Discussion
Our results showed that adolescents with FAO and non-FAO adolescents presented similar aerobic fitness, PAL, peripheral muscle strength, and HRQoL results. However, the adolescents in the FAO group presented higher values of expiratory muscle strength than did the non-FAO group. This study also showed that most adolescents with asthma are physically deconditioned and sedentary, regardless of whether the condition was clinically controlled, and FAO was present.
Aerobic fitness and PAL in children and adolescents with asthma have been studied over the last two decades (23-25), and the association between PAL and lung function remains poorly understood (26, 27). Loponen et al. (2018) showed that a low PAL was associated with a faster decline in lung function in adults with asthma (8). However, no studies have evaluated the association between PAL and the decline in lung function in children and adolescents with asthma. Contrary to our hypothesis, the present study showed that the FAO and non-FAO adolescents have a similar PAL. Two hypotheses can explain our findings. First, both groups had controlled asthma, and this hypothesis is supported by previous studies showing that subjects with controlled asthma experience fewer asthma symptoms and therefore have a higher PAL than do subjects with uncontrolled asthma (28). Second, both groups exhibited a long period of sedentary behavior and a very short period of moderate-to-vigorous physical activity (MVPA).
In our study, most adolescents in the FAO and non-FAO groups presented reduced levels of aerobic fitness, as evaluated by the CPET. All adolescents terminated the CPET due to fatigue in the lower limbs, and they also demonstrated cardiac limitations (reduced cardiac reserve). These results corroborate those in previous studies in either children or adolescents with asthma and those without asthma (7, 21, 22). Taken together, these results suggest that physical deconditioning, rather than asthma or FAO, is the major cause of interruption during maximal exercise. The reduced levels of aerobic fitness observed in our study may be explained by the reduced MVPA levels; however, the opposite result should be observed if the adolescents participate in a physical training program (29).
Children and adolescents with asthma and their nonasthmatic peers have been shown to have similar levels of respiratory muscle strength (30). On the other hand, children and adolescents with severe asthma have reduced maximal inspiratory pressure (MIP) compared to those with nonsevere asthma (31). This difference probably occurs because airway obstruction leads to static hyperinflation and places the respiratory muscle at a mechanical disadvantage (32). Interestingly, we observed that adolescents with FAO exhibit a higher maximal expiratory pressure (MEP) than do non-FAO adolescents. This finding can be explained by the fact that the lower the airway caliber is, the greater the respiratory requirement for exhaling air from the lungs (32).
Several studies have evaluated upper and lower peripheral muscle strength in children and adolescents with asthma (22, 33, 34), and most of them did not observe differences between subjects with asthma and their nonasthmatic peers (22, 33). Our results showed that the upper and lower limb muscle strength values were similar between the FAO and non-FAO groups. In contrast, Lattorre-Romám et al. (2013) demonstrated different handgrip strength results in subjects with asthma. The authors reported that subjects with a lower FEV1 also had poorer handgrip strength (34). The discrepancy between the results presented by Lattorre-Romám et al. (2013) and our results may have occurred because Lattorre-Romám et al. (2013) evaluated adolescents with asthma of steps 1 to 5, while we assessed adolescents with asthma of steps 3 to 5 (mostly severe asthma). Interestingly, in our study, both groups exhibited reduced muscle strength in the upper and lower limbs (approximately 50% of predicted), suggesting that both the FAO and non-FAO groups have muscle weakness. Muscle weakness might be explained by the fact that subjects with moderate-to-severe asthma are more sedentary and present a greater number of exacerbations, leading to an increase in the dose of oral corticosteroids needed (26, 35, 36).
The HRQoL scores were similar between the FAO and non-FAO groups in all the questionnaire domains (physical activity, emotions, and symptoms). Amaral et al. (2014) evaluated HRQoL in adolescents with a lower or normal peak expiratory flow, and the authors did not observe any significant differences between them (37). Our results are supported by those in previous studies demonstrating that adolescents with controlled and partially controlled asthma have a better HRQoL (38, 39). In addition, a worse HRQoL in subjects with uncontrolled asthma is associated with fewer symptom-free days (40). Taken together, these results suggest that HRQoL in adolescents is more strongly related to asthma control than is airflow obstruction.
Our study has limitations. First, because the incidence of FAO in adolescents is low, we had to recruit a large number of adolescents for the FAO group; however, the number of adolescents reached the sample size calculation. Second, we did not include a group of adolescents without asthma; however, the most recent systematic review demonstrated no differences in physical fitness between adolescents with and without asthma (25). Finally, adolescents were selected based on their spirometry results, and complete lung function (volumes and capacities) was not evaluated. However, the LLN, assessed by spirometry, has been reported to be a good index for detecting FAO (3).