<![CDATA[This study aimed to determine the effectiveness of floor gymnastics exercises in improving the balance and flexibility of students at SMAN 1 Demon Pagong. This study employed a quantitative approach with a pre-experimental one-group pretest-posttest design. The research sample consisted of 20 tenth-grade students selected based on predefined criteria. The research instruments comprised the stork stand test to assess static balance and the sit-and-reach test to measure lower-body and trunk flexibility. Data were analyzed using descriptive statistics, the Shapiro-Wilk normality test, the paired sample t-test, and effect size calculation (Cohen's d). The results indicated that the average static balance performance increased from 12.45 seconds to 21.78 seconds (t(19)=18.42; p<0.001; Cohen's d = 4.12), while the average body flexibility improved from 24.10 cm to 32.65 cm (t(19)=16.75; p<0.001; Cohen's d = 3.74). These findings rejected the null hypothesis (H_0) and accepted the alternative hypothesis (H_a). Consequently, structured floor gymnastics training can significantly improve students' balance and flexibility within the evaluated group. ABSTRAK Penelitian ini bertujuan untuk mengetahui efektivitas latihan senam lantai dalam meningkatkan keseimbangan dan kelenturan tubuh siswa SMAN 1 Demon Pagong. Penelitian ini menggunakan pendekatan kuantitatif dengan desain pre-experimental model one-group pretest-posttest design. Sampel penelitian berjumlah 20 siswa kelas X yang dipilih secara terarah sesuai kriteria penelitian. Instrumen yang digunakan adalah stork stand test untuk mengukur keseimbangan statis dan sit-and-reach test untuk mengukur kelenturan tubuh bagian bawah serta punggung. Analisis data dilakukan menggunakan statistik deskriptif, uji normalitas Shapiro-Wilk, uji paired sample t-test, serta perhitungan ukuran efek (effect size). Hasil penelitian menunjukkan rata-rata keseimbangan statis meningkat dari 12,45 detik menjadi 21,78 detik (t(19)=18,42; p<0,001; Cohen’s d = 4,12), sedangkan rata-rata kelenturan tubuh meningkat dari 24,10 cm menjadi 32,65 cm (t(19)=16,75; p<0,001; Cohen’s d = 3,74). Peningkatan tersebut menunjukkan bahwa hipotesis nol (H_0) ditolak dan hipotesis alternatif (H_a) diterima. Dengan demikian, penerapan latihan senam lantai terstruktur dapat meningkatkan keseimbangan dan kelenturan tubuh siswa secara signifikan pada kelompok yang diteliti. ]]>
