service members are often required to carry heavy loads for prolonged periods during both training and combat ( 1 , 21 ) .
loads may include the modular lightweight load - carrying equipment system ( molle ) , army combat helmet , rucksack and equipment , weapon , and body armor such as the improved outer tactical vest ( iotv ) .
the maximum recommended weight of these loads varies depending on whether the load is carried for fighting ( 21.77 kg ) or approach marching ( 32.66 kg ) ( 1 ) .
although the rucksacks are critical to mission success , the rucksack load can impede stability ( e.g. balancing ) ( 22 ) and movement ( e.g. marching or running ) ( 16 ) , making it more difficult to balance and stop or initiate movement .
this may necessitate greater torques at hip and trunk to control motion which has been shown to result in alterations to postural control ( 16 , 22 ) , requiring greater effort to control the load and maintain stability ( 29 ) .
the maintenance of stability may be further challenged in environments of low visibility while wearing a load carriage system .
service members are often required to carry rucksacks in conditions of low visibility ( e.g. night missions or low visibility due to dust and sand ) .
the collective impact of visual condition and load carriage ( specifically rucksack usage ) on service member s stability has received minimal attention . the ability to maintain stability while carrying a load within environments that obscure visual feedback may provide further insight into the prevalence of musculoskeletal injury to the spine ( 15 , 28 , 35 ) .
load induced alterations in postural control are often accompanied by compensatory kinematic changes at the trunk ( 2 , 7 , 11 ) and sagittal plane pelvic angle ( 2 , 12 , 30 ) .
compensatory strategies may include adopting a more forward leaning trunk posture ( 4 , 7 , 11 ) and anteriorly tilted pelvis ( 6 , 30 ) in order to help stabilize the body s center of mass ( 4 ) .
this adaptation may increase the torque on the lower back ( 21 ) and place greater stress on the low back muscles and vertebral disks ( 4 ) .
improved understanding of the biomechanical changes associated with increased load carriage may provide information needed to develop prevention protocols to help minimize injury , lost training / work days and medical costs due to injury in populations involved in regular load carriage .
past work available in this area has focused on experienced service members ( 4 ) . however , musculoskeletal injuries to those with less load carriage experience are very common ( 18 , 19 , 26 , 27 ) , and biomechanical accommodations may differ in those with less experience carrying loads as compared to experienced service members .
musculoskeletal injuries to those new to load carriage are concentrated in the lower extremities and back ( 19 ) .
one review ( 19 ) found that in male army and marine recruits , lower extremity and back injuries accounted for 24.7% to 76.7% .
many attribute these injuries to compensatory strategies elicited by the body in response to carrying loads ( 4 , 6 , 32 ) .
information identifying the biomechanical adaptations in response to load carriage may assist in the development of new standards and procedures for less experienced service members .
thus , the purpose of this study was to examine the effect of three different loads on postural sway , forward trunk lean , and pelvic girdle motion in army reserve officers training corps ( rotc ) cadets under two visual conditions .
we hypothesized that increasing the load in less - experienced military personnel would significantly increase postural sway .
second , we hypothesized that increasing the load would produce significant increases in kinematic adaptations at the trunk and pelvic girdle .
finally , we hypothesized that increases in load would increase postural sway and kinematics significantly more under an eyes closed condition than an eyes open condition .
twenty male army rotc cadets ( age 21  1.1 yrs . ; mass 78.8  9.1 kg ; height 184.2  6.1 cm ) participated in this study .
exclusion criteria included any prior military service , surgery within the past year , acute injury , or any condition that may prevent successful completion of the tasks .
all participants read and signed a consent form that was approved by the university institutional review board . the basic balance master ( neurocom international , inc . , clackamas , or , usa )
the basic balance master ( neurocom international , inc . , clackamas , or , usa ) measures postural control using a portable force plate connected to a computer . for the neurocom protocols ( neurocom international , inc . , clackamas , or , usa ) used in the current project ( described in the procedures )
, the system uses center of gravity ( cog ) sway velocity ( degreessecond ) as the measure of postural control .
the system estimates the position and displacement of the cog , using a simple inverted pendulum approximation from the participant s height and sampled center of pressure data ( 14 ) .
the system calculates the cog sway velocity ( i.e. postural sway velocity ) by taking the ratio of the distance travelled by the cog ( in degrees ) to the time of trial ( 9 , 14 ) .
lower values indicate greater control ( i.e. better balance ) ( 9 , 14 ) .
the basic balance master ( neurocom international , inc . , clackamas , or , usa ) is a common device used by healthcare practitioners to evaluate postural control in a clinical setting ( 34 ) .
the device has been shown reliable for use with various populations ( 10 , 14 ) including healthy adults ( 31 ) .
two cameras [ canon 3ccd digital video camcorder gl2 ntsc ( canon , inc . ,
japan ) ] were positioned in the frontal plane to record the motion of the retro - reflective markers .
additionally , two cameras [ one additional canon and one jvc mini dv ( victor company of japan , limited , malaysia ) ] were positioned in the sagittal plane . in each plane
( i.e. frontal or sagittal ) , one camera recorded the entire body , while the other recorded the motion at the pelvic region .
all cameras recorded at a frame rate of 60 fps with a video resolution of 720 p. reflective markers were placed at the acromion process of the scapula , greater trochanter and lateral femoral condyle of the femur , lateral malleolus of the tibia , and the base of the 5th metatarsal .
a hip belt apparatus was constructed and used to attach reflective markers that indicated the position of the right and left anterior superior iliac spines ( asis ) .
the belt was an adjustable aluminum apparatus that allowed for easier detection and measurement of asis movement ( figure 1 ) .
each participant reported to the sport biomechanics laboratory for one testing day , lasting approximately 90 minutes .
participants reported to the testing session in shorts and shirt , and army - issued boots .
all participants completed a preliminary medical questionnaire , questionnaire on military and load carriage experience , and read and signed a consent form approved by the university s institutional review board .
. leg dominance was determined by gently pushing the participant from behind and recording the foot used to step forward ( 17 ) .
the participant was then fitted with the gear ( per the army field manual 2118 ) ( 1 ) to be worn during testing of the 16.0 kg or 20.5 kg loads .
gear included the modular lightweight load - carrying equipment system ( molle ) , combat helmet , and a mock rifle .
the participant held the mock rifle above the waist , barrel pointing to the ground at about a 45-degree angle .
the rucksack weighed 16.0 kg or 20.5 kg , in accordance with the weights carried during the final ruck marches in basic combat training and one station unit training of entry - level army soldiers .
synchronization of the kinematic and postural sway velocity data was not possible due to the limitations of the technology .
we collected postural sway velocity data on the basic balance master ( neurocom international , inc .
, clackamas , or , usa ) using two neurocom testing protocols : modified clinical testing of sensory interaction and the unilateral stance tests protocols ( neurocom international , inc
the participants performed the modified clinical testing of sensory interaction protocol with and without the eyes open for each of the load conditions ( unloaded , 16.0 kg load , and 20.5 kg load ) using a double leg stance on a firm surface .
the participants completed both an eyes open and eyes closed visual condition for each load before progressing to the next load condition . for each load , the eyes open and eyes closed conditions were separated by a 45-second rest period .
each of the visual conditions lasted for a 60-second period . between the 16.0 kg load and 20.5 kg load ,
the participants were given 2 minutes of rest between load conditions ( figure 2a ) .
for the modified clinical testing of sensory interaction protocol , the participant was placed onto the force platform in a forward facing position , with the participant s feet centered on the force plate . to center the participant s feet on the force plate ,
the medial malleolus of each foot was centered directly over the horizontal line on the force plate , while the lateral aspect of the calcaneus of each foot was position to the short ( s ) , medium ( m ) or tall ( t ) line indicated on the force platform .
the position of the lateral calcaneus was based on participant height : s ( 76 cm to 140 cm ) , m ( 141 cm to 165 cm ) and t ( 166 cm to 2013 cm ) .
participants were instructed to fix their gaze on a mark on the wall positioned at eye level during all eyes open balance tests to unify visual input and minimize the effect on balance ( 8 , 13 ) . the unilateral stance protocol required participants to complete two balance tests ( one with the eyes open and one with the eyes closed ) per load condition ( unloaded , 16.0 kg load and 20.5 kg load ) using a single leg stance on a firm surface for a total of six tests for the protocol .
each balance test was 60 seconds in duration , with a 45-second rest between balance test and a 2-minute rest between each load condition .
all gear was removed between the 16 kg and 20.5 kg load conditions ( figure 2b ) .
the unilateral stance protocol required participants to stand on the dominant limb with the non - weight bearing leg bent at the knee between 45 and 90 degrees of flexion .
a trial was considered a failure if the participant completely stepped off the platform or their foot moved from the appropriate spot on the platform .
the literature indicates that for ages 1836 postural sway velocity values for the eyes open and eyes closed conditions range from 0.70.2s to 1.161.25s ( 3 , 23 ) and 3.962.52s to 6.423.81s ( 3 ) , respectively , depending on the tested limb ( right or left ) and foot posture ( normal , prone or supine ) for the unilateral stance test .
dartfish live ( dartfish , switzerland ) was used to determine trunk and pelvic angles under each load ( 0 kg , 16.0 kg and 20.5 kg ) at the start and completion of each 60-second stance period .
previous research has found that dartfish is a valid reliable 2-d analysis software for measuring joint kinematics ( 20 , 24 ) .
forward trunk lean angles were measured in the sagittal plane and calculated by measuring the angle formed between two lines : the vertical reference line and a line running between the greater trochanter and acromion process ( figure 1 ) .
the marker representing the greater trochanter was positioned directly on the vertical reference line prior to calculating the angle .
the change in angle was determined by finding the difference between the starting and ending angles for each test at the start and finish of the 60-second period .
the asis ( indicated by the marker on the hip belt apparatus ) of the dominant limb was positioned directly on a horizontal referenced line in the sagittal ( anterior - posterior ) or frontal plane ( medial - lateral ) to calculate the start and end angles for each test for both the anterior - posterior ( figure 1 ) and medial - lateral pelvic angles ( figure 3 ) .
the change in angle for the anterior - posterior and medial - lateral pelvic kinematics was determined by finding the difference between the starting and ending angles for each test .
four , 3  2  2 ( load  stance  eyes ) repeated measures anovas with post - hoc comparisons using boneferroni s adjustments were completed to analyze the postural control data and the three kinematic data sets ( forward trunk lean , anterior - posterior pelvic tilt , and medial - lateral pelvic tilt ) .
twenty male army rotc cadets ( age 21  1.1 yrs . ; mass 78.8  9.1 kg ; height 184.2  6.1 cm ) participated in this study .
exclusion criteria included any prior military service , surgery within the past year , acute injury , or any condition that may prevent successful completion of the tasks .
all participants read and signed a consent form that was approved by the university institutional review board .
the basic balance master ( neurocom international , inc . , clackamas , or , usa ) was used to evaluate postural control . the basic balance master ( neurocom international , inc . , clackamas , or , usa ) measures postural control using a portable force plate connected to a computer . for the neurocom protocols ( neurocom international , inc . , clackamas , or , usa ) used in the current project ( described in the procedures )
, the system uses center of gravity ( cog ) sway velocity ( degreessecond ) as the measure of postural control .
the system estimates the position and displacement of the cog , using a simple inverted pendulum approximation from the participant s height and sampled center of pressure data ( 14 ) .
the system calculates the cog sway velocity ( i.e. postural sway velocity ) by taking the ratio of the distance travelled by the cog ( in degrees ) to the time of trial ( 9 , 14 ) .
lower values indicate greater control ( i.e. better balance ) ( 9 , 14 ) .
the basic balance master ( neurocom international , inc . , clackamas , or , usa ) is a common device used by healthcare practitioners to evaluate postural control in a clinical setting ( 34 ) .
the device has been shown reliable for use with various populations ( 10 , 14 ) including healthy adults ( 31 ) .
two cameras [ canon 3ccd digital video camcorder gl2 ntsc ( canon , inc . ,
japan ) ] were positioned in the frontal plane to record the motion of the retro - reflective markers .
additionally , two cameras [ one additional canon and one jvc mini dv ( victor company of japan , limited , malaysia ) ] were positioned in the sagittal plane . in each plane
( i.e. frontal or sagittal ) , one camera recorded the entire body , while the other recorded the motion at the pelvic region .
all cameras recorded at a frame rate of 60 fps with a video resolution of 720 p. reflective markers were placed at the acromion process of the scapula , greater trochanter and lateral femoral condyle of the femur , lateral malleolus of the tibia , and the base of the 5th metatarsal . a hip belt apparatus was constructed and used to attach reflective markers that indicated the position of the right and left anterior superior iliac spines ( asis ) .
the belt was an adjustable aluminum apparatus that allowed for easier detection and measurement of asis movement ( figure 1 ) .
each participant reported to the sport biomechanics laboratory for one testing day , lasting approximately 90 minutes .
participants reported to the testing session in shorts and shirt , and army - issued boots .
all participants completed a preliminary medical questionnaire , questionnaire on military and load carriage experience , and read and signed a consent form approved by the university s institutional review board .
height , weight and leg dominance were measured and recorded . leg dominance was determined by gently pushing the participant from behind and recording the foot used to step forward ( 17 ) .
the participant was then fitted with the gear ( per the army field manual 2118 ) ( 1 ) to be worn during testing of the 16.0 kg or 20.5 kg loads .
gear included the modular lightweight load - carrying equipment system ( molle ) , combat helmet , and a mock rifle .
the participant held the mock rifle above the waist , barrel pointing to the ground at about a 45-degree angle .
the rucksack weighed 16.0 kg or 20.5 kg , in accordance with the weights carried during the final ruck marches in basic combat training and one station unit training of entry - level army soldiers .
synchronization of the kinematic and postural sway velocity data was not possible due to the limitations of the technology .
we collected postural sway velocity data on the basic balance master ( neurocom international , inc . ,
clackamas , or , usa ) using two neurocom testing protocols : modified clinical testing of sensory interaction and the unilateral stance tests protocols ( neurocom international , inc . , clackamas , or , usa ) using balance master ) .
the participants performed the modified clinical testing of sensory interaction protocol with and without the eyes open for each of the load conditions ( unloaded , 16.0 kg load , and 20.5 kg load ) using a double leg stance on a firm surface .
the participants completed both an eyes open and eyes closed visual condition for each load before progressing to the next load condition . for each load , the eyes open and eyes closed conditions were separated by a 45-second rest period . each of the visual conditions lasted for a 60-second period . between the 16.0 kg load and 20.5 kg load ,
the participants were given 2 minutes of rest between load conditions ( figure 2a ) .
for the modified clinical testing of sensory interaction protocol , the participant was placed onto the force platform in a forward facing position , with the participant s feet centered on the force plate . to center the participant s feet on the force plate ,
the medial malleolus of each foot was centered directly over the horizontal line on the force plate , while the lateral aspect of the calcaneus of each foot was position to the short ( s ) , medium ( m ) or tall ( t ) line indicated on the force platform .
the position of the lateral calcaneus was based on participant height : s ( 76 cm to 140 cm ) , m ( 141 cm to 165 cm ) and t ( 166 cm to 2013 cm ) .
participants were instructed to fix their gaze on a mark on the wall positioned at eye level during all eyes open balance tests to unify visual input and minimize the effect on balance ( 8 , 13 ) .
the unilateral stance protocol required participants to complete two balance tests ( one with the eyes open and one with the eyes closed ) per load condition ( unloaded , 16.0 kg load and 20.5 kg load ) using a single leg stance on a firm surface for a total of six tests for the protocol .
each balance test was 60 seconds in duration , with a 45-second rest between balance test and a 2-minute rest between each load condition .
all gear was removed between the 16 kg and 20.5 kg load conditions ( figure 2b ) .
the unilateral stance protocol required participants to stand on the dominant limb with the non - weight bearing leg bent at the knee between 45 and 90 degrees of flexion .
a trial was considered a failure if the participant completely stepped off the platform or their foot moved from the appropriate spot on the platform .
the literature indicates that for ages 1836 postural sway velocity values for the eyes open and eyes closed conditions range from 0.70.2s to 1.161.25s ( 3 , 23 ) and 3.962.52s to 6.423.81s ( 3 ) , respectively , depending on the tested limb ( right or left ) and foot posture ( normal , prone or supine ) for the unilateral stance test .
dartfish live ( dartfish , switzerland ) was used to determine trunk and pelvic angles under each load ( 0 kg , 16.0 kg and 20.5 kg ) at the start and completion of each 60-second stance period .
previous research has found that dartfish is a valid reliable 2-d analysis software for measuring joint kinematics ( 20 , 24 ) .
forward trunk lean angles were measured in the sagittal plane and calculated by measuring the angle formed between two lines : the vertical reference line and a line running between the greater trochanter and acromion process ( figure 1 ) .
the marker representing the greater trochanter was positioned directly on the vertical reference line prior to calculating the angle .
the change in angle was determined by finding the difference between the starting and ending angles for each test at the start and finish of the 60-second period .
the asis ( indicated by the marker on the hip belt apparatus ) of the dominant limb was positioned directly on a horizontal referenced line in the sagittal ( anterior - posterior ) or frontal plane ( medial - lateral ) to calculate the start and end angles for each test for both the anterior - posterior ( figure 1 ) and medial - lateral pelvic angles ( figure 3 ) .
the change in angle for the anterior - posterior and medial - lateral pelvic kinematics was determined by finding the difference between the starting and ending angles for each test .
data were analyzed using statistical software ( spss 19 , ibm corp . , chicago , il , usa ) .
four , 3  2  2 ( load  stance  eyes ) repeated measures anovas with post - hoc comparisons using boneferroni s adjustments were completed to analyze the postural control data and the three kinematic data sets ( forward trunk lean , anterior - posterior pelvic tilt , and medial - lateral pelvic tilt ) .
we identified no significant three - way interaction ( load  visual conditions  stance ) effects for sway velocity , forward trunk lean , or in the anterior - posterior pelvic motion and medial - lateral pelvic motion .
a significant two - way interaction was observed for stance  eyes and load  eyes for mean sway velocity .
post - hoc analysis revealed significant main effects on postural sway velocity . in a double leg stance ,
mean postural sway velocity significantly increased between the 0 kg and 16 kg ( p < 0.001 ) , 0 kg and 20.5 kg
( p < 0.001 ) , and 16 kg and 20.5 ( p = 0.026 ) loads . in a single leg stance ,
mean postural sway velocity significantly increased between the 0 kg and 16 kg ( p < 0.001 ) and 0 kg and 20.5 kg
there was not a significant difference ( p = 0.648 ) in mean postural sway velocity between the 16 kg and 20.5 kg loads while in a single leg stance .
table 1 summarizes the effect stance and vision had on mean postural sway velocity under the three load conditions .
table 2 summarizes the effects of load on forward trunk lean , anterior - posterior pelvic girdle rotation and medial - lateral pelvic girdle rotation .
forward trunk lean , anterior - posterior pelvic girdle rotation , or medial - lateral pelvic girdle rotation were not significantly altered by load .
significant two - way interactions were observed for stance  eyes for forward trunk lean , anterior - posterior pelvic girdle rotation or medial - lateral pelvic girdle rotation .
post - hoc analyses identified significant main effects of vision ( eyes open vs. eyes closed ) and stance ( double leg vs single leg ) on forward trunk lean , anterior - posterior pelvic girdle rotation and medial - lateral pelvic girdle rotation .
the purpose of this study was to examine the effect of three different loads on postural sway , forward trunk lean , and pelvic girdle motion in army reserve officers training corps ( rotc ) cadets under two visual conditions .
the primary finding was that external posterior loads of 16.0 kg and 20.5 kg produced changes in mean postural sway velocity over a 60-second stance period in army rotc cadets .
the same external loads did not result in significant kinematic adaptations over the 60-second stance period . we hypothesized that increasing the load would significantly increase postural sway . in the present study ,
mean postural sway velocity during a double leg stance increased from 0.27s to 0.34s ( 16.0 kg load ) and 0.41s ( 20.5 kg ) under the eyes open condition and from 0.38s to 0.49s ( 16.0 kg load ) and 0.52s ( 20.5 kg ) with the eyes closed .
mean postural sway velocity during single leg stand with eyes open increased from 1.03s to 1.19s ( 16.0 kg load ) and 1.27s ( 20.5 kg ) ; and from 2.20s to 2.69s ( 16.0 kg load ) and 2.83s ( 20.5 kg ) during the eyes closed condition .
the army field manual 2118 ( 1 ) indicates that after proper training experienced soldiers should be expected to carry loads up to 45% of their body weight on a ruck march .
the 16 kg and 20.5 kg loads used in the present study would have represented 20% and 26% of our participants average mean body mass which is more representative of training weights .
future research using a load at the 45% of body weight may produce greater changes .
previous studies on load carriage have used other measures of postural control , making a direct comparison of our results difficult .
however , we did find that investigators using other metrics to quantify postural control ( center of pressure ) reported that load significantly influence balance ( 16 , 29 ) .
posterior center of pressure excursion and medial  lateral excursion increased as much as 54% and 131% , respectively , in college aged females when load conditions went from unloaded to a 18.1 kg load . in the second study
( 29 ) investigators observed that center of pressure excursion using load conditions of 6 kg , 16 kg and 40 kg with enlisted soldiers .
the investigators ( 29 ) observed that , compared with the 6-kg load , the 16- and 40-kg loads increased anterior  posterior center of pressure excursion by 21% and 42% , respectively .
the group ( 29 ) also reported increases of 13% and 56% for the 16- and 40-kg loads , respectively , when compared to 6 kg for medial 
, the researchers had the participants stand in a double leg stance with their eyes open . in the present study , we observed that in a double leg stance with the eyes open mean postural sway velocity also increased significantly .
together with previous research , it appears that external loads greater than or equal to 16 kg result in substantial alterations to postural control .
the present study also examined kinematic changes ( i.e. adaptations across the 60-second stance period ) at the trunk and pelvic girdle due to external load carriage .
we hypothesized that , as the external load increased there would be significant kinematic adaptations at the trunk and pelvic girdle .
however , our results revealed that increasing the external loads did not result in significant kinematic changes over the 60-second stance duration any of the 12 balance tests for forward trunk lean , anterior - posterior pelvic motion , or medial - lateral pelvic motion .
this is perhaps due to the sensitivity of our analysis software . in the current study
however , it may lack the sensitivity to measure smaller angular changes , such as those observed at the trunk and pelvic girdle in the present study . the use of a more sensitive instrumentation such as 3-d motion capture would help provide a clearer insight into the influence of load on trunk and pelvic kinematics during a quiet stance . investigators ( 4 ) using 3-d motion capture reported observing male soldiers ( with an average body mass of 74.9 kg ) soldiers leaned forward at the trunk significantly while walking with loads of 39.95 kg and 50.05 kg compared to loaded conditions under 15.96 kg .
investigators ( 30 ) have also observed that anterior pelvic motion was significantly altered with loads as low as 15% of body mass in college - age females while walking .
future studies should be performed using 3-d motion capture to validate our results in regards to the influence of incremental loading on trunk and pelvic motion during single and double leg standing .
ruck marches are often completed in low visibility conditions ; thus , in the present study cadets performed tasks with the eyes open and closed .
single legged stance was included to better replicate a complete gait cycle ( compared to double legged standing ) . in the single leg stance , significant alteration in forward trunk lean and anterior - posterior pelvic motion with the eyes closed were observed .
a better understanding of the interplay of load and visual conditions will help in the development of effective training interventions .
reflective markers were applied to the skin and clothing above bony processes near a joint .
movement of the skin , clothing or of the hip belt apparatus could have produced errors ( 5 , 25 ) .
however , with the exception of the marker placement on the greater trochanter , this method has been validated in other studies ( 33 ) . dartfish software has limitations when measuring joint angle changes that are 6 which could have impacted our kinematic findings .
we did not assess balance touchdowns in the data analysis , potentially leading to deflated results of the influence of load on postural sway .
in addition , participants in this study had some experience with load carriage as opposed to initial - entry trainees who would have no load carriage experience .
this study suggests that increased load using military load carriage systems influences postural sway in male army rotc cadets with little load carriage experience .
a 16.0 kg to 20.5 kg load increased all postural sway measures in double leg and single leg stances . limiting the participants ability to utilize visual feedback
 musculoskeletal injuries are a common occurrence in military service members . 
 it is believed that the load carried by the service member impedes stability and alters back and pelvis kinematics , increasing their susceptibility to musculoskeletal injuries , specifically in the lower extremities . 
 the purpose of this study was to examine the effects of two different loads on postural sway , forward trunk lean , and pelvic girdle motion in united states army cadets . 
 twenty male army reserve officers training corps cadets participated in this study . 
 each participant performed the modified clinical testing of sensory interaction ( mctsib ) protocol and the unilateral stance ( uls ) protocol under three different rucksack load conditions ( unloaded , 16.0 kg , and 20.5 kg loads ) . 
 mean postural sway velocity was recorded along with 2-d kinematics of the trunk in the sagittal plane and the pelvis in the frontal and sagittal planes . 
 external loads of 16.0 kg ( p < 0.001 ) and 20.5 kg ( p  0.003 ) significantly increased mean sway velocity by 16% to 52% depending on stance and visual condition , but did not produce significant changes in trunk and pelvic kinematics . 
INTRODUCTION
METHODS
Participants
Protocol
Statistical Analysis
RESULTS
DISCUSSION