Design of CF Load Carriage Systems

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Design of CF Load Carriage Systems
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System Anatomy
Fragmentation Vest
Tactical Assault Vest
Rucksack
3
Upper Strap
Load Volume
Lower Strap
Waistbelt
Framesheet
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Design Concepts

• Load Volume
• Suspension System
• Load Control
• Force Distribution

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Injury Potential

• muscle fatigue
• occlusion of blood
• muscle bruise/tear
• discomfort, pain
• damage
• compression
• shear (friction)

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Loading Areas
Shoulders Back Waist
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Anthropometrics and Biomechanics

• Development of static biomechanical model
• Analysis of typical pack geometries
• Comparison of model results to discomfort
  ratings in a test cohort
• Suggested design limits for shoulder and lumbar
  loads

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Biomechanical Model

• Simple model to determine forces distributed to
  torso
• Based on geometry of strapping and lean angle

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Pack Model
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Horizontal Lumbar Force
Transverse Shear
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Shoulder Model
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Input Values

• Strap locations
• Angle of lower strap
• Tension of lower strap
• Lean angle
• Pack weight
• Shoulder angles

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Output Values

• Upper strap force and angle
• Lower reaction force
• Shoulder reaction force
• Shoulder friction force

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Packs Evaluated

• Military Systems (1999 Issue)
• A
• B
• C
• Commercial Systems (Mid-High Range)
• D
• E

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Human Testing

• 6 km march with 32kg
• 20 soldiers, volunteers, informed consent
• Combined ratings for perceived discomfort in
  shoulder and lumbar areas
• Score converted to 100

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Shoulder Force vs. Perceived Discomfort
370
B
350
C
330
Shoulder Force (N)
310
E
A
290
D
289 N
270
250
0
20
40
60
80
Perceived Discomfort ()
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Lumbar Force vs. Perceived Discomfort
B
240
C
220
200
A
D
180
Lumbar Force (N)
E
160
140
135 N
120
100
0
20
40
60
80
Perceived Discomfort ()
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Suggested Design Limits

• Maximum lumbar force of 135 N recommended to
  reduce likelihood of low back discomfort
• Maximum shoulder force of 145 N recommended to
  reduce likelihood of shoulder discomfort

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Standardized Testing

• Objective Methods
• Load carriage simulator
• Stiffness
• Suspension
• Human-based Methods
• FAST Trials

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Dynamic Load Carriage Simulator

• A computer-controlled pneumatic system, able
  to move with three degrees of freedom, displaces
  an instrumented anthropometric human torso
  through a vertical motion representative of human
  gait (normal walking, running, slips and ducks)

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Anthropometric Scaling

• Torso
• 4 anthropometric torsos
• (5th, 50th ile female,
• 50th, 95th ile male)
• fiberglass shell, internal structure,
• distributed body mass representative of humans
• Skin
• range of potential skin analogues evaluated for
  force/displacement and creep properties
• Bocklite selected for reproducibility and
  stability

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Body Forces

• Body fixed system measures hip reaction forces
• Forward lean is adjusted to balance hip moment
• Lean Angle is an input variable

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Force Distribution

• Custom Strap Force Transducers (link with 2
  strain gauges mounted to measure axial force)
• Used as a standardized input, to ensure all LCS
  are tested under similar conditions
• Strap forces set to realistic settings by pack
  users
• Accuracy /- 2
• Highly Linear, R2 lt 0.9995

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Relative Motion of Kit

• ability of load carriage system to track bodys
  motion relates to users perception of load
  control
• relative motion of payloads (e.g., pack frame,
  vest pocket contents) measured during dynamic
  testing
• using Fastrak displacement measurement system
• accuracy confirmed with Optotrak system
• RMS 0.66 mm (static)
• RMS 0.65 mm (dynamic

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Skin Pressure Measurement

• average and peak skin pressures measured in
  shoulder, waist and lumbar regions
• skin pressures can be related to discomfort and
  tissue tolerance levels
• measured using F-Scan pressure measurement
  system
• Standard Error (as of mean)
• 9.6 for average pressure
• 14 for peak pressure
• 9 due to sensor curvature

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Contact Pressures
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Range Of Motion Stiffness Tester

• Description
• 50th ile male torso
• Bocklite skin analogue
• thrust bearing allows trunk rotation
• Universal joint provides forwards and sideways
  leaning
• Outcome Measures
• Torsional Stiffness (N/deg)
• Forward Sideways Bending Stiffness (N/deg)

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Static Suspension System Tester

• Force Distribution
• Biomechanical Modeling
• Design Guidance

Load Cell (6 d.o.f.)
Force Plate (6 d.o.f.)
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Human Performance and Capacity

• Mobility
• Function
• Agility
• Discomfort
• March
• Overall

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Balance

• Bent Balance Beam
• Boulder Hop
• Straight Balance Beam

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Agility

• Fence Climb
• Side Slope

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Load Control

• Pylon Run
• Forward Ramp

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Static Tests

• Hands front/above
• Trunk motion
• Sitting
• Prone
• Doff

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Visual Inspection Battle Order Tasks
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March
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Description of FAST Trials Circuit
March 1 km
Start
Balance Fwd/Bwd
1 km
Balance to Sides
1 km
Over Obstacles
1 km
Under Obstacles
1 km
Side Inclines
1 km
End of Circuit
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Psychophysical Measures

• Discomfort
• 0 (No Discomfort) 9 (Extreme Discomfort)
• Acceptability for balance, agility and load
  control
• 1 (Unacceptable) 6 (Totally Acceptable)

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Anatomical Tissue Tolerance Limits

• Low Back Forces
• Compressive Force
• 3340 N
• Shear Force
• 1000 N
• Moment
• 140 Nm
• Shoulder Contact Forces
• 135 N / Shoulder
• Skin Contact Pressure
• Avg Pressure gt 20 kPa
• Peak Pressure gt 120 kPa (5 sec)

Anatomical Tissue Tolerance Limits

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Musculature SystemBACK SHOULDERS
Strap Force vs. Time
14
12
10
Force in
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Newtons
6
4
2
0
0
1
2
3
4
5
6
7
Time in Seconds
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Validation Study

• Human performance protocol on 28 subjects.
• Male
• Age 25.5 (2.5) years
• Service 5.4 (2.7) years
• Height 1.78 (.07) m
• Weight 82.1 (9.8) kg
• Incomplete block design provided mean value of
  12 assessments for each of four systems.

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Average Shoulder Pressure Limits

• 20 kPa 95 report discomfort
• 18 kPa 90 report discomfort
• 14 kPa Blood Occlusion (TLV)

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Application to Load Carriage System Evaluation

• Establish cutting scores for correlated objective
  measures based on lowest decile performance
• Compare to pack overall ranking for four
  representative systems (A,B,C,D)

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Correlation of Subjective and Objective Parameters
Number of Variables Correlating at plt .05
Specifications
Performance Criteria
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Human Factors Overall Rating FAST Trials
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LC Sim Relative Displacements
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Skin Contact PressuresLower Lumbar Region
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Skin Contact PressuresUpper Lumbar Region
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Skin Contact PressuresFront Top Shoulder
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Stiffness Correlations

• Torsional
• Hands above Head Mobility
• Hands in Front Mobility
• Acceptability in March
• Overall Balance, Comfort, Fit, and
  Maneuverability

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Component Arrangement

• Fragmentation vest
• Load carriage vest
• Battle order
• Marching order

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Accessibility Versatility

• Criticality of kit
• in case of attack (ex. munitions)
• in case of danger (ex. NBC gear)
• Frequency of use of kit
• periodically (ex. water)
• irregularly (ex. rain gear)
• infrequent (entrenchment tool, 3rd socks)

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Focus Groups

• Test
• importance of design features
• Outcomes
• understanding of design
• development of weighting for criteria

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Short Vests
Waist Length
Webbing
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Protected Spaces for Pack
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Effect of Fragmentation Vest


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Effect of LC Vest Design
2
Average Shoulder Pressure (kPa)
1
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Final Recommendations

• LCV3 with modifications
• Padded shoulder design
• Remove conflicts with
• C9 and FRAG collar
• Reduce pressure points
• by Kevlar change at waist

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Thank You