Key words: Kinematics, thoracic wall volumes, ribs motion, swimming.

Data Collection
A group of 15 male swimmers (SG) was compared to a control group of 15 healthy non-athletes (CG). The criteria for inclusion in the SG were the following: a) participation in training activities for swimming competitions for more than 3 years, with this training occurring at least three times a week or covering an average of over 30.000 meters/month; b) effective participation in regional or national competitions. The mean age, weight and height of the swimmers were 20.7 (± 2.4) years, 72.9 (± 5.9) kg and 1.78 (± 0.06) m, respectively. The CG was composed of male volunteers with no cardiopulmonary or postural diseases; they were non-swimmers, although they did exercise regularly. The mean age, weight and height of the non-athletes were 22.1 (± 2.4) years, 72.3 (± 10.5) kg and 1.78 (± 0.06) m, respectively. The university ethics committee approved the research study (181/2003) and informed consent was obtained from all participants.

Fifty three spherical retro-reflective markers (=5mm) were attached to the trunk of the subjects (figure 1) and the three-dimensional coordinates of the markers were obtained with the kinematical analysis system Dvideo (Figueroa et al., 2003), with 6 digital video cameras (JVC-GR 9500) sampled at 60Hz. Synchronization of the camera registers was made using the audio band (Barros et al., 2006) while camera calibration and 3D coordinate reconstruction were based on the direct linear transformation method (Abdel-Aziz and Karara, 1971).

The participants remained seated on a chair without back support, in a position involving abduction of the shoulders, knee flexion and feet on the ground. They were asked to avoid any movement unrelated to breathing during the performance of vital capacity maneuvers (VC): each subject performed 5 cycles of vital capacity breathing, defined as maximal inhaling followed by maximal exhaling.

From the 3D coordinates of the markers, the separate volumes of the chest wall and the ribs rotation angles were calculated as follow. The chest wall was divided in 4 compartments: superior thorax (ST - reflecting the action of neck and parasternal muscles); inferior thorax (IT - reflecting the action of parasternal muscles and diaphragm); superior abdomen (SA - reflecting the action of diaphragm and abdominal muscles); and inferior abdomen (IA - reflecting the action of abdominal muscles) (Figure 1-A). The coordinates of the 12 markers enclosing each compartment defined two irregular dodecahedrons, with 8 vertices each, that could be further divided into 6 tetrahedrons. The volume of each tetrahedron was computed by simple geometric formulas; the sum of these volumes gave the compartment volume while the sum of the volumes of all compartments defined the total volume of the trunk (Tk) (Loula et al., 2004).

After the volumes calculation, the angle of rotation of each pair of rib was calculated: a rib coordinate system was obtained for each pair of ribs (2nd to 10th rib) using the 3D coordinates of the markers positioned at the lateral extremity of the right and left ribs and at the spinous process of the corresponding thoracic vertebra (Figure 1-B), and a trunk coordinate system was calculated using the markers at the posterior superior iliac spines and the first thoracic vertebra. The rotation angle was then calculated around the quasi- transversal axis between the coordinate system associated to each pair of ribs and the coordinate system associated to the trunk, representing the upward and downward motion of the rib (Sarro et al. , 2005).

The correlation coefficient was used to assess the relation between the two variables, measuring the strength of association between the curves of the rotation angles of the ribs and the curves of the separate thoracoabdominal volumes: the angle of each rib was correlated with the volume of each compartment and with the total volume of the trunk. High positive correlation values indicate that the compartment and the rib considered move in phase agreement, meaning the simultaneous increasing of the angle of rotation of the ribs and the compartment volume at each instant and, hence, an efficient behavior to achieve maximum volumes (coordinated pattern). High negative correlation values reveal that the compartment and the rib considered move in opposite phase, meaning that whenever the increasing (or decreasing) of the angle of rotation of the ribs, there is a decreasing (or increasing) of the compartment volume at each instant and, so, an inefficient behavior of the ribs to increase volume (uncoordinated pattern). Values close to zero reveal that the compartment and the rib considered present an uncorrelated variation, varying separately at each instant.

The aim of this experiment was to verify the chest wall coordination of swimmers during breathing from the correlation of the results of two up-to-date methods based on videogrammetry. Using the 3D coordinates of markers positioned on the trunk surface, obtained by kinematical analysis, the rotation angles of the ribs (2 to 10) and the volume of four chest wall compartments were calculated as a function of time. All the rotation angles as much as the volumes of the four compartments presented a signal coherent with breathing cycle and high correlation was found between them, for both control and swimmer groups.

Comparing the groups, the control group presented higher values when the superior thorax was considered, showing that ribs motion is more associated with the volume variation of the superior thorax. Swimmer group presented higher correlation between ribs rotation angles and variation of the volume of superior and inferior abdomen, pointing a better association of ribs motion with these compartments. Although no statistical difference was found between the groups considering inferior thorax, the highest correlation values were found for this compartment in the swimmer group. Since during inspiration the diaphragm is responsible for the displacement of the abdomen and the inferior thorax (which represents the region of apposition between the diaphragm and the rib cage) (Ward et al., 1992), these results could be explained by the prevalent action of the diaphragm of the swimmers during vital capacity maneuvers. Furthermore, the abdominal muscles also play an important role: they deflate the inferior thorax during expiration and allow the abdominal pressure to decrease throughout inspiration, in parallel with pleural pressure, allowing the diaphragm to contract (Aliverti et al., 1997). Based on this assumption, the high correlation found in swimmers between rib motion and abdominal volumes could be attributed to a better coordinated action of both inspiratory and expiratory muscles during vital capacity maneuvers.

According to Masliah, 1999, a coordinated movement is generally recognized as being an efficient movement. Hence, the results showed the improved efficiency of the diaphragm and abdominal muscles of swimmers to displace coordinately the rib cage and abdomen when the respiratory system is submitted to higher efforts like maximal breathings, the most used by the swimmers during training. This optimization of the breathing pattern found in swimmers reinforces the idea that practicing swimming can promote positive changes in the thoracoabdominal motion.

Breathing pattern optimization was found by Barros et al., 2003 in yoga practitioners, correlating the variation of the thoracic and abdominal areas during VC maneuvers. They found that yoga practice induced a pattern in phase agreement in the variation of thoracic area and abdominal area during vital capacity maneuvers.

Aliverti et al., 1997 affirm that the central drive to the various respiratory muscle groups changes during exercise according to exercise workload and is translating into velocity of shortening or force depending on the load against which the muscles act. Although the load imposed to the respiratory muscles by water resistance and by the thorax compression caused by excessive contraction of upper limbs and back muscles, swimmers do not present higher respiratory muscle force when compared to non-athletes or athletes of other modalities (Cordain et al., 1990; Armour et al., 1993). So, the intense requirement of the respiratory system during regular swimming training might change the central drive to the respiratory muscles, leading to a better coordination of the chest wall.

The major contribution of this work was the identification of this optimized breathing pattern in swimmers, not yet reported in the literature. One of the limitations of the study is the fact that correlation does not suggest a cause-effect relationship but only the degree of concomitance between the variables and, so the cause of which may be unknown. Nevertheless, the significance of the finding cited above remains in the fact that this optimized pattern can be necessary to the increment of breathing performance and can also partially explain the higher lung volumes of swimmers reported in literature, calling for new investigations involving breathing patterns and lung volumes.