Keeping upright stance is important to other complex motions like locomotion and running for human beings. The mechanism of balance-keeping control in upright standing is still unknown. This study was conducted to analyze the body sway by using a simple PID (proportional, integral, derivative) control model and to investigate the influence of vision on its gains. Ten healthy subjects took part in the study. The upright body was modeled as one-link inversed pendulum model. While determining the model parameters according to subject's physical statue, the gain of PID parameters, (K_P, K_D, K_I are gains of proportion, derivative, and integral respectively.) could be estimated. Four kinds of visual patterns, (three for central visual field stimulation, one is eyes closed) were designed for visual stimulation. The results showed that the gain of K_D was decreased significantly in eyes closed (131.5±37.6 Nms/rad in eyes open and 90.4±26.0 Nms/rad in eyes closed, p<0.001), and, K_P, K_I were not changed. The results suggested that the PID control model was a promising means for individual balance ability analysis and that the visual effect on balance-keeping control in upright standing was analogized to a damper in the mechanical system.

To restore gait function in paraplegic patients, hip-knee-ankle-foot orthoses are available. Orthoses maintain patient stability when standing and walking by restricting the degree of freedom the lower extremity joints. The disadvantages of orthoses include difficulties in standing and sitting movements, and the large burden placed on the upper extremities in walking. Usage of orthoses in daily living was therefore restricted. We are developing a gait assist robot, which we named WPAL (Wearable Power-Assist Locomotor). WPAL has a high degree of freedom and internal power for flexion-extension direction in each hip, knee, ankle joint. We compared WPAL with the conventional Primewalk orthosis as a preliminary investigation. (1) We compared independent standing ability and walking distance of a walker in WPAL and Primewalk orthosis. All three subjects achieved an independent level in standing and walking in WPAL, despite these users requiring assistance when using the Primewalk. Walking distances with the WPAL were several times greater when compared to walking distance with the Primewalk. (2) We performed a single case study using the patient who made the most entries into WPAL exercise. We compared heart rate, physiological cost index (PCI), modified Borg scale, lateral sway of trunk in 6 minutes walking on treadmill. His heart rate, PCI, and modified Borg scale were significantly lower and lateral sway was significantly smaller in WPAL gait. WPAL which has degree of freedom and internal power in the lower extremities is considered a step toward practical powered orthoses use in gait reconstruction for spinal cord injuries.