Intrinsic viscoelasticity of ankle joint during standing

The ankle joint intrinsic viscoelasticity of standing human was measured using a newly developed system which allows to apply rapid mechanical perturbation to the ankle joint. The viscoelasticity without the contamination of the stretch reflex was identified using the response during first 50 millisecond of the measured step response. The measured elasticity values in 10 normal subjects were 30 Nm/rad in sitting, 95 Nm/rad in standing and 212 Nm/rad in single-leg standing. The functional role of the intrinsic viscoelasticity in postural stability was estimated as about 32%. The eyes-closed standing also showed significant increase of viscoelasticity during normal (121 Nm/rad) and single-leg standing (241 Nm/rad). It strongly suggests the existence of a higher postural control system which modulates the intrinsic viscoelasticity with the environment