||BACKGROUND: Physical activity is beneficial for young children. The aim is to monitor the changes in body posture and balance, as well as in the level of lower limbs mechanical output in six-year-old boys practicing judo (JU) and in a group not practicing that sport (NT). METHODS: In 12 matched pairs of boys (JU+NT) body mass and height were measured and BMI was calculated at three time points (baseline, three months, six months). Body posture was evaluated with the use of Moire method (TT, DTK, MR, LALBC-F, DALBS). The balance was examined by means of the UPST test with the eyes open (EOA) and closed (ECA). Ground reaction and relative power were assessed in standing vertical jump on the dynamometric platform. In the subsequent calculations, the MANOVA with the GLM procedure using 10 dependent above variables (excluding BMI and Pr indices), and next Factorial Repeated Measures ANOVA were used. RESULTS: In MANOVA a significant interaction of the time, and group, factor was ascertained. Next, the series of univariate ANOVAs yielded following results: for MR the difference between groups was significant after three months (with JU having better results than NT group), but not at the baseline or after six months. For UPST EOA, JU group had higher results after three months than those of NT group. For Impulse (J), the interaction and time positive trend were significant. CONCLUSIONS: A six-month practice of judo results in a significant improvement in the quality of body posture, balance, and lower limbs muscle strength impulse.