Analysis of the relationship between muscular strength and joint stiffness in children with Down syndrome during drop landing

BACKGROUND: Children with Down syndrome (DS) have critical biomechanical impairments such as increased ligamentous laxity, muscle hypotonia, and dysfunctional motor coordination, which makes performing everyday tasks challenging. OBJECTIVE: The purpose of the study was to explore the differences in the vertical joint stiffness, plantar force, and range of motion during drop landing for DS and age-matched typically developing children. METHODS: Six young male children with DS and age-matched seven healthy typically developing children (TD) assessed joint strength using an isokinetic dynamometer and performed five trials of single-leg drop jump using force platform and motion capture system. RESULTS: The peak vertical ground reaction force (VGRF), Range of motion (ROM), joint stiffness, and joint strength of lower limb were calculated and compared across DS and TD groups. The results revealed a significantly larger peak VGRF [z=-2.857, p< 0.001] values for the DS group compared to the TD groups. The results of Spearman's correlation analysis showed a negative correlation between hip joint stiffness and knee joint ROM [r=-0.886, p< 0.05] and ankle joint stiffness and knee joint ROM [r=-0.829, p< 0.05] for DS. CONCLUSIONS: The abnormal movements observed among DS was not due to the difference in stiffness of the lower extremity but due to the utilization of different landing mechanisms with changes in ROM.