tanaka1 recommends partial leg bathing 
 because this bathing method can induce hyperthermic effects all over the body while imposing 
 a minimum load .
furthermore , partial leg bathing is more practical : it can be implemented 
 far more easily than full - body bathing .
partial leg bathing can ease pain or stiffness 
 through increased vascular flow in the region induced by topically expanded arteriovenous 
 vessels . according to tanaka1
, these 
 favorable effects are based on the topical supply of oxygen and nutrients as well as the 
 washout of carbon dioxide , lactic acid , and substances responsible for causing inflammation 
 and pain .
partial bathing , such as leg bathing or hand bathing , can enhance systematic 
 circulation through the entire body .
topically captured heat from the blood will circulate 
 all over the body and increase the deep - body temperature by 0.2  c to 0.3  c3 , if given enough time . however , to the best 
 of our knowledge , no study has comprehensively investigated the hyperthermic effects of hand 
 bathing . therefore , to identify the additional benefits of partial bathing beyond those 
 recognized in our previous study4 , the 
 effects of hand bathing using plain water and water supplemented with inorganic salt and 
 carbonated gas were assessed and compared .
we also attempted to increase the effects of warm 
 bathing by introducing finger flexion - extension exercises in the water with inorganic salt 
 and carbonated gas .
sixteen healthy , young male subjects ( mean age  standard deviation [ sd ] , 22.5  2.6  years ) 
 were recruited . according to horikiri5 , 
 the deep - body temperature increases from 0.7  c to 1.0  c after full - body bathing at 41  c 
 for 10 minutes .
the optimal temperature at which a thermal effect is elicited during 
 warm - water bathing is between 38  c and 41  c .
water temperatures higher than 42  c are 
 inappropriate because heat stimulation can lead to sympathicotonia6 .
therefore , the water temperature was set at 41  c , and the 
 duration of bathing was set at 20 minutes . in order to maintain a consistent environment , 

hand bathing constituted the following two conditions : plain - water bathing and bathing in a 
 solution of artificial bath additives including inorganic salts and carbon dioxide 
 ( co2 bathing ) .
a bath additive product named  kikiyu  , manufactured by bathclin 
 co. , ltd . , was added into the water used for co2 bathing . according to the study 
 by watanabe et al.7 ,
30  g of kikiyu bath 
 additive was added per 200 l of water .
the stirring rate for carbon dioxide was 33  ppm . in 
 addition to plain water bathing and co2 bathing
, we also conducted 
 kineto - co2 bathing , which involved flexion - extension exercise of the 
 metacarpophalangeal joints , proximal interphalangeal joints , and distal interphalangeal 
 joints in the same solution used in the co2 bathing condition .
this 
 flexion - extension exercise was performed at 60 laps per minute using all joints of the 
 fingers simultaneously .
finally , in the control condition , the finger flexion - extension 
 exercise was performed outside of water .
the region of the hand involved in the bathing was the area covering one - third of the right 
 forearm .
subjects participated in either hand bathing or control group conditions for 20 
 minutes after resting by sitting quietly on a chair .
subjects were randomly assigned to the 
 hand bathing or control group conditions on different dates . the water temperature for hand 
 bathing
tokyo , japan ) and 
 maintained with sbh-802fvii ( sbh-802fvii kumagai electric co. , ltd . ,
measurements included heart rate , blood pressure , deep - body temperature , skin blood flow , 
 superficial temperature , and subjective heat perception .
the heart rate was measured with the bsm-2401 ( bsm-2401 
 nihon kohden co. , ltd . ,
tokyo , japan ) . both systolic blood pressure and diastolic blood 
 pressure were measured at the left upper arm with a digital automatic sphygmomanometer 
 ( hem-770a omron co. , ltd . ,
deep - body temperature was measured on the basis of 
 the sublingual temperature using the cm-210 .
the alf21n ( alf21n advance co. , ltd , tokyo , 
 japan ) was adapted to measure skin blood flow at the right upper forearm .
superficial 
 temperature was measured at the left hallux using the thermo - hunter hr-1(thermo - hunter hr-1 
 optex co. , ltd . ,
the borg scale ratings of perceived exertion were modified 
 to measure subjective heat perception ( table 
 1table 1.subjective heat perceptionextremely hot1098very hot76hot5slightly hot4just right ( optimal)3lukewarm2very lukewarm1extremely lukewarm0.5dont feel anything0the borg scale ratings of perceived exertion ( rpt ) were modified to assess subjective 
 heat tolerance ) .
the study design was approved by the school of health sciences faculty of 
 medicine kagoshima university ethics committee ( no . 318 ) .
the borg scale ratings of perceived exertion ( rpt ) were modified to assess subjective 
 heat tolerance all subjects signed a consent form after being informed of the study purpose , content , and 
 all risks involved .
all statistical calculations were performed using spss ver.23 ( spss 
 inc . , chicago , il , usa ) software .
the shapiro - wilk test was used to confirm normal 
 distribution of data . if a significant difference was detected after normal distribution was 
 confirmed at all levels , a multiple comparison test was performed . after confirming a normal 
 distribution with the shapiro - wilk test followed by the mauchly s test of sphericity , 
 one - factor repeated - measure analysis of variance ( anova )
was conducted . when the results of 
 one - factor repeated - measure anova produced a p - value < 0.05 , a bonferroni s multiple 
 comparison test was performed . when a normal distribution was not confirmed with the 
 shapiro - wilk test ,
the friedman s test was conducted . if the results of the friedman s test 
 produced a p - value < 0.05 , the multiple comparison with wilcoxon s test was performed 
 after adjusting the values with the bonferroni multiple hypothesis test correction .
post - bath heart rates increased in all four conditions ( plain water bathing , co2 
 bathing , kineto - co2 bathing , and control ) , and the results of the one - way anova 
 demonstrated significant differences ( p<0.05 ) in pre - bath and post - bath values .
the 
 results of the multiple comparison test showed significantly higher ( p<0.05 ) heart rates 
 after kineto - co2 bathing than after plain water bathing , co2 bathing , 
 or the control condition ( table 2table 2.one-way repeated measures analysis of variance and multiple comparisons for each 
 variable before and after bathingexperimental variablesplain water bathingco2 bathingkineto - co2 bathingcontrol groupheart rate ( bpm)7.05.07.76.614.08.03.94.5systolic blood pressure ( mmhg ) 1.27.80.15.63.011.20.13.0diastolic blood pressure ( mmhg)0.05.61.84.95.86.24.82.6sublingual temperature ( c)2.22.2 * 103.62.3 * 104.92.2 * 100.90.4 * 10difference in forearm skin blood flow : after bathing / during 
 rest1.20.41.20.42.40.92.30.8superficial temperature of hallux ( c)4.31.43.21.35.81.1perception of warmth 2.21.63.21.24.21.5compared to plain water bathing values ( p<0.05)compared to co2 bathing values ( p<0.05)compared to kineto - co2 bathing values ( p<0.05 ) compared to control group values ( p<0.05)*co2 bathing involved bathing in a solution of artificial bath additive 
 including inorganic salts and carbon dioxide*kineto - co2 bathing involved the finger flexion - extension exercise in the 
 same solution used in co2 bathing*control group participated in the same finger flexion - extension exercise outside of 
 water ) .
however , the results of the one - way anova were insignificant for systolic 
 blood pressure and diastolic blood pressure ( table 
 2 ) .
sublingual temperature increased in all four conditions ( plain water bathing , 
 co2 bathing , kineto - co2 bathing , and control ) , and the results of 
 the one - way anova demonstrated significant differences ( p<0.05 ) in pre - bath and post - bath 
 values ( table 2 ) .
the results of the multiple 
 comparison test for sublingual temperature exhibited significantly higher values ( p<0.05 ) 
 in the co2 bathing and kineto - co2 bathing conditions than in the plain 
 water bathing and control conditions ( table 
 2 ) . compared to plain water bathing values ( p<0.05 ) compared to co2 bathing values ( p<0.05 ) compared to kineto - co2 bathing values ( p<0.05 )   compared to control group values ( p<0.05 ) * co2 bathing involved bathing in a solution of artificial bath additive 
 including inorganic salts and carbon dioxide * kineto - co2 bathing involved the finger flexion - extension exercise in the 
 same solution used in co2 bathing * control group participated in the same finger flexion - extension exercise outside of 
 water skin blood flow increased in all four conditions ( plain water bathing , co2 
 bathing , kineto - co2 bathing , and control ) in terms of the post - bath values during 
 rest . in particular , the significant difference ( p<0.05 ) in skin blood flow was detected 
 in the conditions where the finger flexion - extension exercise was incorporated ( table 2 ) .
these findings suggest that the finger 
 flexion - extension exercise effectively facilitates circulation in the body .
an average 
 increase in the superficial temperature of the hallux ( by 3.2  c to 5.8  c ) was observed 
 after hand bathing . although the results of the one - factor anova for increased superficial 
 temperature of the hallux did not reach statistical significance for the plain water 
 bathing , co2 bathing , or kineto - co2 bathing conditions , the highest 
 increase in the temperature ( 5.8  c ) was detected in the kineto - co2 bathing 
 condition ( table 2 ) . subjects reported a change in their perception of warmth from  lukewarm  to  slightly hot
the results of the friedman s test for perception of warmth 
 showed a significant difference .
subjects who underwent co2 bathing and 
 kineto - co2 bathing reported significantly greater perception of warmth 
 ( p<0.05 ) than their counterparts in the plain water bathing condition according to the 
 results of the wilcoxon s test adjusted with the bonferroni multiple hypothesis test 
 correction ( table 2 ) .
the heart rates increased by approximately 7 bpm in both the plain water bathing and 
 co2 bathing conditions .
the increase in heart rates might be attributed to the 
 inhibition of the parasympathetic nerves8 
 facilitated by heat - induced vasodilation due to warm water bathing . compared to the values 
 obtained during rest
, the heart rates increased even further by 14 bpm after introducing the 
 finger flexion - extension exercise in the co2 bathing condition .
this striking 
 upturn is due to the mental strain imposed by the finger flexion - extension exercise , 
 excitability of the sympathetic neurons mediated by muscle spindles , and the stimulus to the 
 sinus node itself triggered by right atrial dilatation9,10,11 .
the effects of warm water bathing may have contributed to the 
 lowering of the diastolic blood pressure .
however , no notable differences were detected 
 between plain water bathing and co2 bathing conditions . the deep - body temperature increased by only approximately 0.1  c in the control group , 
 indicating that the finger flexion - extension exercise alone can not increase deep - body 
 temperature .
the deep - body temperature increased approximately 0.2  c in the plain water 
 bathing condition , which was lower than the increase with co2 bathing ( 0.3  c ) . 

thus , these findings suggest that the finger flexion - extension exercise can induce 
 beneficial effects on deep - body temperature only in combination with warm water bathing .
the 
 synergistic effects of heat from the co2 bathing and increased skin blood flow 
 due to the finger flexion - extension exercise contribute to the rise in deep - body 
 temperature .
tanaka et al.2 state that the chloride 
 capsule created over the surface of the skin along with the mirabilite lead to higher 
 deep - body temperature in co2 bathing .
the results of our study are consistent 
 with those of tanaka et al.2 ; similar 
 positive effects concerning deep - body temperature occurred in the co2 bathing 
 condition .
furthermore , the studies by nonaka demonstrated that co2 bathing has 
 the potential to accelerate skeletal muscle regeneration after injury12 by increasing the expression of myogenin13 .
heat conducted through warm water induces 
 the opening of arteriovenous anastomoses , thereby accelerating skin blood flow .
this large 
 volume of heated blood will circulate throughout the body to raise the deep - body 
 temperature . in our study , a significant increase in forearm skin blood flow was 
 demonstrated in the control and kineto - co2 bathing condition compared with the 
 plain water and co2 bathing conditions .
this is due to the increase in the skin 
 blood flow facilitated by muscle pump effects from the finger flexion - extension 
 exercise .
these synergistic effects of increased skin blood flow and deep - body temperature due to 
 co2 bathing may have caused the significant increase in deep - body temperature 
 ( of approximately 0.5  c ) in the kineto - co2 bathing condition compared with the 
 control condition or the plain water bathing condition .
thus , it is likely that incorporating exercise into 
 warm water bathing will improve finger mobility .
furthermore , ueda15 showed that the concentration of lactic acid in blood 
 significantly decreases after incorporation of exercise during co2 bathing . 

therefore , it is reasonable to expect that exercise during warm water bathing would improve 
 finger mobility and exercise tolerance . in our study ,
hand bathing successfully increased 
 the superficial temperature of the hallux by 3.2  c to 5.8  c .
in particular , the highest 
 increase ( 5.8  c ) in the superficial temperature of the hallux was detected in the 
 kineto - co2 bathing condition .
thus , warming the hands in the washroom and 
 engaging in finger flexion - extension exercise could ease the cold feeling in the feet and 
 toes .
finally , the perception of warmth was significantly heightened in the co2 
 bathing and kineto - co2 bathing conditions compared with during plain water 
 bathing .
subjects reported feeling  slightly hot  in the kineto - co2 bathing 
 condition .
thus , the results of our study suggest that incorporating finger 
 flexion - extension exercise into hand bathing provides greater perception of warmth even in 
 this type of simple partial bathing . taken together , the accumulating body of evidence from 
 our study highlights the beneficial effects of finger flexion - extension exercise while hand 
 bathing .
incorporating such exercise into hand bathing therapy can increase deep - body 
 temperature , improve finger mobility and exercise tolerance , as well as ease the cold 
 feeling in the feet and toes .
 [ purpose ] the purpose of our study was to compare the effects of hand bathing using plain 
 water and water supplemented with inorganic salt and carbonated gas and to assess the 
 hyperthermic effects of performing finger flexion - extension exercise while bathing in 
 water with carbonated gas and inorganic salt and without water . 
 [ subjects and methods ] 
 sixteen healthy , young males were subjected to plain water bathing , co2 
 bathing , kineto - co2 bathing , or no bathing . 
 co2 bathing involved 
 bathing in a solution of artificial bath additives including inorganic salts and carbon 
 dioxide . 
 partial bathing of the hand was implemented for 20 minutes at 41  c . 
 the 
 concentration of carbonic gas was set at 33  ppm . in the kineto - co2 bathing 
 condition , 
 finger flexion - extension exercise was performed at 60 laps per minute in the 
 same solution used in co2 bathing . 
 the control group engaged in the same 
 exercise as those in the kineto - co2 bathing group , but without bathing . 
 
 [ results ] a significant increase in deep - body temperature was observed in the 
 co2 bathing and kineto - co2 bathing conditions compared with both 
 the plain water bathing and control condition . 
 [ conclusion ] significantly heightened 
 hyperthermic effects were observed when finger flexion - extension exercise was performed 
 during co2 bathing . 
INTRODUCTION
SUBJECTS AND METHODS
RESULTS
DISCUSSION