Research shows that the involvement of majority of Lithuanian schoolchildren in sports activities is insufficient. Lack of physical activity has an adverse effect on their growth and maturity. Scientists, who have studied the issue of physical activity of children in Lithuania, envisage obviously declining trends of physical activity. Research data show that physical activity of more than 50% of boys and over 60% of girls in Lithuania are not sufficient (Skurvydas, 2017). Physical activity is

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... fined as any bodily movement produced by skeletal muscles that's requires energy expenditure. The objective of the research: to assess the impact of strengthening exercises on the functioning of quadriceps muscle for teenagers children who are involved in sports activity or not involved in sports activity. Conclusions: 1. After application of strength exercises the increase in thigh volume, quadriceps muscle thickness and strength were statistically significant (p<0.05) in both groups: sporting and non-sporting. Quadriceps muscle thickness and muscle strength were statistically significantly greater (p<0.05) in sporting group, and thigh volume increase was statistically insignificant greater (p>0.05) in non-sporting group. 2. Difference in thigh volume, quadriceps muscle thickness and strength between the dominant and non-dominant leg was statistically insignificant (p>0.05) prior to and after the application of strength exercises. 3. The closest correlation was between the quadriceps muscle thickness and thigh volume as well as between the quadriceps muscle strength and thickness. The relation between the quadriceps muscle strength and thigh volume was average or weak.