanics between the backpack and load conditions during walking (p<.05). Notably, relative to the No Load condition, trunk angle decreased approximately 14° while head angle increased approximately 13° for the BP 25% state on average. In contrast, average trunk and head angle differences for the BTP 25% state were approximately 7.5° and 7°, respectively. There was also a significant difference in head angle from pre- to post-walk (p<.05) across backpacks, loads, and time. Taken together, the results indicate that the BTP more closely resembled the participants' natural stance and gait patterns as determined by the No Load condition. The more upright posture supported by the BTP may help reduce characteristics of poor posture and, ideally, help to reduce low back pain while carrying loads.

Self-reported load carriage injuries of military soldiers

The aim of this study was to investigate whether occupational load carriage constitutes a significant source of injury to military soldiers. An online survey was sent to soldiers serving in specific Australian Army Corps known to experience the greatest occupational exposure to load carriage. Of the 338 respondents, 34% sustained at least one load carriage injury. Fifty-two per cent of those injured during initial training reported sustaining an additional load carriage injury. The majority of injuries (61%) were to the lower limbs with bones and joints the most frequently injured body structures (39%). Endurance marching (continuous marching as part of a physical training session) was the activity accounting for most (38%) injuries. Occupational load carriage is associated with military soldier injuries and, once injured, soldiers are at a high risk of future load carriage injury. The bodily sites and nature of self-reported injuries in this study are akin to those of formally reported injuries and those of other militaries.