Traditionally, breathing expansion has been considered solely as a physiological process, neglecting the biomechanical aspects. This expansion does not appear to occur uniformly across the upper and lower levels of the ribcage. Moreover, running performance seems to be a determining factor for the thoracic wall movements. This study aims to analyse how mediolateral and anteroposterior thoracic diameters vary at different levels (upper, middle, and lower) in athletes with different running performance levels at various running intensities. Twenty-two healthy athletes were recorded using optoelectronic plethysmography while performing an incremental running test. Three recordings were taken from each subject at different running intensities, and three running performance groups were created by the final velocity obtained in the incremental test. An increase in the thoracic expansion of anteroposterior and mediolateral diameters at all levels was observed while the exercise intensity rose. However, a mirrored pattern was found while the intensity rose:​ the upper anteroposterior and lower mediolateral diameters appear to expand more during the initial phase of the effort, whereas the upper mediolateral and lower anteroposterior diameters expand more during the final phase of the effort. No significant differences were found between running performance groups at the same exercise intensity, although the mirrored pattern was slightly seen when analysing each group separately. Bigger differences among running performance groups are needed to glimpse thoracic kinematics variances. Further research on thoracic kinematics is required to understand better respiratory disorders, fitness assessments, and respiratory diseases.