# Evaluation of Thorax Diameter Changes through Trunk Morphology and Different Running Intensities
## 4 Discussion
In addition, during high-intensity exercises, the expiration is not forced due to the high breathing frequency that is needed, so the residual volume (air remaining inside the lungs when the usual expiration is done) remains inside the lungs [3]. This can be the reason for the reduced increase of the thorax volume found in this work when compared with the results from studies that do not observe high-intensity activities or even analyse deep inspirations and expirations. Unfortunately, most of the results found in the literature about breathing and exercise have obtained changes in breathing volume or other physiological variables which cannot be compared quantitatively with results in this investigation. To the best of our knowledge, there is no information about the influence of high-intensity running on thorax cross-sections using a motion capture system. Therefore, the data presented in our study about breathing mechanics have great potential for measuring the quantification of fitness status and different respiratory disease symptoms. The only changes between morphology groups occurred on the anteroposterior cross-section, agreeing partially with the results obtained from other studies which conclude that the respiratory biomechanics are altered depending on the depth or flatness of the thorax [33]. However, in our study, deeper thoraxes showed a higher increase in the anteroposterior cross-section when reaching high intensities. Further, [PERSON] et al. (2022) and [PERSON] et al. (2022) determined a statistical relationship between trunk morphology and locomotor performance during running in an incremental test when the subjects exceeded the anaerobic threshold, finding that a smaller anteroposterior cross-section seems to be positive for better performance during endurance exercise [32; 33]. However, [PERSON] et al. in 2022 conducted 3D shape analyses and no explicit information about thorax cross-sections can be compared with data containing size in this study. In the future, 3D sizes and shape analyses should be conducted in the context of exercise [33]. This difference among the anteroposterior cross-section of different morphology groups when reaching high-intensity efforts could be explained as a factor used by the deeper thoraxes to combat their morphological disadvantage. Flatter thoraxes have shown a better predisposition for performing endurance efforts, so deeper thoraxes need to expand their anteroposterior cross-section in a way to increase their air volume to perform exercises such as running, cycling, etc. However, there are some limitations in this study. No physiological variables related to the quality of breathing were analysed. The intensity of the exercise was assessed with a heart rate pulsometer without considering the volume of oxygen consumed. Also, the difficulty of using this technique with female subjects justifies the absence of females in the sample but gives the results a lack of external validity. A method that could be used on females needs to be developed. More investigations into the kinematics of the thorax are needed, especially on the craniocaudal cross-section, for a better understanding of breathing disorders, physical condition evaluations, and breathing diseases. Apart from that, the sample of this study is limited so a bigger one is needed to be recruited for future studies, especially if the effects of morphology need to be evaluated. In this way, the kinematics of the thorax may develop into a parameter in the assessment of quality of life and/or sports performance.