the axillary crutch is an assistive device that supports the gait of patients with lower 
 limb impairments .
however , crutch walking has several unresolved issues , including the 
 appropriate length of the crutch1 , crutch 
 palsy2 , palmar pain during crutch weight 
 bearing3 , and deviation of the axillary 
 pad from the axilla during the crutch stance phase4 .
in particular , the axillary pad often deviates from the axilla 
 during the crutch stance phase , and this is one of the main issues for safe crutch walking . 

if the axillary pad is not held firmly against the chest wall , the crutch tends to be easily 
 displaced from the axilla , possibly leading to falls .
therefore , the cause of the axillary 
 pad deviation from the axilla needs be identified to ensure user safety .
we previously 
 showed that external rotation of the crutch throughout the crutch stance phase ensured that 
 the axillary pad was held tightly against the chest wall during single - crutch walking5 .
however , this observation is thought to 
 apply only to single - crutch walking .
thus , the aim of this study was to analyze the crutch 
 position in the horizontal plane to confirm the stability of the axillary pad during 
 double - crutch walking .
twelve healthy young subjects were enrolled in this study ( 6 males and 6 females ; mean age , 
 22.1  0.5  years ; mean height , 168.7  8.0  cm ; mean weight , 63.3  11.3  kg ) .
this study was 
 performed in accordance with the ethical principles of the declaration of helsinki and was 
 approved by the ethics committee of kyushu university of nursing and social welfare .
all 
 subjects received a description of this study and provided their written , informed consent 
 before participating in this study . before carrying out the measurements ,
the crutches were 
 adjusted to the appropriate length for each subject , and adequate practice of crutch walking 
 was done .
the appropriate crutch length was determined using the standard method described 
 by rusk and lowman6 .
double - crutch walking 
 was performed using a three - point gait ( swing - through gait ) with a pseudo - affected left 
 leg . for this study
, a new crutch was created that had stepless adjustments for the height of 
 the hand grip and the crutch length to enable its adaptation to all subjects .
the new crutch 
 had an extension that was attached to the axillary pad so that a straight line connecting 
 the two infrared reflective markers indicated the long axis of the axillary pad ( fig .
schematic diagram of the crutch position the measuring system consisted of a three - dimensional motion capture system ( vicon nexus , 
 uk ) , comprising nine infrared cameras and six force plates ( amti , usa ) and a digital video 
 camera ( jvc , japan ) .
the sampling rates were 100  hz . the crutch position in the horizontal 
 plane was defined as the angle between the sagittal ( yz ) plane and the long axis of the 
 axillary pad .
the direction of the crutch position was determined as internal rotation when 
 the crutch turned toward the body , and external rotation when the crutch turned away from 
 the body ( fig .
the crutch position data were obtained using four infrared 
 reflective markers attached to both crutches ( two markers each on the right and left 
 crutches ) , and the results were calculated using the data collected by the motion capture 
 system .
the positions of both crutches , that were measured during the crutch walking , were 
 calculated using the data of the ground reaction forces .
all subjects were asked to perform a 5-m straight 
 walk using double - crutch walking .
repeated - measures analysis of variance and tukey s honest significance test were used to 
 compare crutch angles across three time points during the crutch stance phase ( crutch 
 contact , mid stance , and crutch off ) of each subject .
cross - correlation coefficients were used to examine the symmetry of 
 the left and right crutch angles during the crutch walking cycle of each subject .
table 1table 1.crutch angles of each phase of the crutch walking of each subjectsubjectcrutch angle ( degrees)direction of crutch rotationbetween crutch contact 
 and crutch offcrutch contactmid stancecrutch off1rt.9.3
 1.93.9  2.1 * 0.3  2.3*external rotationlt.7.8  2.06.7  0.92.7  2.6 * 2rt.9.2  1.70.0  1.1 * 5.4  1.4*lt.11.7  2.25.0  1.5*0.3  4.9 * 3rt.14.0  1.55.2  1.2*1.4  2.0*lt.12.8  1.53.2  1.2 * 0.7  2.0 * 4rt.5.2  1.42.0  1.9 * 2.6  1.2*lt.5.6  1.31.7  2.1 * 2.6  4.1 * 5rt.1.5  2.73.8  1.4 * 6.1  2.7*lt.1.0  2.13.9  1.7 * 9.9  2.1 * 6rt.4.8  1.90.1  5.9 * 5.9  3.4*lt.7.5  2.41.1  1.1 * 4.7  2.0 * 7rt.6.3  1.90.7  2.7 * 3.9  3.1*lt.11.5  3.62.9  4.6 * 2.4  3.9 * 8rt.9.1  3.13.2  2.5 * 9.2  2.9*lt.3.1  2.01.6  2.09.5  2.4 * 9rt.3.8  2.13.9  1.7 * 12.2  5.2*lt.5.0  2.92.1  1.2 * 6.2  1.6 * 10rt.7.0  1.02.0  3.5 * 2.8  2.0*lt.4.7  0.52.0  2.4 * 12.8  1.7
* 11rt.12.9  1.710.4  1.25.4  2.2*lt.14.5  0.511.4  1.110.2  3.4 * 12rt.7.3  2.51.8  3.5 * 3.9  5.1*external rotationlt.0.7  2.05.9  5.05.6  3.6no rotationdata are means  sd . *
shows the crutch angles of both sides in each phase of the crutch stance of 
 each subject .
crutch angles of each of the two crutches were significantly different across 
 the crutch stance phase of 11 of the 12 subjects .
post hoc 
 analysis showed significant differences for 6 of the 11 subjects ( subjects 2 , 3 , 6 , 7 , 9 , 
 and 10 ) in three comparisons ( crutch contact vs. mid stance , crutch contact vs. crutch off , 
 and mid stance vs. crutch off ) on both sides .
four other subjects ( subjects 1 , 4 , 5 , and 8) 
 showed significant differences in three comparisons on one side , whereas the other side 
 showed significant differences in two of three comparisons ( significance was always observed 
 in the comparison of crutch contact vs. crutch off ) .
the remaining subject ( subject 11 ) 
 showed significant differences in one or two of three comparisons on both sides 
 ( significance was always observed in the comparison of crutch contact vs. crutch off ) . 

finally , subject 12 showed significant differences in three comparisons on one side , but the 
 other side did not show significant differences in these three comparisons .
correlation coefficients and phasic differences between the left and right crutch 
 angles of each subjectsubjectccphasic difference10.851%20.950%30.950%40.906%50.911%60.930%70.930%80.980%90.950%100.890%110.901%120.870%cc : correlation coefficient .
phasic difference as a percentage of the whole crutch - walking cycle . shows the correlation coefficients and phasic differences between the left and 
 right crutch angles during the crutch - walking cycle of each subject .
a strong correlation 
 was observed between the left and right crutch angles in all subjects .
however , phasic 
 differences were shown by 4 of the 12 subjects ( subjects 1 , 4 , 5 and 11 ) , and subject 4 had 
 an especially large phasic difference of 6% .
sasaki et al . reported that , when the shoulder joint was positioned in an internally 
 rotated position , the axillary pad was not fixed against the chest wall and could easily 
 slide backward from the axilla4 . in other 
 words , it is very important for safe crutch walking that the axillary pad is kept in the 
 axilla during crutch walking . in an earlier study
, we suggested that analysis of the crutch 
 position in the horizontal plane during single - crutch walking would be necessary to ensure 
 safe crutch walking5 .
we also showed that 
 external rotation of the crutch throughout the crutch stance phase ensured that the axillary 
 pad was held tightly against the chest wall during single - crutch walking .
the reason for 
 this is that external rotation of the crutch throughout the crutch stance phase is related 
 to the shape of the chest wall ( fig .
2.relationship between the chest wall and the position of the axillary pad during the 
 crutch stance phase ) .
that is , because the chest wall is rounded , the axillary pad moves along the chest 
 wall .
therefore , the aim of this study 
 was to analyze the crutch position in the horizontal plane to confirm the stability of the 
 axillary pad during double - crutch walking .
relationship between the chest wall and the position of the axillary pad during the 
 crutch stance phase this study found that the crutch rotated externally in the horizontal plane during the 
 crutch stance phase of 11 subjects , although there was inter - subject variation in the angle . 

we consider that these variations are dependent on : crutch tip contact position , transfer 
 and/or position of center of gravity , and difference of crutch walking pattern ( step length , 
 walking speed ) .
regarding the crutch tip contact position , the crutch angle is posited to 
 become externally rotated when the width between the left and right crutch tips becomes 
 wider .
conversely , when the width becomes narrower , the crutch angle tends to be internally 
 rotated . in both cases , external rotation of the crutch throughout the stance phase probably 
 ensured that the axillary pad was held tightly against the chest wall during double - crutch 
 walking , as in single - crutch walking . in the one atypical subject ( subject 12 )
, the crutch 
 rotated externally on one side , but did not rotate on the other side during the crutch 
 stance phase . in this case , it is likely the crutch deviated from the chest wall on the side 
 that did not rotate externally , and the subject s crutch gait was unstable . in a review of 
 the video recordings ,
subject 12 was one of the subjects who lacked skill in crutch walking , 
 even though sufficient training was performed in advance ; i.e. , subject 12 exhibited many 
 failures of the measurement and large body sway during crutch walking .
therefore , external 
 rotation of the crutch throughout the stance phase appears to be very important for keeping 
 the axillary pad in the axilla , not only during single - crutch walking , but also during 
 double - crutch walking . in double - crutch walking
, the symmetry of the right and left crutch positions appears to be 
 an important factor of safe crutch walking , in addition to external rotation of the 
 crutches .
3.the ideal crutch transition during crutch walking ( conceptual illustration ) shows the ideal crutch transition during double - crutch walking . as shown in fig .
3 , the symmetry of the right and left crutch 
 positions is required for straight walking throughout double - crutch walking .
if the 
 positions of both crutches are not synchronized , straight walking becomes unstable because 
 the direction of the force holding the axillary pad against the chest wall becomes 
 asymmetrical .
the present study confirmed that there was a similarity between the left and 
 right crutch angles during crutch walking of all subjects .
incidentally , assuming the crutch - walking cycle is the same as the normal walking cycle , the 
 phase from crutch contact to mid stance is about 30% of whole crutch - walking cycle .
further , the length of this phase is 
 less than one second . as shown in table 2 , a 
 phasic difference of 6%
it means that there was a 
 considerable discrepancy in the symmetry of the left and right crutch angles in this phase 
 for subject 4 .
therefore , it is likely that the crutch stance phase of subject 4 was 
 negatively impacted because this subject lacked skill in crutch walking .
it is our opinion 
 that symmetry of crutch positioning is one of the important factors for safe double - crutch 
 walking .
the ideal crutch transition during crutch walking ( conceptual illustration ) in conclusion , the crutch positions of both sides were measured in the horizontal plane in 
 order to confirm the stability of the axillary pad during double - crutch walking .
external 
 rotation of the crutch throughout the stance phase is very important for keeping the 
 axillary pad in the axilla , not only during single - crutch walking , but also during 
 double - crutch walking .
in addition , symmetry of crutch positioning appears to be one of the 
 important factors for safe double - crutch walking .
however , we think our findings are not sufficient to recommend 
 their application in clinical settings , because the subjects were healthy . therefore , the 
 selection of subjects requiring crutches is needed to confirm our findings in further 
 studies .
 [ purpose ] the aim of this study was to analyze the crutch position in the horizontal 
 plane to confirm the stability of the axillary pad during double - crutch walking . 
 [ subjects ] twelve healthy young subjects ( 6 males and 6 females ) . 
 [ methods ] the subjects 
 were asked to walk in a straight line , using double crutches , for a distance of 5 m , 5 
 times . 
 crutch position data were obtained using four infrared reflective markers attached 
 to both crutches ( two markers each on both crutches ) . 
 the crutch angles of each subject 
 were compared across three time points during the crutch stance phase ( crutch contact , mid 
 stance , and crutch off ) . [ results ] crutch angles of each of the two crutches were 
 significantly different across the crutch stance phase of 11 of the 12 subjects . 
 phasic 
 differences were shown by 4 of the 12 subjects , and a strong correlation was observed 
 between the left and right crutch angles of all subjects . 
 [ conclusion ] external rotation 
 of the crutch throughout the stance phase was very important for keeping the axillary pad 
 in the axilla , not only for single - crutch walking , but also for double - crutch walking . in 
 addition , 
 symmetry of the crutch positioning is one of the most important factors for safe 
 double - crutch walking . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION