Effects of different heights and task conditions on the biomechanical characteristics of children's lower limbs when descending steps
Authors: Teng Jinsheng , Yang Huijun , li xu 
Published: September 2022

Abstract
Background: The ability of the human body to balance control is influenced by the cooperation of muscle strength and the nervous system. For steps of differing heights or different cognitive tasks, there are changes in neural control and sensory integration that change the balance control ability and movement control strategy of lower limbs. To explore the change rule of biomechanical characteristics of lower limbs of children with steps of differing heights and cognitive tasks, thereby revealing the characteristics and corresponding mechanisms of balance control when descending steps of differing heights and performing different cognitive tasks, thus providing much useful reference to aid the prevention and reduction of children's fall-related injuries from descending the steps.
Method: This study enrolled 20 volunteers from grades three to six who attend Southwest University Primary School (5 volunteers from each grade). Using kinematics (JVC9800 high-speed camera), dynamics (BIOFORCEN dynamic balance training evaluation system), electromyography (JE-TB0810 eight-channel electromyography tester), and other tools, the biomechanical parameters of step movements of subjects at differing step heights and performing different cognitive tasks were synchronously collected. The SPSS23.0 statistical analysis software was used to process the obtained parameters, and the significance level of all indexes was set as α = 0.05.
Result: 1. The angle of the knee and ankle joints at the first step under a single task were significantly less than those at the first step under a double task, and less than those at the third step while performing double tasks. 2. The changes of foot pressure (COP) root mean square of the left-right displacement under the first step and single-task step, root mean square of anterior-posterior displacement, total trajectory length of swing, and 95% confidence ellipse area were significantly < the COP root mean square of the left-right displacement under the third step and single-task step, the root mean square of the anterior-posterior displacement, the root mean square of swing trajectory length, 95% confidence ellipse area < the COP root mean square of the left-right displacement, the root mean square of anterior-posterior displacement, total swing trajectory length, 95% confidence ellipse area < the COP root mean square of the left-right displacement, the root mean square of anterior-posterior displacement, total swing trajectory length and 95% confidence ellipse area under the third step double-task step. 3. The mean EMG amplitude of biceps femoris, rectus femoris, tibialis anterior, and medial gastrocnemius from 200ms before touching the step to the moment the forefoot touched the step at the first step under a single task were significantly greater than those at the third step under a single task, which was also greater than those at the first step under double tasks, which was greater than those at the third step under double tasks.
Conclusion: Different step heights and cognitive tasks hindered the transmission of central nervous system information, reduced the expected posture adjustment of the body, and affected the pre-activation of lower limb muscles when descending steps, thereby affecting children's ability to balance control, and gait stability, thus increasing the risk of the child's fall.