in daily life , walking is an essential movement and it is the most frequently performed 
 action by human being .
therefore , stable walking is a very important element in people s 
 lives . as people age
, the muscles related to walking weaken and , as a result , the elderly 
 are more exposed to injuries in comparison to young people in fall . the proportion of the 
 elderly , defined as adults aged 65 or older , is increasing worldwide because of extended 
 lifespan due to the development of medical facilities and improved diet .
particularly , in 
 south korea , the baby boom generation is entering the old age and old population is becoming 
 a larger share of the entire population than ever before .
various injuries of the elderly , 
 as a result of the increasing older population , can cause serious economic and social 
 problems , unlike those of younger people , and are thought to be a social priority in many 
 developed countries .
fall is one of the most common accidents experienced by the elderly , 
 and it can cause serious injury which may even lead to death1
. falls by the elderly are closely related to declining muscle strength of the lower limbs 
 and deteriorating balance2,3,4,5 , and it has been reported that fall risk factors of the elderly can 
 be reduced by strengthening the muscles of the lower limbs6,7,8 .
many researchers have designed various ways for improving the 
 muscle strength of the lower limbs in order to reduce the risk factors of falls for the 
 elderly . oneil
et al.9 and schlicht et 
 al.10 reported that the elderly showed 
 significant increases of muscle power and had a positive effect on the lower limb muscle 
 strength after 8 weeks of lower body muscle strength training , respectively .
in addition , 
 tsouslou et al.11 and kim and shin12 revealed that aquatic exercise for 24 
 weeks and 12 weeks decreased fall risk factors for the elderly .
considering the findings of many previous studies , continuous exercise for lower limb 
 muscle strength and aquatic exercise have a positive effect on preventing falls by the 
 elderly .
however , from the perspective of the elderly who have weaker physical strength , 
 muscle strength training may cause excessive stress or pain on the musculoskeletal system , 
 while an aquatic exercise reduces the burden on the joints because of reduction of 
 bodyweight as a result of buoyancy .
it is , therefore , thought that an aquatic exercise is 
 more beneficial for the elderly than muscle strength training .
there are , however , two potential limitations to studies investigating training for 
 preventing falls by the elderly .
first , the elderly mostly experience falls when balance is 
 lost , such as when ascending stairs or crossing an obstacle .
compared to level walking , 
 obstacle crossing walking increases the joint range of motion and translation , and raises 
 the body , which significantly reduces gait stability13 .
however , most previous studies have been conducted using level 
 walking without an obstacle to examine the effects of training .
these findings , therefore , 
 can not properly reflect the psychological withdrawal of fall due to an obstacle and function 
 of the lower limb muscle in crossing an obstacle .
second , in selecting variables to evaluate 
 the effect of training , many previous researches attempted to prove the effect of training 
 using general gait variables such as gait speed and stride . to be sure
, gait speed may be a 
 valid variable in general gait analysis , but perhaps more detailed variables are needed for 
 analysis of the training effects . in other words , variables that can represent the 
 relationship between trailing foot ( swing foot ) and an obstacle in crossing an obstacle must 
 be defined in order to prove the effect of training .
therefore , the purpose of this study was to examine the fall - prevention effect of 12 weeks 
 aquatic training for the elderly using variables that are representative of the relationship 
 between the trailing foot and an obstacle .
ten healthy elderly participants , who lived in c city , were recruited for this study . they 
 had no history of orthopedic abnormalities in the past one year .
all participants finished 
 the 12 weeks of aquatic training , and their average age , height , body mass , and bmi were 
 77.155.21  years , 149.873.54  cm , 57.44 6.74  kg , and 25.582.98  kg / m 
 respectively . before starting the study ,
three of them are left foot dominant people and the others are left 
 foot dominant ones .
this study was approved by the institutional review board of korea 
 national sport university , and prior to their participation , informed consent was received 
 from each participant . the aquatic exercise program used in this study was conducted by a qualified trainer and it 
 was performed three times a week , 60 minutes per session for 12 weeks .
the exercise was 
 composed of 10 minutes warm - up , 10 minutes cool - down , and 40 minutes main exercise .
the 
 warm - up was mostly stretching and light walking , and the main exercise consisted of side 
 walking and running , forward walking and running , cross - stepping , and lunge .
the cool - down 
 consisted of stretching and balance exercise , and for exercise intensity the rpe was 
 adjusted to between 12 and 13 . to determine the effect of the 12 weeks aquatic training , the subjects performed two 
 obstacle - crossing tests , one week before the beginning of the exercise program and one week 
 after the program had finished . before each test
, the subjects were explained about the 
 procedure of the test and performed enough warm - up exercise in order to allow natural 
 movements during the test .
the obstacle used in this study was adjustable in height and was 
 adjusted to 30% of each subject s leg length in order to exclude the influence of the 
 subject s height . after sufficient warm - up , the subjects were asked to start walking at 5  m 
 from the obstacle , cross the obstacle , and then walk three more meters .
a force plate was 
 installed before the obstacle , and the subjects were not given an explanation about the 
 force place so that they would naturally step on it when crossing the obstacle . when 
 crossing the obstacle , the subjects used their dominant foot as the supporting foot ( leading 
 foot ) and non - leading foot as the swing foot ( trailing foot ) . in order to minimize potential 
 physical discomfort caused by a fixed walking speed , the subjects used a self - selected speed 
 when walking14 , 15 .
the obstacle - crossing of each subject was recorded using seven 
 infrared cameras ( oqus , qualysis ) at a recording speed of 100  hz .
the recorded data were 
 processed using a fourth - order butterworth low - pass filter , to reduce experimental random 
 error , and a cutoff frequency of 6  hz . for the outcome variables , we calculated maxcop and meancop
. maxcop was defined as the 
 maximum distance from average cop of the stance phase to cop of at any instant of the stance 
 phase .
meancop was defined the mean distance between average cop and cop of each instance 
 during the stance phase .
toe clearance ( tc ) , heel clearance ( hc ) , and maximum vertical heel clearance ( mvhc ) were 
 measured in this study .
they are variables that indicate the relationship between the 
 trailing foot and an obstacle when crossing an obstacle .
tc was measured as the horizontal 
 distance between the toe and obstacle before crossing the obstacle when the vertical 
 positions of both were the same .
also , hc was measured as the horizontal distance between 
 the obstacle and the heel after crossing the obstacle at the point where the vertical 
 position of the heel was the same as the height of the obstacle .
mvhc was defined as the 
 vertical distance between the heel and obstacle when the heel was directly above the 
 obstacle . crossing velocity ( cv )
was calculated by measuring the horizontal distance between 
 the point where the lead foot touched the ground before crossing the obstacle and the one 
 where it landed on the ground after crossing it , and dividing it by the time it took . to examine the effect of the 12 weeks aquatic training on the safety of obstacle - crossing 
 in gait by the elderly , the paired t - test was used . in this study ,
the obstacle clearance parameters and gait stability parameters between pre - training and 
 post - training are presented in tables 1table 1 .
obstacle clearance parameters between training periodstc ( cm)hc ( cm)mvhc ( cm)cv ( cm / s)pre - training23.1  1.8 * 71.0  12.0 * 15.2  4.4 * 57.7  3.2*post - training31.7  2.087.9  14.520.4  3.879.8  5.9values are group mean  standard error .
* significant difference between pre- and 
 post - aquatic training ( p<0.05 ) . and 2table 2 .
gait stability parameters between training periodsmaxcop ( mm)meancop ( mm)sl ( step length ) ( cm)ct ( crossing time ) ( s)pre - training233.3  22.9 * 55.4  4.6 * 81.3  3.7 * 1.4  0.1*post - training118.6  29.737.4  5.899.7  5.91.3  0.1values are group mean  standard error .
* significant difference between pre- and 
 post - aquatic training ( p<0.05 ) .. for the obstacle clearance parameters , significantly increased patterns were 
 found for all variables ( table 1 , p<0.05 ) . 

for the all gait stability parameters , statistically significant training effects were found 
 ( table 2 , p<0.05 ) .
values are group mean  standard error . * significant difference between pre- and 
 post - aquatic training ( p<0.05 ) .
values are group mean  standard error . * significant difference between pre- and 
 post - aquatic training ( p<0.05 ) .
in this study , in order to assess the effect of 12 weeks of aquatic training for the 
 elderly , variables that represent the safety and stability of obstacle - crossing were 
 calculated .
the elderly showed quicker cv after the 12 weeks of 
 aquatic training ( table 1 , p<0.05 ) .
cv is 
 calculated by dividing step length ( sl ) with crossing time ( ct ) , and to increase this value , 
 either sl needs to increase or ct needs to decrease . in this study ,
the elderly used an 
 increased step length ( 81.3  cm  99.7  cm ) and a decreased ct ( 1.4s 
  1.2s ) to increase their obstacle - crossing speeds .
this increased speed 
 seems to be resulted from the increased lower limb muscle strength from a continuous aquatic 
 exercise .
this finding also agrees with the results of previous studies that concluded 
 improved gait ability reduced the risks of fall for the elderly16 , 18 . in the results of this study ,
the elderly showed statistically significant increases in tc , 
 hc , and hmvc , which are representative of efficient obstacle - crossing , after the 12 weeks of 
 aquatic exercise ( table 1 , p<0.05 ) .
tc and hc 
 represent safe distances before and after crossing the obstacle , and are reported to be 
 closely related to safe obstacle - crossing19,20,21 . 

we think the increase in tc observed in this study ( 23.07  cm < 31.74  cm ) resulted from 
 effort by the elderly to secure a more safe distance in order to safely cross the obstacle . 

we also think the increase in hc after the exercise ( 71.65  cm < 84.74  cm ) was the result 
 of an effort to secure a wider base of support by landing the foot at a point further from 
 the obstacle for safer obstacle - crossing . also , the increase found in hmvc after the 
 exercise ( 15.57  cm < 19.23  cm ) agrees with the findings of chen et al.18 and yoon16 , who reported that increased hmvc is necessary for safe obstacle 
 landing of the elderly .
we suggest that the improvements seen in all of the results related 
 to the trailing foot ( swing foot ) were due to the aquatic exercise strengthening the lower 
 limb muscles of the elderly and these findings indicate that continuous aquatic exercise can 
 help to prevent falls by the elderly . in this study ,
both maxcop and meancop , which were calculated to evaluate the stability of 
 obstacle - crossing by the elderly , showed statistically significant decreases ( table 2 , p<0.05 ) .
unlike the other variables , the 
 cop parameters calculated in this study were representative of the movement of the lead foot 
 ( supporting foot ) during crossing of the obstacle .
that is , increases in cop would indicate 
 that the lead foot was being supported less stably when crossing the obstacle , while 
 decreases in cop would indicate more stable support with less shakiness .
therefore , the 
 decreases of maxcop ( 232.26  mm to 118.58  mm ) and meancop ( 55.39  mm to 37.39  mm ) seen in this 
 study indicate that the supporting foot of the elderly became more stable when crossing the 
 obstacle , and that the effect of the aquatic training can reduce the risk of fall for the 
 elderly . in conclusion , a 12 weeks of aquatic exercise can improve the stability of the elderly when 
 crossing a height obstacle , which is the most frequent cause of falls by the elderly , and 
 help to prevent falls by the elderly .
   [ purpose ] the purpose of this study was to examine the fall - prevention effect of 12 
 weeks of aquatic training for the elderly using variables representative of the 
 relationship between the trailing foot and an obstacle . 
 [ subjects ] ten healthy elderly 
 participants , who lived in c city , ( age : 77.155.21 yrs , height : 149.873.54  cm , body 
 mass : 57.44 6.74 , and bmi : 25.582.98  kg / m2 ) , participated in this study . 
 [ methods ] to determine the effect of 12 weeks aquatic training , 3-d motion analysis with 
 7 infrared cameras and one force plate , was performed . 
 [ results ] tc , hc , mvhc , and cv 
 significantly increased after intervention . for the all gait stability parameters , 
 statistically significant training effects were found . 
 [ conclusion ] in conclusion , 12 
 weeks aquatic exercise can help the elderly become more stable when crossing a height 
 obstacle , which is the most frequent cause of falls by the elderly . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION