New horizons for aerobic fitness normalization in children: Breaking the paradigm

Traditional methods to normalize aerobic fitness (VO2max) have been considered inadequate for not properly adjusting the effects of body size in children, adolescents and adults. Allometric scaling (Y=aXb) has emerged as an efficient method to compare individuals of different body dimensions. Studies aimed to compare VO2max in individuals with different body sizes should use methodology that properly accounts for the body size effect, thereby avoiding misinterpretation and reaching the potentially wrong conclusion. The use of allometric scaling for studies aimed to compare aerobic fitness in children and adolescents who dramatically vary in body size is strongly recommended.


INTRODUCTION
The oxidative and anaerobic capacity exhibited by skeletal muscle during the growth course and biological maturation is a subject of considerable debate 1,2 , as the effects of growth and biological maturation on energy metabolism are not yet clear.Exercise physiologists are faced with the challenge of determining whether changes in physiological performance variables, such as VO 2max , are more related to normal growth and development or can be significantly altered by changes in physical activity levels.Indeed, such comparisons between groups or individuals' performance and their peers have been confounded by the body size effect [3][4][5][6] .
A recent study published in the Brazilian Journal of Kinantropometry and Human Performance determined the influence of sexual maturation and physical activity level on cardiorespiratory fitness of schoolchildren of both sexes aged 9-14 years 7 .This study showed the influence of maturation on aerobic fitness.Estimated VO 2max values were different among maturation stages in both sexes, and between sexes at the same maturation stage.Absolute VO 2max (L .min -1 ) values increased with the advance of biological maturation, while relative VO 2max (ml .kg -1.min -1 ) values showed a decreasing trend, in girls only.The influence of biological maturation on VO 2max was estimated at 7%.The authors hypothesized that the influence of sexual maturation on VO 2max may be related to lower PFK activity, a key glycolytic enzyme.However, this hypothesis could only be accepted if the prepubertal group had presented lower values of relative estimated VO 2max compared to pubertal and postpubertal groups.In addition, this study did not examine whether the ratio standard (ml .kg -1.min -1 ) appropriately adjusted for the body size effects on VO 2max .For this reason, it is not possible to determine whether biological maturation truly had an effect on VO 2max or whether the results were affected by a phenomenon called scaling denominator.
One must be very careful in interpreting the effects of biological maturation on aerobic fitness, as many studies have failed to use an appropriate method to adjust or control for body size effects 5,8 .Body size has paramount importance for the understanding the effects of biological maturation and growth on aerobic fitness because it serves as the main normalizing variable.Nevertheless, there are many theoretical and statistical limitations to the standard ratio used to adjust confounding effects associated with body size.With ratio standard analysis, it is assumed that VO 2max is normalized for body mass effects.However, VO 2max is often overestimated in individuals with low body mass and underestimated in those with high body mass 4,6,9 .The main reason for this is because oxygen consumption increases at a slower rate than body mass 3,4 .
For this reason, some authors have suggested using allometric scaling (Y=aX b ) to facilitate body size comparisons of performance variables between heterogeneous groups [3][4][5]8 . Exonent b can be estimated by means of linear regression analysis after calculating the logarithm of the power function according to log Y = log a + b log X, where Y corresponds to the value of the dependent variable (in this case, absolute VO 2max ), X corresponds to the independent variable (body size), a is a scaling constant, and b is the exponent value relative to body size [3][4][5]8,9 .
The purpose of this point-of-view was to highlight the importance of appropriately normalizing aerobic fitness in heterogeneous populations, such as children and adolescents.Breaking the paradigm of traditional ratio standards using body mass as the denominator would avoid possible misinterpretations in the development of aerobic fitness during growth.

DISCUSSION TOPICS
Although the traditional method for VO 2max normalization (ml .kg body mass -1.min -1 ) is broadly used in literature, is this the best way to compare and identify the effects of biological maturation, growth, and sex on aerobic fitness in children and adolescents?The traditional method to normalize aerobic fitness has been found to be inefficient and may lead to misinterpretations when physiological variables between groups of individuals with different body sizes and composition are compared [3][4][5][6]8,9 .
Currently, there are alternatives that seem to be more appropriate to avoid the confounding effects of body size and body composition on performance variables 10 .To elucidate the VO 2max behavior during childhood, Welsman et al. 3 used standard and allometric scaling to adjust the effects of body mass on VO 2max in prepubescent, pubescent, and adult individuals.No difference among groups was found when VO 2max was normalized by the standard value, and this result corroborated the literature at that time.Conversely, the allometric analysis (ml .kg -0.80.min -1 ) found a progressive increase in VO 2max among groups, and these findings altered the conventional interpretation of the behavior of VO 2max during childhood and adolescence.Similarly, Cunha et al. 4 showed that absolute VO 2max (ml .min -1 ) was higher in post-pubescent than in pubescent boys.Biological maturation and body mass can explain 73.5% of the variance of absolute VO 2max .Although VO 2max relative to body mass (ml .kg -1.min -1 ) was similar among groups, the effects of body mass on the variation of VO 2max values remain even after this normalization (5.1%).This result demonstrated that VO 2max (ml .kg -1.min -1 ) is not independent of body mass.On the other hand, VO 2max normalized by a derived allometric exponent (ml .kg -0.90.min -1 ) was higher in postpubertal compared to pubertal boys and the values were considered independent of body mass.Nevertheless, these differences could not be attributed to biological maturation or to any other variable that compose the multiple linear regression analysis.The authors concluded that VO 2max (ml .kg -0.90.min -1 ) was independent of biological maturation, chronological age, body mass, stature, and years of training.
Some studies have used muscle volume (MV), free fat mass (FFM), total muscle mass or lower limb muscle mass as normalizing variables in the attempt to adjust for differences in body dimensions 6,8,11,12 .These variables were chosen to reflect the reality that under maximal intensity exercise, 90% of blood flow and VO 2 are allocated to ATP synthesis during muscle contraction.Therefore, the active muscle mass during exercise may be an even more appropriate variable to normalize VO 2max 13 .Similarly, other studies have used MV to normalize VO 2max , since MV better represents the active muscle mass during exercise 8,11,12 .Welsman et al. 11 found that VO 2max and power were higher among boys compared to girls using body mass, whereas no significant difference was found between sexes when data were normalized by MV using either ratio or allometric scaling.These authors suggested that boys and girls exhibit similar aerobic and anaerobic performance when data were adjusted for MV using the following scaling factors: Liters of O 2 . L MV -0.55.min -1 and Watts .L MV -1.2 , respectively.Tolfrey et al. 8 also analyzed the influence of body size on VO 2max in boys and men using body mass, FFM, and lower limb MV as normalizing variables for VO 2max .In boys, VO 2max /kg body mass 0.79 and VO 2max /kg FFM 1.00 did not completely remove the effect of these variables on VO 2max .Conversely, when VO 2max was normalized to MV (VO 2max /L MV 0.64 ), this effect was fully adjusted.In this study, allometric scaling to MV was the most appropriate approach to normalize VO 2max in boys and men.
Recent studies have shown that biological maturation has no effect on aerobic fitness variables in children and adolescents 4,14,15 , when body dimensions are appropriately normalized 4,14,15 .The evidence shows that the historical approach to normalizing VO 2max to body mass (ml .kg -1.min -1 ) seldom completely accounts for its effect on fitness.

FINAL COMMENTS
We strongly recommend the use of allometric scaling for studies whose aim is to investigate the impact of growth on aerobic fitness of children and adolescents.