## 4 Discussion  
The main finding of this study was that 3-weeks of the supplementary body-weight training program performed along with soccer training during the season were effective in decreasing lower limb muscle activity imbalance during high-speed running in professional soccer players. The statistically significant decrease in imbalances was found in the hamstring and gluteal muscles in comparison to baseline values. To the authors' knowledge, this is the first study to examine a short-term supplementary body-weight training program performed during the competitive season in professional soccer players. Considering that there was no control group in the study, it cannot be confidently stated that the results are related to the performed supplementary body-weight training program and not to the soccer training itself. Nevertheless, significant knowledge for future research and training clues can be derived from the current data. The fact that a very short-term (3 weeks) supplementary body-weight training program performed along with soccer training during the season was so effective in decreasing the muscle activity imbalance of the lower limbs during high-speed running in professional soccer players is of high practical value. This is because in most professional sports, coaches have to allocate the necessary time to improve technical skills, team tactics, and physical fitness. Moreover, the training regimen presented in this study does not require any spe \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Muscle [\%]**} & \multicolumn{2}{c}{**Pre-Intervention**} & \multicolumn{2}{c}{**Post Intervention**} & \multirow{2}{*}{**ES**} \\ \cline{2-2} \cline{4-5}  & **Mean \(\pm\) SD** & \(\pm\)**95\% CI** & & **Mean \(\pm\) SD** & \(\pm\)**95\% CI** & \\ \hline
**Q** & 8.13 \(\pm\) 3.8 & 6.03–10.23 & 7.6 \(\pm\) 3.68 & 5.56–9.64 & 0.14 \\
**H** & 17.07 \(\pm\) 10.35 \# & 11.34–22.8 & 6.47 \(\pm\) 4.69 \({}^{\star}\) & 3.87–9.06 & 1.32 \\
**G** & 21.53 \(\pm\) 11.42 \# & 15.21–27.86 & 12.13 \(\pm\) 8.8 \({}^{\star}\) & 7.26–17.01 & 0.92 \\
**T** & 15.67 \(\pm\) 5.02 \# & 12.88–18.45 & 8.67 \(\pm\) 4.03 \({}^{\star}\) & 6.43–10.9 & 1.54 \\ \hline \hline \end{tabular}  
* Quadricepes muscles (Q), hamstring muscles (H), gluteal muscles (G), total activity (T); SD—standard deviation, CI—confidence interval; ES—effect size;* significant difference between pre- and post-intervention values \(p\) < 0.05; \# significant difference within recorded muscle activity values \(p\) < 0.05.  
\end{table} 
Table 1: Difference in muscle activity imbalance between and within pre- and post-intervention for the quadriceps (Q), hamstrings (H), gluteal muscles (G), and for the total activity of these muscles (T). cialized equipment requirements. Taking this into account, a short-term, supplementary body-weight training program such as the one used in the present study would offer an efficient way to improve lower limb muscle activity balance during running with a minimal time and equipment investment. Significant imbalances between the right and left lower limbs are caused by the differences in the strength of particular muscle groups. The dominance of one side of the body over the other causes differences in the strength of various muscles activated during specific actions required in the sport. Studies of soccer players have shown differences in strength and flexibility between the dominant and non-dominant limbs [2] and biomechanical asymmetries between each other [3]. It has been proven that muscle strength deficiencies and asymmetries in the lower limbs increase the risk of thigh muscle injuries [4]. [PERSON] et al. [32] attempted to define whether strength and flexibility asymmetries were present in the legs of soccer players. They found significant differences between the dominant and non-dominant limbs in the knee flexors and for the dynamic control ratio. In both cases, the knee flexors of the preferred leg were weaker than those of the non-preferred leg.