One limiting factor of this study concerns the protocol
used to evaluate the combined effect of aerobic exercise
and smoking on mucociliary clearance and cardiac autonomic control. The way this protocol was implemented
may have attenuated the effect of one of the stimuli and
therefore may not have reflected the sum of both. Another
limiting factor is the variation in the characteristics of the
smokers, which may have led to the lack of significant
differences between groups regarding heart rate variability
indexes. Categorization according to smoking conditions
can make future samples homogeneous and therefore contribute to different findings. One should also bear in mind
that the assessment of cardiac autonomic modulation was
performed in the time interval in which mucociliary clearance was evaluated to investigate the relationship between
the autonomic nervous system and mucociliary clearance.
Therefore, the moment used to evaluate the autonomic
nervous system in the 4 protocols varied among subjects,
which may have influenced the response of the heart rate
variability indexes.