Introduction. Such sports as fitness have high demands on the motor system, mainly on the foot arch. Repeated movements can expose the ankle joint and the sole of children practicing fitness sports to high loading. Aim of Study. The aim of this study was to determine the incidence of incorrect body posture, flexibility and flatfoot in children attending competition fitness classes. Material and Methods. The study population was composed of 18 children aged 8.9 to 10.3 years of age. Body posture was assessed using the eye-viewing method according to Klein and Thomas. Backbone flexibility was measured using tests of Schober´s and Stibor´s symptoms, right and left backbone lateral flexibility, Thomayer´s symptom, Otto´s inclination and the reclination symptom. Podoscopy was used to assess flatfoot degrees. All indicators were evaluated individually and statistical significance of the relationship between individual dimensions of body posture and flat feet were assessed using the chi-square (χ2) test. Results. The highest proportion of incorrect body posture in all tested children was recorded in the dimension of hips and shoulders (78%), backbone curvature (61%) and shoulderblades (55.6%). Flat feet were diagnosed according to Kapandji in 27.8% and 11.2% cases according to Srdečný. When investigating the relationship of flat feet in the test of Kapandji and Srdečný to the dimension of body posture statistical significance was recorded between flat feet and the dimension of hips and shoulders and backbone curvature. In the test of Srdečný a significant relationship was observed between flat feet and the dimensions of head, shoulderblade and abdomen. Conclusions. This study focused on the verification of the impact of the training process in fitness sports practiced by children on their body posture and flatfoot incidence. We recommend observing children for flatfoot symptoms, which can negatively influence the postural function in young athletes.