Table 2.

Posture model parameters and definition of variables.

Model block or variable	Definition	Block parameters
Inverted pendulum body (B)	Laplace transform: B(s) = 1/(Js2 − mgh)	J: body moment of inertia about ankle joint axis (kg m2).
		m: body mass not including feet (kg).
		h: body CoM height above ankle joints (m).
		g: acceleration due to gravity (m/s2).
Neural controller (NC)	Laplace transform: NC(s) = KP + KDs	KP: gain factor converting orientation error position to active torque (Nm/rad).
		KD: gain factor converting orientation error velocity to active torque (Nms/rad).
Torque feedback (TF)	Laplace transform: TF(s) = KT/(TTs + 1)	KT: gain factor converting corrective torque to position error (rad/Nm).
		TT: time constant of lowpass filter in torque feedback path (s).
Muscle/tendon mechanics (MT)	Laplace transform: MT(s) = Kpas + Bpass	Kpas: stiffness factor generating passive torque in proportion to muscle/tendon stretch (Nm/rad).
		Bpas: damping factor generating passive torque in proportion to muscle/tendon stretch velocity (Nms/rad).
Time delay (TD)	Laplace transform: TD(s) = $e^{-s{\text{τ}}_{\text{d}}}$	τd: feedback loop time delay (s).
Vestibular system	Block representing the ability of nervous system to derive an estimate of body-in-space orientation from vestibular sensory information; assumed to be equal to unity
Prop. system	Block representing the ability of nervous system to derive an estimate of body relative to the feet orientation from proprioceptive sensory information; assumed to be equal to unity.
WV	Vestibular weighting factor indicating the proportion of orientation information derived from the vestibular system and used by the postural control system
WP	Proprioceptive weighting factor indicating the proportion of orientation information derived from the proprioceptive system and used by the postural control system
SS actuator	Support surface servomotor dynamics (lowpass filter)
SSin	Ideal PRTS stimulus command input to the support surface actuator
SS	Actual angular rotation of the support surface
BS	ML body CoM angular tilt relative to earth-vertical
BF	ML body CoM angular tilt relative to the feet (and to the support surface)
bs	Internal representation of BS derived from vestibular sensors
bf	Internal representation of BF derived from proprioceptive sensors
e	Sensory orientation error derived from vestibular, proprioceptive, and torque sensors
Ta	Active corrective torque generated from muscle activation in relation to sensory feedback
Tp	Passive corrective torque generated from muscle/tendon stretch
Tc	Sum of Ta and Tp