# Evaluation of Thorax Diameter Changes through Trunk Morphology and Different Running Intensities
## 1 Introduction  
During quiet breathing, the human thorax seems to be static, but during exercise, it expands or contracts through inspiration and expiration [1, 2]. The increase and decrease of the thoracic volumes are due to the movements of the costal and sternum bones produced by the synergic contraction of the intercostal, abdominal, and diaphragm muscles [3, 4]. Some authors have documented that the thoracic volume is one of the key factors in determining the correct functioning of the respiratory system [5, 6]. Not all individuals have the same respiratory pattern: women exhibit a breathing pattern with a greater increase in the anteroposterior cross-section in the upper partand a more thoracic breathing character; in men, it is mixed (both in the upper and lower parts); in children, it is abdominal; and in the elderly, it is characterized by spinal curvature, leading to increased abdominal breathing [3; 7]. Moreover, the BMI (body mass index) modifies the breathing pattern and may even reach hypoventilation-obesity syndrome [8]. Traditionally, the spirometry technique and expiration gas analyser are methods used for measuring respiratory variables [9; 10]. However, these techniques are unable to detect changes in the kinematics on the chest wall and also impede normal breathing activity because of the mask and tubes that are required [6; 10; 11; 12; 13; 14]. [PERSON] and [PERSON] (2002) considered that physical condition evaluation will have higher validity when the evaluation of adaptation of the organism to a specific physical activity enables reproducing the technique of a movement more specifically [14]. The analysis of chest wall movements during breathing and thoracoabdominal volume calculation allows pathologies to be viewed from another perspective, providing the latest information for medical treatments or rehabilitation [15]. Developing methods for evaluating the physical condition and health of athletes based on the thoracic range of movement (ROM) in specific situations is a promising research topic [14; 15; 16; 17; 18; 19]. Some studies have demonstrated that the cross-sections could be simple and effective variables for analysing the thoracic ROM in different body postures or from maximum inspiration to maximum expiration [20; 21; 22; 23]. Meanwhile, handbooks of physiology have reported the behaviour of the three cross-sections of the thorax during breathing, but to date, no study has measured them during high-intensity exercise [3]. We contend that the use of cross-sections for evaluating exercise intensity levels or establishing the relationship between them has potential as a new method for evaluating the functioning of the thorax and quantifying fitness status. One of the most frequent non-invasive techniques is the use of optoelectronic systems, which register and represent the position of reflective markers dispersed over different body parts. The use of this technique for analysing breathing patterns started in 1994 with the first study that used four cameras and thirty-two reflective markers for estimating the thorax volume [24]. This technique has evolved to create optoelectronic plethysmography (OEP) [9; 15; 17; 18; 19; 25], which detects every single movement of the thorax [11]. The OEP uses infrared cameras and a geometrical model to calculate breathing variables [4; 9; 11; 15; 16; 17; 18; 19; 26; 27] and to analyse respiratory pathologies and other physiological disorders [6; 13; 16; 18; 28; 29]. In numerous studies, the results of the OEP technique have been compared with spirometry, validating the OEP technique as a gold standard for analysing breathing patterns [6; 13; 15; 17; 18; 19; 21; 26; 27]. Because of the complexity of the thorax, with OEP methodology, the position of the markers is crucial to compute the variables required [15; 17; 18; 19]. The number of markers varies among different studies from 89 to 12 markers [4; 17; 18; 19; 28; 30].