the elderly , defined as individuals aged 65  years and older , are rapidly increased due to 
 the advances in medical facilities and the improved diet of modern society
. falls by the 
 elderly frequently occur and often result in injuries requiring hospitalization .
it has been 
 estimated that one third of the elderly population experiences at least one fall per year in 
 the united states , and 50% and 65% of the elderly population have been reported to 
 experience falls and injury in australia and south korea , respectively1 , 2 .
injuries from 
 falls by the elderly are a cause of rapidly increasing medical expenses .
in united states , 
 the total direct cost of fall - related injuries for the elderly reached $ 20 billion , and it 
 is expected to climb to $ 44 billion by the year 20203 .
falls by the elderly are more serious because they happen not only during some special 
 movement , but also during activities of daily living , due to age - related declines in balance 
 control . even though the injury resulting from a fall
may be small , the elderly may suffer 
 more seriously than young people , and sometimes falls by the elderly may result in severe 
 injury or even death , depending upon their physical status4 , 5 .
moreover , if an elderly 
 person falls and experiences difficulty in walking , he or she may easily tend to lose 
 confidence , become lethargic , face a deteriorated living quality , and disengage from the 
 society6 .
therefore , the fall injuries 
 of the elderly should be taken seriously and receive an immediate social response .
the fall risk factors of the elderly include gender , race , past fall experience , illness 
 history , sensory organ damage , weak leg muscles and poor sense of balance7 . among the risk factors , weakening leg 
 muscles and sense of balance
have received a lot of research attention since it is thought 
 possible to improve them through regular exercise8,9,10,11 . to prevent falls by the 
 elderly , diverse training methods
the most frequent 
 ones are exercises on the ground such as aerobic and leg muscle strength workouts .
many 
 researchers have reported that such regular ground exercises help to improve elderly 
 people s leg muscular strength , bodily function and postural stability , upgrading their 
 walking safety and preventing falls9 , 11,12,13 .
although these ground exercises have plenty of strong points , aerobics or muscular workouts 
 may cause excessive stress or pain to physically feeble elderly , and such concerns have been 
 reported to reduce the amount of exercise by the elderly14 .
unlike ground exercise , aquatic exercise lowers joint stress , thanks to underwater buoyancy 
 that lightens body weight , while offering higher density to cut the risk of fall injuries . 

underwater exercise also causes body resistance , helps isokinetic muscle contraction and 
 balanced development between agonist and antagonist muscles15 .
therefore , compared to ground exercise , aquatic exercise is deemed 
 more appropriate for physically weak elderly people , and regular aquatic exercise would also 
 be appropriate for them .
most researches , however , seem to have more fundamental problems than choosing a type of 
 exercise ( ground vs. aquatic ) .
the majority of studies have measured gait stability gained 
 as a result of training only in a static status ( i.e. , duration of standing on one leg , 
 sit - to - stand time , berg balance scale ) .
such an approach may not fully explain walking 
 stability since walking is a dynamic situations3 .
diverse efforts have been expended by many researchers to address this issue and measure 
 walking stability in dynamic situations where actual walking stability is compromised16,17,18 .
they have reported that the center of 
 pressure ( cop ) , which represents the biomechanical trend of the supporting foot in the 
 stance phase , and the center of mass ( com ) or the velocity of center of mass ( comv ) , which 
 represents human body movement , are closely related to body stability in walking . 

especially , the inclination angle between the vertical axis and the vector connecting the 
 position of cop to the position or velocity of com has been found to be very accurate and 
 useful for evaluating walking stability .
these studies also reported that for walking 
 stability evaluation , the frontal medial - lateral (
m - l ) inclination angle was more sensitive 
 than that of the sagittal plane .
since previous studies have examined either only young 
 people , or groups of healthy elderly and the elderly with diseases , it is hard to find a 
 study which has examined the effect of exercise in this regard . therefore , the purpose of this study was to investigate the effect of 8 weeks aquatic 
 exercise on the gait stability of the elderly using dynamic factors ( cop , com , and 
 comv ) .
eleven elderly participants , who stayed at the seoul k welfare center , were recruited for 
 this study .
all participants had no history of orthopedic abnormality within the past 1  year 
 and completed the aquatic exercise program which lasted for 8 weeks .
their average age , 
 height , body mass , and leg length were 77.18  4.96 yrs , 149.48  3.61  cm , 56.94  6.62  kg , 
 and 82.36  2.98  cm , respectively . before the training , each participant was asked to kick a 
 soccer ball three times to check their dominant leg , and the foot they used to kick the ball 
 was determined as their dominant leg . in this study ,
this study was 
 approved by the institutional review board of the korea national sport university . the aquatic exercise for the study
the exercise took place 3 times a week and each class lasted about 60 minutes for 
 class .
the class started with 10 minutes warm - up exercise and finished with 10 minutes 
 stretching , and the main exercise was performed for 40 minutes .
the main exercise included 
 side walking , forward walking , jogging , backward walking , cross stepping , and joint rotation 
 movement .
the program intensity was gradually elevated every 3 weeks with rpe being adjusted 
 to 1216 .
before the program began , we offered the subjects pre - notice , safety education and 
 information , and received their written consent to participate in the exercise program and 
 the experiment .
, b : tue , thu , sat ) to 
 choose from for subjects convenience .
the training was performed in a swimming pool heated 
 to 27  c , with a depth of water of 130150  cm . to identify the 8-week aquatic training effect , we conducted walking tests with obstacles 
 for the subjects twice : one week before , and one week after the aquatic exercise . the 
 obstacle used in the study had an adjustable height . to control effects related to subjects 
 height
an obstacle was placed 
 about 10  m away from the starting point and one force plate was put right before the 
 obstacle .
after sufficient warm - up , each participant was asked to start walking 10  m away 
 from the obstacle , and to continue walking for an additional 5  m after overcoming the 
 obstacle .
subjects were asked to overcome the obstacle with their dominant leg as the 
 leading ( supporting ) foot and the non - dominant leg as the trailing ( swing ) foot . 

participants self - selected walking speed was used during the experiment to minimize 
 potential discomfort to their body , which could have possibly been caused if the speed has 
 been pre - set19 , 20 .
one force plate ( 9286aa , kistler ) was used to measure the cop position of the leading foot 
 while crossing over the obstacle ; a sampling frequency of 1,000  hz was used . for the 
 calculation of center of body weight ( com ) ,
a 14- segment ( both hands , forearms , upper arms , 
 feet , lower legs , thighs , torso and head ) body model was used .
a total of 26 reflection 
 markers were attached to the body , and each subject s obstacle crossing was recorded by 7 
 infrared cameras ( proreflex mcu 240 , qualysis ) at a recording speed of 100  hz . to reduce 
 experimental random errors ,
acquired data were filtered with a fourth order butterworth 
 low - pass filter with a cutoff frequency of 6  hz . the recorded videos and cop data were 
 synchronized using qtm ( qualysis tracking manager , qualysis ) software . to evaluate dynamic stability during obstacle crossing , this research employed the formulas 
 of lee and chou18 and chang and yoon17 for the inclination angles of the sagittal 
 and frontal planes , respectively .
the paired t - test was conducted to test the differences in elderly obstacle - crossing 
 stability after 8 weeks aquatic exercise .
the peak com - cop and comv - cop inclination angles between pre - training and post - training are 
 presented in tables 1 and 2table 1 .
peak com - cop inclination angles before and after the training periodposterior swing phaseanterior swing phasemedial inclinationangles ( deg)posterior inclinationangles ( deg)medial inclinationangles ( deg)anterior inclinationangles ( deg)before11.71  1.66 * 16.27  2.998.08  1.17 * 14.26  0.65after6.62  1.0024.57  2.974.86  1.0915.10  0.53values are
com - cop is the vector connecting cop position to com 
 position . * : a significant difference between pre- and post - aquatic training 
 ( p<0.05 ) .. during the swing phase of the foot while walking , whole body com moves 
 forward and passes through the horizontal cop location . in this study
, we defined the 
 posterior swing phase as the period from the start of the swing in crossing the obstacle 
 until the horizontal position of the whole body com coincided with the cop horizontal 
 position , and the anterior swing phase as the period from the end of the posterior swing 
 phase to the time when the trailing foot touched the ground .
values are group mean  se . com - cop is the vector connecting cop position to com 
 position . * : a significant difference between pre- and post - aquatic training 
 ( p<0.05 ) . for the com - cop inclination angles , significantly decreased medial inclination angles were 
 shown in both the posterior and anterior phases ( table 
 1 , p<0.05 ) .
for the comv - cop inclination angles , decreased medial inclination 
 angles were shown in both the posterior and anterior swing phases , but a significance was 
 found only in the posterior phase ( table 
 2table 2 .
peak comv - cop inclination angles before and after the training periodposterior swing phaseanterior swing phasemedial inclinationangles ( deg)posterior inclinationvangles ( deg)medial inclinationangles ( deg)anterior inclinationangles ( deg)before12.25  1.82 * 14.43  1.696.62  1.1524.74  1.11after7.02  0.8616.64  1.085.40  1.0626.60  0.74values are
comv - cop is the vector connecting cop position to com 
 velocity . * : a significant difference between pre- and post - aquatic training 
 ( p<0.05 ) . ,
values are group mean  se . comv - cop is the vector connecting cop position to com 
 velocity . * : a significant difference between pre- and post - aquatic training 
 ( p<0.05 ) .
in this study , we calculated the inclination angles of the sagittal and frontal planes 
 during obstacle crossing to identify changes in the walking stability of the elderly induced 
 by aquatic exercise .
we found statistically a significantly decreased medial inclination 
 angle in the anterior swing phase after the aquatic exercise ( table 1 ) .
the starting point of the posterior swing phase shows 
 the lowest stability in walking , since the support moves from two feet to one foot .
also , at 
 this moment , since base of support ( bos ) is narrowed , and the whole body com becomes easier 
 to move out of the bos , the ground reaction torque can work most adversely on the human 
 body12 .
that is , when the whole body 
 com is within the bos , the ground reaction torque functions in the same direction to support 
 the body , further stabilizing the move , but if the com moves out of the bos , the torque 
 starts to drag the body down toward the ground .
therefore , people need to overcome the 
 torque using their lower extremity muscles at this moment , or change their posture so that 
 the com does not move out of the bos , in order to prevent falls .
our findings for the 
 obstacle crossing are consistent with those of previous studies17 , 18 , 21 which have reported that the frontal plane s com - cop 
 inclination angle is a very accurate guide to understanding the fall risk of elderly 
 subjects .
previous studies have also reported that as the angle becomes larger the stability 
 of gait declines .
chang and yoon17 also 
 reported the frontal plane s xcom - cop inclination angle ( xcom includes com speed factors ) is 
 a good measure of unstable conditions of gait .
thus , the reduced medial inclination angle 
 observed after the intervention can be interpreted as proving the beneficial effect of 
 aquatic exercise on the gait stability of the elderly . in this study , we found that the average anterior - posterior ( a - p ) inclination angle was not 
 significantly changed in in both the posterior and anterior swing phases after the exercise . 

however , unlike previous training studies12 , 14 , we found no statistically significant 
 training effects .
one possible explanation for this could be the difference in research 
 design ( i.e. training period ) .
yoon et al.12 conducted walking training for the elderly for 6 months and found 
 significant changes in the 6th month .
the intervention they used was a ground walking 
 exercise , and the a - p inclination angle was tested every 3 months ( 0 , 3 , 6 months ) . in their 
 study , the a - p angle increased as the training period progressed but significant differences 
 were only found after 6 months .
kim and shin14 reported that the elderly showed improved gait parameters after 3 
 months of their training periods .
accordingly , we would expect to find significant changes 
 in gait parameters , if the training period of the aquatic exercise were extended .
the 
 increased a - p inclination angle , reported in previous studies , seems to be the result of 
 older people trying to put the whole body com closer to the cop while crossing the obstacle , 
 by straightening the upper body , using their training - strengthened lower extremity muscles , 
 to overcome the physically unstable conditions ( com movement in a smaller basal area ) and 
 prevent falls .
so , we think subsequent studies need to extend the training period to gain 
 statistically significant results for the effect of aquatic exercise .
   [ purpose ] the purpose of this study was to investigate the effect of 8 weeks aquatic 
 exercise on the gait stability of the elderly using dynamic factors : center of mass ( com ) , 
 velocity of center of mass ( comv ) , and center of pressure ( cop ) . 
 [ subjects ] eleven elderly 
 participants ( age : 77.18  4.96 yrs , height : 149.48  3.61  cm , body mass : 56.94  6.62  kg , 
 and leg length : 82.36  2.98  cm ) , participated in this study . [ methods ] to identify the 
 8-week aquatic training effect , 3-d motion analysis with 7 infrared cameras and one force 
 plate , was performed . 
 [ results ] for the com - cop inclination angles , significantly 
 decreased medial inclination angles were shown in both the posterior and anterior swing 
 phases . 
 for the comv - cop inclination angles , decreased medial inclination angles were 
 shown in both the posterior and anterior swing phases , but significant difference was 
 found only in the posterior phase . 
 [ conclusion ] the results suggest that 8 weeks aquatic 
 exercise is effective at improving the gait stability of the elderly . 
 further studies 
 should extend the training period to gain statistically significant results for the effect 
 of aquatic exercise in the anterior - posterior direction . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION