The main objective of the present study was to verify the level of CRF (VO2max) that discriminates working memory in adolescents. Results showed that boys need minimum of 45.03 ml.kg− 1.min− 1 and girls 36.63 ml.kg− 1.min− 1 to be classified as “normal”. Additionally, thresholds for VO2max presented significant differences for working memory, which demonstrated higher values of working memory for group with high CRF.
Comparing CRF groups with academic achievement, adolescents classified in the Health Fitness Zone (HFZ), FITNESSGRAM®, presented higher academic grades [1, 27]. When the relationship between CRF and academic achievement in children was evaluated, Hansen et al. [11] found non-linear relationship, in which increases in academic performance occur up to 22 laps (~ 47.5 ml.kg− 1.min− 1) for spelling and 27 (~ 49.7 ml.kg− 1.min− 1) laps for math scores, after that, performance reached a plateau.
The relationship between CRF (pacer laps) and working memory performance, in children, presented significance, analyzing the reaction time (r= -0.13), and working memory accuracy (r = 0.14) [28]. Analyzing relationships longitudinally and controlling other variables (grade, sex, maternal education, BMI), CRF can explain by 7.5% the working memory accuracy [29]. In addition, improvements in CRF are associated with improvements in the cognitive control of the working memory of preadolescents [30]. According to results of the present study, higher CRF values, classified by the created threshold, indicated better working memory for boys.
The threshold values for VO2max to discriminate low and normal working memory in the present study are similar to that used by FITNESSGRAM®, developed by Welk et al. [14], which determined the presence of metabolic syndrome through VO2max measured by submaximal treadmill test. For boys, the HFZ value was ≥43.6 ml.kg− 1.min− 1, a difference of 1.7 ml.kg− 1.min− 1 from the present study; for girls, the HFZ value was≥39.1 ml.kg− 1.min− 1, a difference of 2.47 ml.kg− 1.min− 1.
Ruiz et al. [15], established CRF cut-off points to determine a cardiovascular health profile in adolescents, finding values of 43.8 ml.kg− 1.min− 1 for boys and 34.6 ml.kg− 1.min− 1 for girls. Likewise, these values are similar to those found in the present study, demonstrating that cardiovascular health and mental functions are affected in similar CRF intensities in adolescents.
The low sensitivity and high specificity found for boys can be explained by the number of boys with high CRF in this sample. Borderline adolescents showed median VO2max value of 47.39 ml.kg− 1.min− 1 (39.38–52.52). Using the classification proposed by an international normative [31], the borderline group had subjects in the 50th percentile but some individuals could reach the 90th percentile. On the other hand, the high specificity of created cut-off points (91.8%), implies that individual with low CRF have high probably to be classified as borderline. This pattern was not found for girls, in which the similar sensitivity and specificity could be justified by the homogeneous VO2max distribution.
It is important to highlight the changes that CRF can cause in the cerebral morphology, and these changes are related to the working memory performance. It is noteworthy that well-developed prefrontal cortex [32, 33] and greater hippocampal volume [34] are associated with better working memory. In addition, children with higher CRF have greater hippocampal volume [34, 35].
Neuroelectric indexes are also related to working memory, and evidence suggests that higher P3 amplitude, an event related to the neuronal activity and linked to attentional processing, is associated with better working memory [6]. Comparing to individuals with high and low CRF by Event-Related Potentials, the results demonstrated better P3 indexes for children with high CRF [36]; and higher functional connectivity [19].
This study has some limitations such as the relatively small sample and the creation of thresholds only for adolescents aged 14–16 years. Another limitation was not using academic achievements for the development of thresholds to compare the working memory. However, a strong point is the creation of thresholds for working memory, which is possibly one of the first studies with this objective. In addition, this study used a sample composed of adolescents, and many of studies used children in the second infancy [37].
In summary, VO2max can be used to discriminate adolescents classified as borderline or normal working memory. In this sense, these results can complement normative health data or be useful in school programs, since working memory can improve academic performance.