足関節粘弾性の直立姿勢維持における機能分担

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  • ANALYSIS OF ANKLE JOINT VISCOELASTICITY IN HUMAN UPRIGHT POSTURE

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The factors that determine human postural stability are not only the feedback postural control system but also the stretch reflex and joint viscoelasticity. It is expected that there is a theoretical functional sharing ratio among these components. To determine the functional sharing ratio, it is necessary to measure these components separately, but, few studies have been done. Intrinsic viscoelasticity was therefore measured using a newly developed system which allows rapid mechanical perturbation to be applied to the ankle joint of a standing subject. The viscoelasticity without stretch reflex was estimated by recursive calculation from the ankle joint angle and the torque during the first 50 ms of the measured step response. The steady-state elasticity was calculated from the torque difference at two different joint positions, as a reference value. The error between the estimated elasticity and the reference was 39 percent with plantarflexion disturbance and 32 percent with dorsalflexion disturbance. The experimental results in 10 normal subjects showed 30 nm/rad in sitting, 95 nm/rad in standing and 212 nm/rad in single-leg standing. The differences were statistically significant. Closing the eyes also showed significantly increased viscoelasticity during normal (95/121 nm/rad) and single-leg standing (212/241 nm/rad). The viscoelasticity sufficient to stabilize human upright posture without a feedback control system can be calculated as about 300 nm/rad using a pendulum model. It was experimentally confirmed that the human upright posture is stabilized by both viscoelasticity and the feedback postural control system, because the measured value was less than 300nm/rad. The increase in viscoelasticity with eye closing showed a complementary increase in viscoelasticity to compensate for the decrease in the feedback postural control gain. This suggests that the functional sharing ratio between viscoelasticity and the postural control system could be controlled, probably by a higher postural control system.

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