Local dynamic stability of the trunk segments and lower extremity joints during backward walking.

Backward walking has become a popular training method in physical exercise and clinical rehabilitation. For the sake of safety, it is important to keep a stable gait during backward walking. However, the gait stability during backward walking was rarely studied. This study investigated the effects of walking direction on local dynamic stability of the trunk segments (neck, torso and pelvis) and lower extremity joints (hip, knee and ankle joint). The maximum Lyapunov exponents (λ(s)) of 17 young healthy male adults were calculated while they were walking under three conditions: backward walking with preferred walking speed (BW), forward walking (FW) with the same speed determined by BW, and forward walking with normal speed (FWN). We found that compared with FW, BW showed significant higher values of λ(s) in the trunk segments in vertical (VT) direction (p<0.05). The torso segment also displayed a higher value of λ(s) in anterior-posterior (AP) direction (p<0.01); Higher values of λ(s) during BW were found in the rotation (RT) motion of hip and knee joint (p=0.036, and p=0.009, respectively), and in the abduction/adduction (AB/AD) motion of knee and ankle joint (p=0.013, and p=0.021, respectively). The significant effect of walking speed was found between FW and FWN condition in VT direction (p<0.01). These findings indicate that backward walking did impair the local dynamic stability in trunk segments and lower extremity joints. Especially, the negative effect of BW on the poor gait stability in the AP direction of torso segment, and AB/AD and RT motion of knee joint should not be neglected.