the rising number of children classified as overweight and obese has become a primary health concern in the united states ( 6,9,14,18,19,22 ) . according to body mass index ( bmi ) , over 33% of children / adolescents , ages 6  19 years , have been classified as overweight or obese ( bmi  85th percentile ) ( 6,18,19 ) . if current trends continue the prevalence of overweight in children is expected to double by the year 2030 ( 22 ) . in an attempt to monitor this growing epidemic , school districts across the country use bmi to assess a child s risk of weight - related health problems ( 16 ) . the bmi method does not actually provide an estimate of percent body fat ( % bf ) but rather compares the child s weight and height ratio against population specific norms . because the bmi measurement is easy to perform , inexpensive and noninvasive , it is more often used than direct measures of body fat which can be time - consuming , invasive and costly ( i.e. air - displacement plethysmography , hydrostatic weighing , skinfold method ) . although convenient , bmi results must be interpreted with caution due to the inconsistent pattern of growth and development in this age group ( 6,22 ) . a method of assessing % bf that school districts may find as an attractive alternative to the bmi measurement is leg - to - leg bioelectrical impedance analysis ( lbia ) . during the lbia assessment , a low level electrical current is introduced into the body and impedance , or resistance to the current flow is measured as the participant stands on a scale - like analyzer ( 17 ) . using the calculated impedance value , % bf can be estimated using preprogrammed prediction equations that have been specifically developed for children ( 10,17 ) . lbia is simple to perform , quick ( less than 30 seconds ) , noninvasive and portable ; all characteristics that increase its appeal . it can be used in diverse settings including private clinicians offices , wellness centers , and hospitals and across a spectrum of ages , body weights , and disease states ( 10,17 ) . several studies have examined the validity of lbia in children and their findings are inconsistent ( 7,11,13,20,21 ) . in normal weight children , ages 5  10.9 years , a high correlation ( r= 0.94 ) was found between % bf by lbia and dual - energy x - ray absorptiometry ( dxa ) ; however , while lbia overestimated mean % bf by 2.5% ( 21 ) . hosking et al . ( 11 ) also reported high correlations between % bf by dxa and lbia for boys ( r= 0.92 ) and girls ( r= 0.95 ) but again mean % bf values were significantly different ( boys : % bfdxa  % bflbia = 0.9% ; girls : % bfdxa ( 7 ) reported significant correlations between lbia and dxa % bf , but found large intra - subject variations in overweight / obese adolescents ( 13 ) and preadolescents ( 7 ) . conversely , sung et al . ( 21 ) reported no mean % bf differences between lbia ( 28.9  12.8% ) and dxa ( 27.3  10.3% ) in chinese children ages 7  16 years . a potential limitation to the utilization of lbia technology in school districts may be the need to follow a series of pretesting guidelines designed to control for fluctuations in hydration status , which could affect the accuracy of measurement ( 5,10 ) . for instance , no physical exercise 12 hours prior to the lbia assessment is a recommended guideline ( 10 ) . if necessary , this restriction significantly reduces the usability of these analyzers in schools where controlling a child s pre - testing behavior may be quite difficult . a few investigations have examined the effect that aerobic exercise has on lbia - measured % bf values in children . reductions in % bf have been reported following both laboratory - controlled treadmill ( 1,2 ) and cycle ergometry ( 8) exercise . more recently , andreacci et al . ( 3 ) reported that mean lbia - determined % bf was reduced following a 60-min after - school exercise bout in thirty - three children . although statistically significant , the % bf reductions following the after - school exercise bout were minimal ( female = 1.6% ; male = 0.4% ) . given the relatively small magnitude of change , the authors suggested that adhering to the pre - test exercise guideline may be unnecessary in instances such as a school physical where a 1.02.0% measurement error may have little effect on the adiposity classification of a child ( 3 ) . however , data comparisons against adiposity - based health risk percentiles were not preformed in that study . given the potential use of lbia in the school setting , the impact that a structured in - school physical education class has on lbia % bf measurements and the childs adiposity - based health risk classification is in need of clarification . therefore , the purposes of this investigation were to determine : 1 ) whether participation in a structured in - school physical education class altered the lbia - determined % bf value ; and 2 ) whether the potential exercise - induced % bf magnitude of change altered the health risk classification of the child . seventy - six girls ( age : 121 y ; height : 153.9  7.5 cm ; body mass : 51.9  15.5 kg ; bmi : 21.7  5.4 kg / m2 ) participated in this investigation . prior to participation , parents informed written consent and subjects written assent were obtained according to the requirements established by the bloomsburg university institutional review board and the shikellamy area school district . during each physical education class , prior to beginning the exercise activity , participants were led through 35 min of structured warm - up / stretching . following the warm - up / stretching , participants performed a variety of exercises based on the unit in physical education class for approximately 2530 min . each unit ( i.e. daily lesson ) included both fitness - based ( i.e. circuit based exercise ) and skill - based game / activity ( i.e. field hockey and soccer ) . all units were designed towards increasing physical fitness / activity and meeting state regulated recommendations . the circuit - based exercises ( e.g. jumping rope , push - ups , sit - ups , squats , bicep curls , etc ) were done preceding the skill - based game / activity . at the end of each exercise class participants were led through a cool - down period of stretching , lasting approximately 23 min . all participants had their lbia body composition assessed twice ( pre- and post - exercise ) while participating in the physical education class . body composition was assessed using a lbia analyzer ( tanita model # tbf-300a ; tanita corporation of america inc . , lbia measures of % bf were obtained immediately before and within 5-min of completing the physical education class . prior to the lbia assessment , height was determined using a detecto ( webb city , mo ) physician s scale . gender , age , and height ( cm ) were entered into the lbia system . the  standard mode was used for all lbia measurements , as recommended by the manufacturer . leg - to - leg impedance of the lower extremities and body weight ( kg ) were measured simultaneously while the child stood on the scale . the % bf was then automatically calculated using the analyzer s pre - programmed prediction equations . in order to assess the intensity of the physical activity experienced during the physical education class , girls were randomly selected to wear a heart rate monitor ( polar electro , inc . , heart rate was measured continuously throughout the physical education class and averaged , for each child , over the entire session . statistical analyses were performed using spss 16.0 for windows ( spss inc , chicago , il ) . statistical significance was established a priori at p<0.05 . paired sample t - tests ( pre- vs. post - exercise ) were used to examine the lbia body composition and heart rate data for the girls . bland - altman plots ( 4 ) were used to assess individual differences in % bf and impedance from pre- to post - exercise . children and adolescents ( 15 ) were used to determine whether the % bf change post - exercise altered the subjects adiposity - based health risk classification . as suggested by mueller and colleagues ( 15 ) , the 85th percentile was considered the cutoff point to determine cardiovascular risk related to adiposity level ( > 85th percentile = unhealthy body fat ) . seventy - six girls ( age : 121 y ; height : 153.9  7.5 cm ; body mass : 51.9  15.5 kg ; bmi : 21.7  5.4 kg / m2 ) participated in this investigation . prior to participation , parents informed written consent and subjects written assent were obtained according to the requirements established by the bloomsburg university institutional review board and the shikellamy area school district . during each physical education class , prior to beginning the exercise activity , participants were led through 35 min of structured warm - up / stretching . following the warm - up / stretching , participants performed a variety of exercises based on the unit in physical education class for approximately 2530 min . each unit ( i.e. daily lesson ) included both fitness - based ( i.e. circuit based exercise ) and skill - based game / activity ( i.e. field hockey and soccer ) . all units were designed towards increasing physical fitness / activity and meeting state regulated recommendations . the circuit - based exercises ( e.g. jumping rope , push - ups , sit - ups , squats , bicep curls , etc ) were done preceding the skill - based game / activity . at the end of each exercise class participants were led through a cool - down period of stretching , lasting approximately 23 min . all participants had their lbia body composition assessed twice ( pre- and post - exercise ) while participating in the physical education class . body composition was assessed using a lbia analyzer ( tanita model # tbf-300a ; tanita corporation of america inc . , arlington heights , il , usa ) . lbia measures of % bf were obtained immediately before and within 5-min of completing the physical education class . prior to the lbia assessment , height was determined using a detecto ( webb city , mo ) physician s scale . gender , age , and height ( cm ) were entered into the lbia system . the  standard mode was used for all lbia measurements , as recommended by the manufacturer . leg - to - leg impedance of the lower extremities and body weight ( kg ) were measured simultaneously while the child stood on the scale . the % bf was then automatically calculated using the analyzer s pre - programmed prediction equations . in order to assess the intensity of the physical activity experienced during the physical education class , girls were randomly selected to wear a heart rate monitor ( polar electro , inc . , heart rate was measured continuously throughout the physical education class and averaged , for each child , over the entire session . statistical analyses were performed using spss 16.0 for windows ( spss inc , chicago , il ) . statistical significance was established a priori at p<0.05 . paired sample t - tests ( pre- vs. post - exercise ) were used to examine the lbia body composition and heart rate data for the girls . bland - altman plots ( 4 ) were used to assess individual differences in % bf and impedance from pre- to post - exercise . children and adolescents ( 15 ) were used to determine whether the % bf change post - exercise altered the subjects adiposity - based health risk classification . as suggested by mueller and colleagues ( 15 ) , the 85th percentile was considered the cutoff point to determine cardiovascular risk related to adiposity level ( > 85th percentile = unhealthy body fat ) . the lbia body composition data are presented as a function of time in table 1 . significant reductions ( p<0.0001 ) were observed for % bf , impedance , body mass and fat mass in the girls when compared to pre - exercise values ( table 1 ) . significant increases ( p<0.0001 ) were observed for fat free mass when compared to pre - exercise values ( table 1 ) . bland - altman plots ( 4 ) exploring for individual differences in % bf are depicted in figure 1 . the difference in % bf from pre- to post - exercise is plotted against body mass ( figure 1 ) . according to the bland - altman plot ( 4 ) , body mass had no apparent influence on the magnitude of the % bf change post - exercise . total sample difference in % bf pre- to post - exercise ( mean  sd ) was 0.7  0.9 % bf . as shown in figure 1 , participating in the physical education class resulted in a % bf reduction in the majority of the girls ( 73.7% ) . however , in 19.7% the % bf value increased and 6.6% of the girls experienced no change pre- to post - exercise . more specifically , the % bf magnitude of change was 1.0 % bf in 64.5% of the girls , between 1.1 and 2.0 % bf in 23.7% of the girls , and by more than 2.0 % bf in 11.8% of the girls ( figure 1 ) . prior to the start of the exercise class : 12 girls were classified as  underfat  ( < 5th percentile for body fat ) , 43 girls were classified as ( 5th to 85th percentile for body fat ) , 7 girls were classified as  overfat  ( 85th  95th percentile for body fat ) and 14 girls were classified as  obese (  95th percentile for body fat ) , following the exercise class : only 2 girls had changed classification categories : 1 from the  healthy - fat  to  underfat  category and 1 from the  obese  to  overfat  category . despite these two changes , when using the 85th percentile as the cutoff point to determine cardiovascular risk related to adiposity level ( > 85th percentile = unhealthy body fat ) , both girls remained in the same adiposity classification category ( healthy vs. unhealthy ) following exercise . the primary finding of this investigation was that although the structured in - school physical education class reduced lbia - determined % bf estimates these changes had no effect on the health risk classification of the girls . our finding of a small , but statistically significant reduction in mean % bf ( ~0.6% ) observed in the girls is consistent with the % bf reductions ( range 1.2  1.6% ) that have been previously reported in the laboratory setting ( 1,2,8 ) . recently , andreacci et al . ( 3 ) also reported that mean % bf decreased following an after - school exercise program in eighteen girls . the magnitude of the mean % bf reduction following the after - school exercise session was greater than found presently ( 1.6% vs. 0.6% ) . previous research suggests that the exercise - induced % bf alterations may be influenced by the intensity and/or duration of the exercise bout ( 1,2 ) . possible causes for lbia - determined % bf changes include increases in blood flow to active muscle tissue , cutaneous blood flow , and skin temperature during the exercise bout ( 12 ) . given that the intensity of the after - school and in - school exercise bouts were similar , 76% and 73% of age - predicted maximal heart rate , the larger reduction in mean % bf following the after - school exercise bout may have been due to the differences in duration . in the present study , the lbia post - exercise assessments were conducted immediately following the physical education class , which lasted approximately 35 min , whereas the after - school exercise program was approximately one - hour in length . the comparison of this data suggests that exercise duration may influence the magnitude of % bf change . it appears that exercise beyond 35 min may cause a larger physiological disruption and % bf alteration ; however , most physical education programs will be completed within the duration examined presently . although mean % bf data provides information regarding the entire group , it does not necessarily give accurate estimates for all individuals within that group . as such , the bland - altman ( 4 ) method was used to better explore individual variability for our sample . overall , performing exercise prior to the lbia assessment resulted in a reduction in % bf for the majority of girls ( ~74% ) whereas no change ( 6% ) or an increase ( 20% ) was observed in the others . regardless of whether % bf was increased or decreased the % bf alteration was less than  2.0 % bf in 88% of the girls in this study . similar percentages have been reported for girls in the after - school exercise study and in previous laboratory - designed research , 93% and 86% , respectively ( 13 ) . it is apparent from these investigations that performing sub - maximal and maximal intensity exercise prior to the lbia assessment will result in % bf alterations of less than 2.0 % bf for most girls . to determine whether this magnitude of change had any practical significance we compared the pre- and post - exercise % bf values to previously published body composition percentiles . mueller and colleagues ( 15 ) , developed reference percentiles for % bf by ethnicity , sex and age in u.s . as suggested in that paper , values of % bf above which cardiovascular risk variables increase are located at the 85th percentile ( 15 ) . as such , this percentile was used as the cut - off point for the definition of excessive ( unhealthy ) body fat in this study . despite the exercise - induced % bf alterations observed pre- to post - exercise , all 76 girls remained in the same health - risk classification category ( either above or below the 85th percentile ) following the physical education class . this was not a lbia validation study and the current body of evidence is limited and the findings are inconsistent ( 7,11,13,20,21 ) . as such , we are not recommending that school districts replace the bmi assessment with lbia technology at this time . additional lbia validation preferably against multi - component models ( e.g. three - component , four - component ) is needed to more clearly determine the accuracy of lbia technology in this population . only then may it be considered as an alternate or supplemental method to the current health - risk assessment program . another limitation to the current study was that this investigation included girls ranging in age from 10  14 years . given the complexity of growth and development that is observed during this time period , these findings can not be generalized to children who differ by age or gender than those examined presently . in summary , our laboratory previously reported minimal changes in % bf as a result of laboratory - based exercise and a field - based after - school exercise bout in children . the present investigation was an extension of the use of lbia into an actual in - school , structured , physical education setting . the intensity of the physical exercise class was moderate corresponding to approximately 73% of the girls age predicted maximal heart rate . this study demonstrated that although exercise altered the lbia - determined % bf estimate , the magnitude of change was relatively small and had no impact on the child s subsequent adiposity - based health risk classification category ( healthy vs. unhealthy body fat ) . as such , adhering to the pre - test exercise guideline appears unnecessary when using lbia to categorize the health - risk of an adolescent girl . this information is important for those that currently use or are considering the use of this technology in adolescents .
the purposes of this investigation were to determine : 1 ) whether a structured in - school physical education exercise bout altered the leg - to - leg bioelectrical impedance analysis ( lbia ) determined percent body fat ( % bf ) value ; and 2 ) whether the potential exercise - induced % bf magnitude of change altered the health risk classification of the child . 
 seventy - six girls ( age : 12.21.0 yr ; height : 153.97.5 cm ; body mass : 51.9 15.5 kg ; bmi : 21.75.4 kg / m2 ) participated in this investigation . 
 lbia measured % bf values were obtained immediately before and within 5-min after completing a structured , in - school , physical education class . 
 significant reductions ( p < 0.001 ) in mean % bf ( 25.010.2 vs. 24.410.3 % ) were observed for the group following the physical education class . for the majority of the girls ( 88% ) , 
 the % bf alteration was less than  2.0 % bf . 
 more specifically , the % bf magnitude of change was 1.0 % bf in 64.5% of the girls , between 1.1 and 2.0 % bf in 23.7% of the girls , and by more than 2.0 % bf in 11.8% of the girls . 
 regardless of the % bf magnitude of change , all girls remained in the same adiposity classification category ( healthy vs. unhealthy body fat ) following exercise . 
 adhering to the pre - test exercise guideline appears unnecessary when using lbia to categorize the health risk of an adolescent girl .