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`
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`Compilation Part Notice
`ADPO 11008
`TITLE: A Static Biomechanical Load Carriage Model
`
`DISTRIBUTION: Approved for public release, distribution unlimited
`
`This paper is part of the following report:
`TITLE: Soldier Mobility: Innovations in Load Carriage System Design and
`Evaluation [la Mobilite du combattant: innovations dans la conception et
`l'evaluation des gilets d'intervention]
`
`To order the complete compilation report, use: ADA394945
`
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`[he context of the overall compilation report and not as a stand-alone technical report.
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`UNCLASSIFIED
`
`Petitioner Ex. 1059 Page 1
`
`

`
`25-1
`
`A Static Biomechanical Load Carriage Model
`R.P. Pelot', A. Rigby2 , J.M. Stevenson2 and J.T. Bryant2
`'Department of Industrial Engineering,
`Dalhousie University, P.O. Box 1000,
`Halifax, Nova Scotia, CANADA, B3J 2X4
`
`ics Research Group & Clinical M echanics Group
`Queen's University
`Kingston, Ontario, CANADA, K7L 3N6
`
`SErgonom
`
`Summary
`
`A two-dimensional biomechanical model of a backpack has been developed which incorporates the primary
`forces at the shoulder and waistbelt contact points. The model had been validated using instrumented
`manikins in laboratory experiments. The computer-based formulation allows the user to specify parameters
`for certain pack features, such as pack mass and volume, and it predicts the resulting contact forces on the
`bearer. By treating some parameters as decision variables, such as the location of attachment of the shoulder
`straps to the pack, the model can be used as an optimization tool to achieve a specified objective, such as
`minimizing the total forces on the bearer. A base case analysis and some variants illustrate this type of
`analysis. For the example provided, it is not possible to find a feasible solution within the prescribed
`shoulder-to-waist load ratio. By freeing up other variables, several alternative solutions are presented. This
`model can be used to easily examine trade-offs in certain pack design decisions.
`
`Introduction
`
`Backpacks are common devices to increase human load carriage capabilities, but when heavily loaded can
`still place a great burden on the bearer. Many design improvements have been made over the past decades,
`but more research is still required to fully understand the implications of the associated static and dynamic
`forces. Parametric analysis of personal load carriage systems allows for increased understanding of
`relationships between system design characteristics and the impact of these design features on the bearer. A
`computer-based static biomechanical model of a backpack has been developed to represent the interaction
`between the pack and the bearer at the principal contact points.
`
`Optimization of the biomechanical model yields the best location for attaching the suspension system
`components. Various objectives can be considered, such as achieving the best load balance between the
`shoulders and waist, or minimizing the transverse shear at the lumbar level, which is often associated with
`discomfort and pain. In the current formulation, the objective is to minimize the sum of the three primary
`forces acting on the bearer by the pack: the normal force at the shoulders, the vertical force on the hips and
`the lateral shear on the back at the wasitbelt. A limited set of runs applied to a Base Case backpack
`illustrates the trade-offs inherent in design decisions.
`
`Literature Review
`
`The literature on personal load carriage is quite broad, and generally falls into one of three categories:
`physiological studies, biomechanical studies, and subjective appraisal studies. Most of the biomechanical
`studies concentrate on gait analysis (e.g. DeVita et al., 1991). As there are several comprehensive survey
`articles on various aspects of load carriage (e.g. Rorke, 1990; Haisman, 1988; Pelot et al., 1995), the
`following review focuses on some articles directly relevant to the model described in this paper.
`
`Paper presented at the RTO HFM Specialists' Meeting on "Soldier Mobility: Innovations in Load Carriage System
`Design and Evaluation ", held in Kingston, Canada, 27-29 June 2000, and published in RTO AIP-056.
`
`Petitioner Ex. 1059 Page 2
`
`

`
`25-2
`
`Almost all studies consider the effects of load carriage on the subject through experimentation, and the
`backpack is part of the pack/person system. Articles examining the isolated pack as a system (static or
`dynamic) are almost non-existent, however Bobet and Norman (1984) develop a free-body diagram of the
`trunk/pack system while examining the effects of load placement using EMG. Furthermore, few studies
`concern themselves with load carriage design details. Exceptions include Bloom and Woodhull-McNeal
`(1987) who compare internal and external frame packs, and other researchers who consider a double-pack
`system (e.g. Kinoshita, 1985; Johnson et al., 1995). Certain pack elements are evaluated in isolation, such as
`the shoulder model presented by Holewijn (1990). Field trials comparing pack features are commonly
`reported in relevant magazines (e.g. Jenkins, 1992).
`
`In order to establish limitations on contact forces, information is required on the effects of these pressures on
`the bearer. An article by Sanders et al. (1995) provides an overview of skin response to mechanical stress,
`while particular injuries arising from load carriage pressures are described in several articles (e.g. Bessen et
`al., 1987). Studies by Stevenson et al. (1996) have measured strap forces and pressures and correlated them
`with measures of human discomfort, thereby establishing threshold values on the force levels that may cause
`discomfort.
`
`The body lean angle under load carriage depends on several factors including pack mass, pack design, level
`of fatigue, and terrain. Results of such investigations include those by Bloorn et al. (1987) and Stevenson et
`al. (1996). Five to ten degrees is a typical range, but the user may specify this parameter in the model
`described in this paper.
`
`Since the goal of this biomechanical model is to choose values for certain variables that will optimize an
`objective, such as minimizing total contact forces, the reader may consult a text such as Winston (1996) to
`review optimization and formulation in general, linear programming
`in particular, and non-linear
`programming, as some optional constraints in the present model introduce non-linear relationships.
`
`Biomechanical Model
`
`A free body diagram of a rigid model of a typical rucksack is shown in Figure 1. The notation is defined at
`Table 1. The suspension system elements have been numbered from the top down for convenience. Thus
`the upper shoulder strap's location (d1), attachment angle (01) and tension (TI) are consistently subscripted.
`The subscript '2' refers to the lower shoulder strap portion, and '3'
`is reserved for certain waistbelt
`variables. The entire figure and its associated reference coordinates are angled at 3 degrees from the
`vertical to reflect the normal body lean that occurs under heavy loading conditions.
`
`When conducting a parametric analysis, many of the values in the diagram may be treated as variables, to
`determine the impact of changing them. For the evaluation of a specific pack under given loading
`conditions, all fixed parameters must be specified and the model is solved for the unknown forces T 1, T2 , Fz
`and Fx. To solve for these using the three force balance equations, note that a relationship exists between T,
`and T,. By modelling the shoulder as a pulley with friction, T, and T 2 are related by the friction coefficient
`and the wrap angle, as shown by equation (1) below (see MacNeil, 1996). The wrap angle a depends on
`several pack dimensions, notably the attachment points of the upper and lower shoulder straps, shoulder
`radius, and shoulder-pack distance, as shown in Figure 2 and equations (6) through (10).
`
`Petitioner Ex. 1059 Page 3
`
`

`
`Table 1. Notation for Static Biomechanical Model
`
`25-3
`
`Suspension System Element
`Orientation
`
`Pack Container
`
`Bearer
`
`Waistbelt
`
`Shoulder Straps
`
`Lumbar area
`
`Notation
`X
`Z
`W
`Vx, Vz
`hx, hz
`d 4
`d5
`r
`rH
`13
`Ti
`72
`T3
`d3
`T3c
`T3C
`T3cf
`FBz
`AB
`t
`hB
`T,
`T 2
`d,
`
`d2
`
`01
`0 2
`cx
`
`9s
`SN
`F,
`Fx N
`Fxf
`F z
`I9L
`F,
`
`Definition
`coordinate along pack depth (positive out)
`coordinate along pack height (positive up)
`the force of the mass of the pack
`position of Centre of Mass
`dimensions of pack container
`distance: waistbelt centre to shoulder centre
`distance: pack back to shoulder centre
`radius of shoulder
`radius of hips
`body lean angle
`anatomical lower back angle from vertical
`anatomical hip angle from vertical
`tension in waistbelt
`distance of waistbelt from bottom of pack
`compressive force that T3 applies around the hips
`component of T 3c normal to the hips
`force of friction due to T3c
`lift provided by waistbelt resting on hips
`coefficient of friction of waistbelt on hips
`thickness of waistbelt
`height of waistbelt
`tension in upper shoulder straps (LHS and RHS summed)
`tension in lower shoulder straps (LIIS and RIIS summed)
`distance: waistbelt centre to attachment point of upper
`shoulder stra
`distance from waistbelt centre to attachment point of
`lower shoulder strap
`upper shoulder strap angle from pack normal
`lower shoulder strap angle from pack normal
`angle subtended by contact of strap wrapped around
`shoulder
`coefficient of friction of strap on shoulder
`net force acting normal to the shoulder
`reaction force of lower back on pack in X-direction
`component of Fx normal to the lower back
`force of friction due to Fx
`lift on the pack from friction and angle at lower back
`coefficient of friction of lumbar pad on lower back
`total lift force at lumbar contact point of pack
`
`Petitioner Ex. 1059 Page 4
`
`

`
`25-4
`
`\ I
`
`I
`
`I
`\
`
`.
`
`-
`
`Figure 1. Rucksack free-body diagram with trunk lean
`
`Petitioner Ex. 1059 Page 5
`
`

`
`25-5
`
`Equilibrium equations:
`
`Pulley equation for shoulder wrap:
`
`T2
`Sum of the forces in the X-direction:
`Fx =WsinP+3{ePS -cosO1 +cos0 2}-T 2
`
`(2)
`
`Sum of the forces in the Z-direction:
`'S .sin0, +sin0 2}.T 2
`Fz =WcosfP-{e
`
`(3)
`
`Sum of the moments about the center of mass of the pack:
`(4)
`(Vz - d 2 - d 3)- cos192 - vX- sin 0 2 + e"- -((d1 + d 3 - vz)- cosQ0 - vx sin 01 )T
`
`(vZ - d 3 )- FX -vX FZ =0
`
`2
`
`Isolate T2 by substituting (2) and (3) into (4) and simplifying:
`=W- [vx cos P - (vz - d 3 ) . sin fi]
`T2
`(5)
`els" • d, • cos 0, + d 2 -cos 02
`
`-
`
`Shoulder Wrap angle:
`
`a =)T +01 -02
`
`(6)
`
`01 =tan-'jlid
`
`(7)
`
`+(d4 - d)
`-2d2 (d 4 -dI)-2drd2
`2(r 2 -d2)()
`
`e1=
`
`2 -r 2
`
`&2 :tanj1e2]J
`
`(9)
`
`5 2d-(±
`
`d 5
`2(r2 d 2 )
`
`(d 4
`
`2 ) 2
`
`(8dd
`1)
`
`Normal force on shoulders (sum of both sides):
`-
`S0 =t 2 -esa
`cos' 1 ±T2 cos0 2
`
`SN' = T2 "e"a -sin0 1 +±T2 -sin0 2
`Sv = T2 j(cosd4 2 -)+ e
`cos0) 2 + (sind 2 +
`
`sin0 1 )2
`
`(11)
`
`(12)
`
`(13)
`
`Waist Belt Force:
`
`Tension in belt vs. compressive force on hips, based on hoop stress: T = T3c 1(21r)
`Lift due to hip angle (i.e. cone effect) and friction: F = 2rT3 -cosy -(sinY2 +B
`List due to lumbarpad: FzJ =Fl -*cosy1 -(siny 1 +p, -cosy1 ) (16)
`(17)
`Total lift at waist: Fz = Fz" + Fz
`
`(14)
`" COSy 2 ) (15)
`
`Petitioner Ex. 1059 Page 6
`
`

`
`25-6
`
`' 14
`-
`
`I
`
`TI
`
`lean
`
`Figure 2. Shoulder wrap angle relations
`
`Petitioner Ex. 1059 Page 7
`
`

`
`25-7
`
`___
`
`PACK_
`
`_
`
`T3CTT
`
`T(cid:127)
`
`N
`
`Y2
`
`Nf
`
`Fx
`F(cid:127)
`
`N
`
`Fx
`
`Low B ck
`Side View
`
`Yi zt~
`
`X
`
`Figure 3. Waistbelt and lumbar pad models
`
`Petitioner Ex. 1059 Page 8
`
`

`
`25-8
`
`The pack static equilibrium equations for force in the X direction, force in the Z direction, and moments
`about the centre of gravity can be simplified to the forms given in equations (2) through (5). These
`expressions can be solved for all of the unknown forces illustrated in Figure 1. However, another quantity of
`interest is the resultant normal force on each shoulder, Sy (see equations 11 to 13). Finally, the forces at the
`waist include contributions fi-om the lumbar pad and waistbelt (Rigby, 1997) as shown in Figure 3 and
`equations 14 to 17. The key assumption is that the lumbar pad provides the maximum possible lift, with the
`waistbelt contributing the remaining support in the Z direction, if required to maintain static equilibrium of
`the pack.
`
`The validity of these equations was examined by measuring the forces on several different pack designs
`mounted on instrumented manikins (Rigby, 1997). Given the respective input parameters for each pack,
`using the mnodel to predict the unknown forces was quite good in almost all cases, falling within 10% of
`measured values. The exceptions only occurred in a couple of instances, where the forces were relatively
`low, and although the absolute error was small, the relative error exceeded this 10% threshold. This
`relatively simple rigid, two-dimensional model provides valid outputs for the packs and parameters tested.
`
`Optimization of biomechanical model
`
`The first issue is to determine the decision variables, or those variables that may be altered by the designer.
`To put this in context, there are three categories of values involved in the modelling process:
`
`"* parameters: externally determined values, which are input to the program, and not changed during the
`optimization;
`
`"* decision variables: values which can be changed during the optimization process to best achieve the
`specified objective;
`
`"* state variables: values that are calculated explicitly as functions of the parameters, decision variables
`and/or other state variables.
`
`There is some latitude in selecting decision variables, depending on the purpose of the modelling run. As an
`initial scenario, assume that only the "heights" of the suspension systems attachment points can be varied
`(i.e. dI, d2, and d3).
`
`The next step is to formulate the objective function. Various definitions can address the ultimate goal of
`improving comfort for the bearer. Since there is no unique characterization of the most comfortable load
`distribution, various alternatives can be considered, with a typical version presented below. Minimizing the
`normal force on the shoulder, S", is used as a surrogate for shoulder comfort. The transverse force on the
`lower spine has been significantly correlated with pain and discomfort (Stevenson et al., 1996), which can be
`mitigated by reducing Fx. Finally, excessive vertical forces at the waist should be avoided as a general rule
`by lowering Fz. To achieve this, one objective involves minimizing the weighted sum of these three forces,
`leaving it to the analyst's discretion to set the relative weights. This objective function is presented in the
`formulation below.
`
`The relationships established by the biomechanical model described in the preceding section act as
`constraints on the design process. That is, any variable that is altered may affect many other quantities, so
`that these equations limit the feasible ranges for parameter changes. These relevant constraints are listed in
`the formulation below.
`
`To complete the model, certain other bounds must be applied to ensure a reasonable result. Note that the
`biomechanical model formulation incorporates several implicit assumptions, some of which can be relaxed
`as model analyses progress. First of all, the moment equation was derived on the basis that the upper
`shoulder strap is attached above the centre of gravity, while the lower shoulder strap and the waistbelt lie
`below the C of G. Consequently, these dimensions (dI, d2 and d3) are restricted accordingly in the
`
`Petitioner Ex. 1059 Page 9
`
`

`
`25-9
`
`constraints in the formulation below, although future models can easily circumvent this issue. In any case,
`the upper shoulder strap must be attached no lower than the lower strap (i.e. dl -Ž d2). In practice, a finite
`buffer could be required between them. The lower shoulder strap may be affixed below the centre of the
`lumbar pad (i.e. effective force application point in Figure 1), but not below the bottom of the rucksack.
`Similarly, the upper shoulder strap attachment is limited by the height of the pack. Finally, modelling the
`shoulder as a pulley with friction assumed that the tension is higher in the upper part of the strap (i.e. T1 -
`T2). There is no explicit control over this in the model, as this assumption guarantees a solution with T,
`larger (if a solution exists). Computer runs may also be conducted where the converse assumption is made,
`to see if the former case is always valid.
`
`Finally, threshold limits for certain values may be recommended. Previous studies suggest an upper bound
`of 135 Newtons should be placed on Fx to remain within the comfort zone (Stevenson et al., 1996).
`Similarly, SN may be constrained to lie below 280 Newtons. Rules of thumb over many years of experience
`have also implied that the support for heavy loads be split such that the waist bear about twice the amount of
`weight than do the shoulders (Pelot, 1995). This guideline does not account for the angle of the resulting
`normal force on the shoulders, so as a first approximation it is applied simply to the ratio of SN over Fz. The
`degree to which this condition is satisfied can be controlled by requiring the ratio to lie within a prescribed
`range centered on (2/3) as shown in the constraints below. Continuous improvements in pack suspension
`system designs may render this prerequisite obsolete.
`
`Optimization formulation
`
`Objective function:
`
`minimize C 1 . SN + C2 . Fx + C 3 .Fz
`where: C', C2 , and C3 are user-specified coefficients
`
`Subject to these constraints:
`Equations 1, 2, 3, 5, 6, 7, 8, 9, 10, 15, 16 and 17 (from above)
`
`Additional constraints:
`S'v <280
`F, <135
`d2 >_ -d3
`d3 <-ýVZ
`
`SA, _2 !
`Fz
`3ý
`
`.
`
`Base Case analysis
`
`di -d 2 >0
`di + d3 >- Vz
`
`di +d3 <hz
`d2 + d3 <! VZ
`
`Representative data from a typical commercial pack are presented in Table 2. Aside from pack dimensions,
`anthropometric data and friction coefficients were established during laboratory experiments (Rigby, 1997).
`The mass of 30 kg (66 lbs) represents a reasonable load for a typical military mission, although computer
`runs can be conducted to evaluate the effects of much heavier weights sometimes borne by the soldier. By
`default, the C of G is assumed to be at the volumetric centre of the pack. Original data is input in specified
`units, then converted for use in the model. The decision variables are set to the current pack dimensions
`initially.
`
`Giving equal weight of 1.0 to each force coefficient C1, C 2 and C3 when minimizing the objective function
`yields the results shown in Table 3 for several variations on the Base Case. The optimization procedure does
`not find a feasible solution for the Base Case itself. In other words, for the given parameters, there is no
`choice of the three decision variables that satisfy all of the constraints. Further analysis indicates that the
`restriction being violated is the upper bound on the transverse force at the lumbar level. With the given
`configuration, it is not possible to keep Fx below 135 Newtons. Removing this constraint, and running the
`model again results in a feasible solution, listed as Run 2 in Table 3. The minimumn Fx attained is 155.3 N.
`
`Petitioner Ex. 1059 Page 10
`
`

`
`25-10
`
`To achieve this, the shoulder straps are attached to the pack as high as allowed (recall that the lower strap
`cannot rise above the Centre of Gravity), and the waistbelt as low as possible. Note that d 3 = 0 does not
`mean that the waistbelt is lowered relative to the body, since the waistbelt-to-shoulder distance d 4 is
`constant, but rather that the bag is raised so that the bottom is flush with the centre of the lumbar pad. The
`rminirnum objective value results from the sum of its three force constituents. Thus the model lowers SN, Fx
`and Fz as much as possible. The ratio of shoulder to waistbelt lift is within its prescribed tolerance of (2/3)±
`0.1, which means that this constraint is redundant for the conditions of this run. The ratio falls naturally near
`the desired value. It is clear that the attachment locations of the upper strap and waistbelt in this scenario are
`too close to the pack edges to be practical, but the purpose of these evaluations is to understand the
`fundamental design trade-offs. In a more realistic analysis, allowable ranges on the attachment region for
`each strap can be included in the model.
`
`Table 2. Base Case Data
`
`Biomechanical Load Carriage Model: Base Case
`
`Description
`mass of pack + load
`depth of pack
`height of pack
`CofG from back
`CofG from bottom
`shoulder strap top position from WB
`shoulder strap bottom position from WB
`waistbelt position from pack bottom
`waistbelt to shoulder centre
`pack back to shoulder centre
`shoulder radius
`body lean angle
`low back angle
`hips angle
`
`shoulder friction coefficient
`low back friction coefficient
`waistbelt friction coefficient
`
`ORIGINAL
`Data Units
`30.000 kg
`34.000 cm
`42.000 cm
`17.000 cm
`21.000 cm
`43.333 cm
`2.000 cm
`6.667 cm
`43.000 cm
`14.300 cm
`7.000 cm
`10.000 deg
`7.000 deg
`10.000 deg
`
`0.35 ---
`0.35 ---
`0.35 ---
`
`Notation
`W
`h,
`hz
`Vx
`Vz
`d,
`d2
`d3
`d4
`d5
`r
`0
`Y,
`
`'(
`
`Rs
`AL
`
`9B
`
`CONVERTED
`Data Units
`294.3 Newtons
`0.3400 m
`0.4200 m
`0.1700 m
`0.2100 m
`0.4333 m
`0.0200 m
`0.0667 m
`0.4300 m
`0.1430 m
`0.0700 m
`0.1745 rads
`0.1222 rads
`0.1745 rads
`
`0.35 ---
`0.35 ---
`0.35 ---
`
`Table 3. Optimization results for Base Case (BC) and some variations
`
`Run Conditions
`
`d,
`(cm)
`
`d 2
`(cm)
`
`d3
`(cm)
`
`Vx
`(cm)
`
`Vz
`(cm)
`
`SN
`(N)
`
`Fx
`(N)
`
`Fz
`(N)
`
`Obj
`(N)
`
`SN/Fz
`
`1
`
`2
`
`3
`
`4
`
`Base Case (BC)
`
`infeasible
`
`BC (no limit onFx) 42.0
`
`21.0
`
`BC with CofG free
`
`32.2
`
`-4.8
`
`0.0
`
`4.8
`
`17
`
`10
`
`21
`
`37
`
`127.8
`
`155.3 215.8 498.9 0.592
`
`109.6
`
`103.6
`
`193.5 406.7
`
`0.566
`
`BC with CofG free
`& no limit on SN/Fz
`
`38.5
`
`37.0
`
`0.0
`
`10
`
`37
`
`32.7
`
`77.5
`
`270.6
`
`380.8 0.121
`
`Petitioner Ex. 1059 Page 11
`
`

`
`25-11
`
`It is interesting to examine the impact of allowing the Centre of Gravity to move. Reasonable bounds are
`imposed by restricting the distance of the C of G from the back to vary between 10<Vx<30 cm., and the
`position from the bottom of the bag to lie between 10<Vz<37 cm. The output is shown as Run 3 in Table 3.
`To minimize the forces, the load C of G falls as close to the back and as high as possible. This is consistent
`with empirical observations in field studies (Hinrichs et al, 1982). The objective value is lower than in the
`previous run, since allowing the C of G to move corresponds to more degrees of freedom. Notably, each of
`the three target forces has a reduced magnitude. The lower shoulder strap is attached below the waistbelt,
`hence the negative distance. The fact that d 2 is equal in value and opposite in sign to d 3 indicates that the
`shoulder strap is secured right at the bottorn of the pack. Both the lumbar transverse force and the shoulder
`normal force are within the recommended threshold values. The shoulder/waist split constraint is binding at
`the optimum, which means that the 2:1 ratio is approximately maintained only because of the explicit
`condition included in the formulation.
`
`Relaxing this last requirement results in the output labeled Run 4 in Table 3. The suspension system
`attachment points have changed, dramatically in the case of the lower shoulder strap. The effect of raising
`the shoulder strap attachment points is to remove much of the vertical load from the shoulder, which is then
`transferred to the hips, resulting in a higher Fz, and a markedly reduced shoulder-to-waist force split. The
`transverse lumbar force is significantly reduced and the overall objective function is much lower. Thus
`artificially promoting a "desirable" shoulder-to-waist load ratio may result in significantly higher forces
`being exerted on the bearer.
`
`Summary
`
`These optimization results provide an overview of the types of issues that may be explored through this
`biomechanical model. A particular pack may be represented using the appropriate parameters, and the
`model can predict the changes associated with specific design changes. Alternatively, monographs may be
`produced showing the optimal solution for a wide range of combinations of the decision variables. Such a
`comprehensive set of tests would provide as complete a picture as possible of the interactions inherent in the
`biomechanical model, which ultimately can enhance the design process. Different objective functions can
`be introduced, since there is no single answer to the question of what is the "best" combination of forces for
`the bearer. Finally, the model can be used to perform sensitivity analyses on one or more input parameters.
`
`Acknowledgement
`
`The work described in this paper was funded by the Department of National Defence, Defence R&D
`Canada, and was performed for the Defence and Civil Institute of Environmental Medicine (DCIEM) by
`Queen's University, under a number of different PWGSC contracts.
`
`References
`
`Bessen, J.B., Belcher,V.W. and Franklin,R.J., "Rucksack Paralysis With and Without Rucksack Frames",
`Military Medicine, 152(7), pp.372-375.
`
`Bloom, D. and Woodhull-McNeal,P. (1987) "Postural adjustments while standing with two types of loaded
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`
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`Petitioner Ex. 1059 Page 13