Objective: To investigate the effects of poor sitting postures on muscle fatigue in schoolage children, so as to provide evidence for developing healthy sitting guidelines. Methods: In May 2024,30 children aged 6-12 were recruited from Changping District in Beijing. A combination of surface electromyography (sEMG) and psychophysical scales were used to assess muscle fatigue at varying deviation angles under four sitting postures (forward head tilt, lateral head tilt, uneven shoulder height, and forward trunk inclination). Oneway ANOVA and LSD post hoc multiple comparisons were employed to analyze the differences in electromyographic (EMG) data among various deviation angles under different sitting postures. Results: The mean integrated electromyography (IEMG) of representative muscles showed statistically significant differences (P<0.05) across deviation angles in all postures, with muscle fatigue worsening as deviation angles increased. Forward head tilt:significant IEMG differences were observed in the left/right sternocleidomastoid and left/right splenius capitis (F=13.74, 13.21, 5.43, 6.11,P<0.05). Lateral head tilt:significant differences were found in the right sternocleidomastoid muscle (SCM), right splenius capitis, and left trapezius (F=5.13, 4.73, 12.13, P<0.05). Uneven shoulder height:significant differences occurred in the right SCM, bilateral splenius capitis, and left trapezius (F=12.46, 12.56, 32.49, 5.98, P<0.05).Forward trunk inclination, significant differences were identified in the left/right SCM, left/right splenius capitis, and left/right lumbar muscles (F=4.45, 9.84, 14.49, 26.44, 8.02, 18.34, P<0.01). Temporal analysis revealed varying fatigue onset times across postures:severe fatigue occurred earlier in lateral head tilt and forward trunk inclination, while excessive forward head tilt, lateral head tilt, and uneven shoulder postures predominantly induced mild to moderate fatigue. Conclusions Under different sitting postures, both the integrated IEMG of relevant muscles and subjective evaluations in schoolaged children increase with greater postural deviattion. Head tilt and trunkleaning postures require particular intervention, with emphasis on unevenshoulder alignment and trunk inclination control.