Load Placement Research Articles

Load carrying during locomotion in the barnacle goose ( Branta leucopsis): The effect of load placement and size

Load carrying has been used to study the energetics and mechanics of locomotion in a range of taxa. Here we investigated the energetic and kinematic effects of trunk and limb loading in walking barnacle geese ( Branta leucopsis). A directly proportional relationship between increasing back-load mass and metabolic rate was established, indicating that the barnacle goose can carry back loads (up to 20% of body mass) more economically than the majority of mammals. The increased cost of supporting and propelling the body during locomotion is likely to account for a major proportion of the extra metabolic cost. Sternal loads up to 15% of body mass were approximately twice as expensive to carry as back loads. Given the key role in dorso-ventral movement of the sternum during respiration we suggest that moving this extra mass may account for the elevated metabolic rate. Loading the distal limb with 5% extra mass incurred the greatest proportional rise in metabolism, and also caused increases in stride length, swing duration and stride frequency during locomotion. The increased work required to move the loaded limb may explain the high cost of walking.

Short-term effects of backpack load placement on spine deformation and repositioning error in schoolchildren

Backpack weight of 10–15% has been recommended as an acceptable limit for schoolchildren. However, there is still no clear guideline regarding where the backpack centre of gravity (CG) should be positioned. The changes of spinal curvature and repositioning error when carrying a backpack loaded at 15% of body weight at different CG locations (anterior or posterior at T7, T12 or L3) in schoolchildren were analysed. Both spinal curvature and repositioning error were found to be affected by backpack anterior–posterior position and CG level. A relatively smaller change was observed during anterior carriage with the least change when the backpack CG was positioned at T12. The results also suggested that alternative carriage by changing the backpack position occasionally between anterior and posterior positions might help to relieve the effects of backpack on spine. However, future study is recommended to further substantiate the beneficial effects of alternative carriage on children. Statement of Relevance: Anteriorly carried backpack with centre of gravity positioned at T12 was shown to induce relatively less effect on spinal deformation and repositioning error in schoolchildren. Changing backpack carriage position occasionally may help to relieve its effects on spinal deformation. The findings are important for ergonomic schoolbag design and determining a proper load carriage method.

Walking in Simulated Martian Gravity: Influence of the Portable Life Support System’s Design on Dynamic Stability

With human exploration of the moon and Mars on the horizon, research considerations for space suit redesign have surfaced. The portable life support system (PLSS) used in conjunction with the space suit during the Apollo missions may have influenced the dynamic balance of the gait pattern. This investigation explored potential issues with the PLSS design that may arise during the Mars exploration. A better understanding of how the location of the PLSS load influences the dynamic stability of the gait pattern may provide insight, such that space missions may have more productive missions with a smaller risk of injury and damaging equipment while falling. We explored the influence the PLSS load position had on the dynamic stability of the walking pattern. While walking, participants wore a device built to simulate possible PLSS load configurations. Floquet and Lyapunov analysis techniques were used to quantify the dynamic stability of the gait pattern. The dynamic stability of the gait pattern was influenced by the position of load. PLSS loads that are placed high and forward on the torso resulted in less dynamically stable walking patterns than loads placed evenly and low on the torso. Furthermore, the kinematic results demonstrated that all joints of the lower extremity may be important for adjusting to different load placements and maintaining dynamic stability. Space scientists and engineers may want to consider PLSS designs that distribute loads evenly and low, and space suit designs that will not limit the sagittal plane range of motion at the lower extremity joints.

Effects of backpack load placement on pulmonary capacities of normal schoolchildren during upright stance

Backpack carriage affects posture, physiological costs and physical performance. Limited literature concerning the effects of backpack load placement on pulmonary capacities of schoolchildren has been reported. The objective was to assess the effects of backpack load placement on pulmonary capacities of normal schoolchildren. Forced vital capacity (FVC), forced expiratory volume in 1 s (FEV 1), peak expiratory flow (PEF), and forced expiratory flow (FEF 25–75%) were measured in 22 normal schoolchildren with a mean age of 12 years during free standing and when carrying a backpack of 15% bodyweight with its center of gravity positioned at T7, T12 and L3. The main effect of load was found to be significant for FVC and FEV 1. However, no significant effect of load placements on the pulmonary function of schoolchildren was found. Manipulation of load placements may not alleviate the restrictive effects exerted on the pulmonary function resulted from backpack load carriage. Relevance to industry Daily carriage of a school backpack on the musculoskeletal health of children and adolescents has become an area of concern. Restrictive effects on the pulmonary function due to backpack carriage were reported and it is useful to explore whether these effects could be alleviated by manipulating the backpack center of gravity level.

Lower limb dynamics change for children while walking with backpack loads to modulate shock transmission to the head

Backpack load carriage increases ground reaction forces and increases the stiffness in the upper extremity that can cause transmission of higher amount of forces from the lower extremity to the head. This study investigated the effect of load carriage and placement of load on the back on the shock transmission mechanisms amongst children. Fifteen primary school boys with mean age 10.01 (±1.31) years, mean height 136.40 (±10.08) cm and mean mass 31.83 (±7.13) kg completed the study. Subjects carried 10%, 15% and 20% bodyweight (BW) loads on two locations on the back, namely upper and lower. Results showed a significant reduction in pelvic and trunk rotation in the transverse plane and an increase in the upper body stiffness for loads exceeding 15% of BW. The lower limb results showed a reduction in the first peak force and cadence and a significant change in the walking velocity and time to the first peak force for 20% load. No significant differences were found for the load configuration but the upper configuration showed slightly higher shock transmission. The changes in the lower limb dynamics indicated that there are locomotion mechanisms in place amongst children to modulate shock transmission to the head.

Effect of backpack load placement on posture and spinal curvature in prepubescent children

Parents, educators and researchers have expressed concern about the long term impacts of children carrying excessive loads in their backpacks on a daily basis. Although many researchers have investigated appropriate weight limits for children's packs, little research has been conducted on the design of children's backpacks. The purpose of this study was to evaluate the changes in children's trunk forward lean (TFL), cranio-vertebral angle (CVA) and spinal lordosis angle (LA) that occurred with high, medium and low load locations during standing and walking. Ten-year-old children (n = 15) completed a repeated measures designed study while carrying 15% of each child's body weight in a typical backpack with only shoulder straps. A special instrumented backpack (IBP) was designed that allowed the weight to be placed in the proper location and continuously measure changes in spinal curvature. TFL and CVA postures were captured on digital video at five intervals including: standing without a backpack prior to a 1000 m walk; standing with a backpack at the beginning and end of a 1000 m walk; and walking with a backpack at the beginning and end of a 1000 m walk. Results indicated that significant changes occurred in TFL and CVA when the backpack was loaded to 15% body weight. The low load placement in the backpack produced fewer changes in CVA from the initial standing baseline measure than the high and mid placements. When all measures were assessed collectively, there were fewer changes in LA in the low load placement. These findings indicate that future backpack designs should place loads lower on the spine in order to minimize children's postural adaptations.

A Rolling Horizon Model to Optimize Aerodynamic Efficiency of Intermodal Freight Trains with Uncertainty

Aerodynamic efficiency of intermodal freight trains can be significantly improved by minimizing the adjusted gap lengths between adjacent loads. This paper first develops a static model to optimize load placement on a sequence of intermodal trains that have scheduled departure times. This model applies when full information on all trains and loads is available. Then, we develop a dynamic model to account for the more realistic situation in which there is incomplete or uncertain information on future trains and incoming loads. This paper develops methodology to balance between (i) the advantage from optimizing multiple trains together and (ii) the risk of making suboptimal decisions due to incomplete future information. We propose a rolling horizon scheme to address this challenge, where exponentially decreasing weights are assigned to the objective functions of future trains. Numerical results based on empirical data show significant aerodynamic efficiency benefits from these optimization models.

Path Planning Reflecting the Changing Working Environment Through the Cargo Load Placement and Retrieval by Omnidirectional Transporter and Radio Frequency Identification

The warehouse transport environment changes with load movement, e.g., the area where a load is placed becomes an obstacle for a carrier and the area from which a load is removed becomes area once more useable for movement. These considerations make path planning an important aspect of work planning. Transport system is suggested to manage environment change using radio frequency identification (RFID) which is put on a load and records the data of load size and its position. When the environment data of RFID changes by placing or removing the load, new information is read by reader of load carrier and sent to map management server. A three-omnidirectional vehicle is used to make transport more efficient.

Thermal Conduction Switch for Thermal Management of Chip Scale Atomic Clocks (IMECE2006-14540)

We report the first use of a bimetallic buckling disk as a thermal conduction switch. The disk is used to passively alter the thermal resistance of the package of a chip scale atomic clock. A vertical-cavity surface-emitting laser and a cesium vapor cell, contained in the clock, must be maintained at 70±0.1°C even under an ambient temperature variation of −40°Cto50°C. A thermal test vehicle has been developed to characterize a sample package with a thermal conduction switch. Three cases are presented for the temperature control of the test vehicle under different load placements and environmental conditions: (1) a heating load with a good conduction path to the switch in a vacuum package; (2) the same loading as in Case 1, but packaged in air; and (3) a heating load insulated from the switch, in a vacuum package. At 38°C, the switch snaps upward to reduce the package’s thermal resistance. As a result, the heating power needed to maintain a constant temperature, 63.9±0.1°C, is increased from 118to200mW for Case 1. Such a significant change of the thermal resistance demonstrates the effectiveness of the thermal switch. However, the switch becomes less effective with air filling the gap, as in Case 2, and the switch is not effective at all if the heating load does not have a good conduction path to the switch as in Case 3. The steady state response of this novel thermal conduction switch has been well characterized through experimentation and finite element analysis.

Chart for edge stress in plate on elastic solid

A computerized chart is described for determination of the maximum bending stress, σe, under edge loading developing in a slab on an elastic solid. The solution used is akin to a closed-form solution. A predictive equation for σe is established, similar in form to the dense liquid formula of Westergaard. The variation of the load placement effect factor, i.e., the ratio of σe to the maximum stress under interior loading, is examined. This is shown to be solely a function of the load size ratio. The new solution is incorporated in a computer program (H51ES), which requires a relatively small memory core and can be implemented on a personal computer. The program can accommodate both single- and multi-wheel gears and promises to be useful to airport and highway pavement engineers.Key words: rigid pavements, elastic solid, edge loading, numerical solutions, slabs.

Backpack load positioning and walking surface slope effects on physiological responses in infantry soldiers

The primary purpose of this study was to survey a sample of infantry soldiers to determine their preferred backpack load distribution. The second objective was to examine the effect of backpack load position, walking speed and surface grade on the physiological responses of infantry soldiers. In the first phase of the study, analysis of 500 questionnaires indicated that backpack item arrangement was typically determined by convenience and habit. About 33.6% and 30.6% of the soldiers reported placing heavier items in the lower and higher positions, respectively. In the second experimental phase of the study, five male infantry soldiers who had completed basic training (including 30 km tactical road march) each performed eight treadmill walking bouts at one of two speeds (i.e., 3.2 and 6.4 km/h) and two surface grades (0% and 6%) carrying 15% of their body weight in a backpack with two chambers (upper and lower). Respiratory frequency, minute ventilation, oxygen consumption and heart rate were recorded using the Quark pulmonary system. The MANOVA results indicate that the effect of walking speed was strongly significant for all physiological indices. In addition, as might be expected, walking on a 6% grade was associated with higher oxygen consumption and heart rate compared to level going. In terms of load position, the mean respiratory frequency was significantly higher where the load was carried in the upper position. Furthermore, there was a significant interaction between load position and walking grade on oxygen consumption. Where participants were walking at zero incline, there was no significant difference in mean oxygen consumption. By contrast, the mean oxygen consumption was significantly higher where loads were carried in the upper position and negotiating the 6% grade. Previous research has found that, ideally, load carriage should complement stability, bringing the load's center of gravity as close to the body as possible and making use of the larger muscles. However, carrying heavy loads close to the trunk can affect lung function. Thus, load placement is an important factor in physiological response to load carriage, and optimum choice of upper or lower position when distributing items in a backpack may be dependent on the walking grade. Relevance to industry The need to carry heavy loads for long distances is common to a range of human endeavor, with physical transport used by soldiers, various types of workers and recreational hikers. Load distribution and walking gradient are important factors in terms of the efficiency of load carriage and should be taken into consideration in both the design and loading of backpacks. Thus, the findings of the present study will be relevant to infantry soldiers, mountain climbers or of manual material handling (MMH) in industry, providing practical implications for tasks associated with exposure to uneven walking/working surfaces.

Finite element stress analysis and fatigue behavior of cast circumferential clasps

Deformation and fracture of cast circumferential clasps may be a result of stresses induced during mastication. Most biomechanical clasp studies have been performed only under static conditions. There is little information regarding behavior of clasps over time. The purpose of this study was to evaluate stress distribution on cast circumferential clasps, and the displacements or deformations, depending on the load placement and range. Fatigue analysis was then conducted to evaluate the behavior of clasps over time. Static stress values and distribution induced in cast circumferential clasps were calculated and studied using 3-dimensional finite element experimental models for Co-Cr cast circumferential clasps. Average loads between 20 and 35 N were applied vertically along the clasp components to simulate static stress distribution during translation and rotation of the denture. After determination of stress concentration areas, the fatigue behavior of clasps was studied using finite element analysis during simulated cyclic masticatory loads (loads between 0 and 20 N included in 4500 masticatory cycles over 24 hours). For the translation simulation, the maximum stress was 310.27 MPa, located near the lower margin of the retentive arm, and for the rotation simulation, the maximum stress was 310.31 MPa, located near the upper margin of the same arm. Under simulated static load, the magnitude of stresses found in the clasps was under the reported yield strength (640 MPa) of the Co-Cr alloy. The maximum stresses indicated the area of highest fracture risk, but fractures occurred only under a simulated cyclic mastication load representing 5.5 years of service. Within the limitations of the simulation study, static stress analysis of cast circumferential clasps indicated the location of greatest fracture risk to be at the junction of the clasp arm with the body, for all situations. In addition, fatigue analysis estimated clasp degradation over time and the survival rate of the same clasps, which was found to be 5.5 years, on average.

Contribution of loading conditions and material properties to stress shielding near the tibial component of total knee replacements

This communication reports important preliminary results of a parametric analysis into the stress shielding effects of loading conditions and material properties of a total knee replacement (TKR) prosthesis. A previously developed finite element (FE) model of the proximal tibia that incorporated orthotropic and heterogeneous bone properties was used. Tibiofemoral joint compression and soft tissue (ligament and muscle) forces were also included to better represent the loading condition in the tibia. Stress shielding effects were studied for a prosthesis similar to a commercially available model. Results from the model show that the hypothesis of relatively higher Young's modulus of implant compared to bone as the primary cause of stress shielding is not sufficiently descriptive. Loading conditions as a result of altered bone or implant condylar surface geometry, load placement on the condylar surface, and load pattern created by the TKR are at least as important or, in some cases, more important factors in observed stress shielding immediately post-operation. This finding can be used to focus new implant design on altered loading conditions as well as material selection.

Sensitivity of Live Load Distribution Factors to Vehicle Spacing

Two slab-on-girder bridge superstructures are analyzed using grillage models. Different live load placement configurations are investigated to determine the sensitivity of live load shear and moment to vehicle spacing. Results from both bridges show that the distribution factors are relatively insensitive to vehicle spacing. Therefore significant computational speedups are available when applying vehicle loads on an influence surface with a fixed spacing.

THE CORRELATION OF ELECTRICAL PROPERTIES OF PRESTRESSED UNIMORPHS AS A FUNCTION OF MECHANICAL STRAIN AND DISPLACEMENT

ABSTRACT Prestressed Unimorph-type actuators are being adapted for industrial applications previously not envisioned for piezoelectric based technology. These include electronically controlled diesel valve injectors, non-resonance fluidic pumps and low-stroke linear motors. To expand the range of applications and refine the design of these actuators for additional technologies, the effects of load placement, force distribution and mounting must be investigated in order to retrofit existing devices and systems. This research uses several sets of multi-strain gauged THUNDER actuators mounted in a fully compliant test fixture equipped with an LVDT and weight system and to characterize the room temperature strain and displacement as a function of mechanical loading. The results will be used to refine computational models that aid in the design of these actuators for future applications.

The Physiological And Performance Effects Of Varying Load Center Of Mass Position During Load Carriage At 7.5 Km.h-1

PURPOSE The aim of the study was to determine and compare the physiological and performance effects of varying center of mass positions during load carriage at a speed of 7.5km.h−1. METHODS Ten male volunteers (20.54 ± 0.92 yrs, 74.32 ± 7.60 kg) exercised on a level treadmill at 7.5 km.h−1 carrying a 25 kg load. Each participantr repeated the trial 3 times, for the 3 center of mass positions of high, mid or low. These positions were randomly assigned for each trial. The physiological variables measured included oxygen consumption (VO2), heart rate (HR), respiratory exchange ratio (R), rating of perceived exertion (RPE). Performance testing was conducted using the Standing Broad Jump and Zig-Zag Agility Run prior to the 45 minute load carriage exercise protocol and at the end of 15 minute rest interval post exercise. RESULTS VO2 was significantly lower in the high position (37.02 ± 7.41 ml.kg−1·min−1) compared to the mid and low positions (38.35 ± 6.03 ml.kg−1·min−1, 39.37 ± 4.65 ml.kg−1·min−1 respectively). HR, RPE and R did not change significantly between the high (176.69 ± 23.02 bpm, 15.67 ± 1.63, 1.13 ± 0.13 respectively) and low load positions (176.11 ± 24.61 bpm, 15.50 ± 1.52, 1.06 ± 0.06 respectively). Post rest jumps and run times were not significantly different from pre-exercise jump and run times. CONCLUSION The results of this study suggest that load placement is an important consideration affecting the physiological, subjective and performance responses to load carriage. At 7.5 km.h−1, it was found that packing heavier items at the top portion of the backpack might be the most energy efficient model of load carriage. Performance measures of standing broad jump and agility run times did not appear to be affected by the exercise bout with load carriage and indicate that the 15-minute compulsory rest may be sufficient to allow for full recovery.

The Physiological And Performance Effects Of Varying Load Center Of Mass Position During Load Carriage At 7.5 Km.h-1

PURPOSE The aim of the study was to determine and compare the physiological and performance effects of varying center of mass positions during load carriage at a speed of 7.5km.h−1. METHODS Ten male volunteers (20.54 ± 0.92 yrs, 74.32 ± 7.60 kg) exercised on a level treadmill at 7.5 km.h−1 carrying a 25 kg load. Each participantr repeated the trial 3 times, for the 3 center of mass positions of high, mid or low. These positions were randomly assigned for each trial. The physiological variables measured included oxygen consumption (VO2), heart rate (HR), respiratory exchange ratio (R), rating of perceived exertion (RPE). Performance testing was conducted using the Standing Broad Jump and Zig-Zag Agility Run prior to the 45 minute load carriage exercise protocol and at the end of 15 minute rest interval post exercise. RESULTS VO2 was significantly lower in the high position (37.02 ± 7.41 ml.kg−1·min−1) compared to the mid and low positions (38.35 ± 6.03 ml.kg−1·min−1, 39.37 ± 4.65 ml.kg−1·min−1 respectively). HR, RPE and R did not change significantly between the high (176.69 ± 23.02 bpm, 15.67 ± 1.63, 1.13 ± 0.13 respectively) and low load positions (176.11 ± 24.61 bpm, 15.50 ± 1.52, 1.06 ± 0.06 respectively). Post rest jumps and run times were not significantly different from pre-exercise jump and run times. CONCLUSION The results of this study suggest that load placement is an important consideration affecting the physiological, subjective and performance responses to load carriage. At 7.5 km.h−1, it was found that packing heavier items at the top portion of the backpack might be the most energy efficient model of load carriage. Performance measures of standing broad jump and agility run times did not appear to be affected by the exercise bout with load carriage and indicate that the 15-minute compulsory rest may be sufficient to allow for full recovery.

Tunable randomization for load management in shared-disk clusters

We develop and evaluate a system for load management in shared-disk file systems built on clusters of heterogeneous computers. It balances workload by moving file sets among cluster server nodes. It responds to changing server resources that arise from failure and recovery, and dynamically adding or removing servers. It also realizes performance consistency---nearly uniform performance across all servers. The system is adaptive and self-tuning. It operates without any a priori knowledge of workload properties, or the capabilities of the servers. Rather, it continuously tunes load placement using a technique called adaptive, nonuniform (ANU) randomization. ANU randomization realizes the scalability and metadata reduction benefits of hash-based, randomized placement techniques, while avoiding hashing's drawbacks: load skew, inability to cope with heterogeneity, and lack of tunability. ANU randomization outperforms virtual-processor approaches to load balancing, while reducing the amount of shared state among servers and the amount of load movement.

New approaches for designing microstrip filters utilizing mixed dielectrics

A strategy is developed for designing capacitively loaded microstrip filters on low-temperature co-fired ceramic (LTCC) substrates with inclusions or superstrate layers of higher permittivity dielectrics. Finite-difference time-domain simulations of the field distribution at resonant frequencies are used to determine the optimal locations and size of capacitive loads. It is demonstrated that strategic capacitive load placement enables altering the center and attenuation pole frequencies, the shape and width of the passband, and input impedance of the filter by modification of selected resonant modes. Capacitive loading with higher permittivity dielectrics is shown to be very efficient in decreasing dimensions of microstrip filters with low-permittivity substrates. The designs of novel compact resonators and filters have been developed and the prototypes fabricated by using LTCC technology. The results of prototype measurements agree with the simulation results, which validates the proposed approach

Effect of load position on physiological and perceptual responses during load carriage with an internal frame backpack

The purpose of this study was to determine the effect of load position in an internal frame backpack on physiological and perceptual variables. Ten female participants walked on a level treadmill for 10 min carrying 25% of their body weight in a high, central, or low position. The variables measured included oxygen consumption (VO2), heart rate (HR), respiratory exchange ratio (R), respiratory rate (RR), minute ventilation (VE), and rating of perceived exertion (RPE). VO2, VE, and RPE were significantly lower in the high position (18.6 ± 2.3 ml/kg/min, 31.7 ± 5.0 l/min, 2.8 ± 0.8, respectively) compared to the low position (22.2 ± 3.0 ml/kg/min, 38.6 ± 7.5 l/min, 3.7 ± 1.0, respectively). HR, R, and RR did not change significantly as the load was moved from the high (129.8 ± 16.8, 0.89 ± 0.06, 30.3 ± 4.2, respectively) to the low position (136.0 ± 25.3, 0.92 ± 0.04, 33.8 ± 5.2, respectively). The results of this study suggest that load placement is an important factor in the physiological and perceptual responses to load carriage, and that packing heavy items high in the backpack may be the most energy efficient method of carrying a load on the back.