Dynamic stability of superior vs. inferior segments during walking in young and older adults.

Active control of trunk motion is believed to enable humans to maintain stability during walking, suggesting that stability of the trunk is prioritized over other segments by the nervous system. We investigated if superior segments are more stable than inferior segments during walking and if age-related differences are more prominent in any particular body segments. Eighteen healthy older adults and 17 healthy young adults walked on a treadmill for two trials of 5 min each at their preferred speed. 3D kinematics of the trunk, pelvis, and left thigh, shank, and foot were recorded. Local divergence exponents and maximum Floquet multipliers (FM) were calculated to quantify each segment's responses to small inherent perturbations during walking. Both older and younger adults walked with similar preferred walking speeds (p=0.86). Local divergence exponents were larger in inferior segments (p<0.001), and larger in older adults (p<0.001). FM was larger in the superior segments (p<0.001), and larger in older adults (p<0.001). The age-associated difference in local divergence exponents was larger for trunk motion (interaction p=0.02). Thus, superior segments exhibited less local instability but greater orbital instability. Trunk motion was more sensitive to age-associated differences in dynamic stability during gait. Trunk motion should be considered in studying age-related deterioration of gait.