Postural Control for Initiation of Lateral Step and Step-up Motions in Young Adults

The purpose of this study was to clarify the postural control of the initiation of lateral step and step-up motions in healthy young adults (24.3 ± 1.8 years: mean ± SD). The tasks involved the lateral step (step lengths, 10 cm and 20 cm) and lateral step-up motions (10-cm high stool; step lengths, 10 cm and 20 cm). The variables for analysis included motion duration (weight-shift phase and swing phase), shifts of the center of pressure (CoP) and the center of gravity (CoG), displacement and inclination of the shoulder and the pelvis, and root mean square electromyographic amplitude of the erector spinae, gluteus medius (GM) and adductor longus (AL) bilaterally. The CoP shift toward the stepping side was larger in the step-up task than in the step task, and it was smaller when both tasks were performed in the long lengths. The CoG shift toward the supporting side and the displacements of the shoulder and pelvis were larger in the step-up task than in the step task. However, the magnitude of GM activity of the supporting leg was larger in the step task than in the step-up task, and increased when both tasks were performed in the long lengths. In the stepping leg, the magnitude of AL activity was larger in the step-up task than in the step task, and decreased when both tasks were performed in the long lengths. These results suggest that GM activity of the supporting leg and AL activity of the stepping leg control the shift of CoG in the frontal plane. Our results indicate that the increase in GM activity does not depend on the height of stepping but on the length of stepping in lateral step and step-up motions.