Fig. 1.

Scheme of the NMS model. d(t): external force disturbance which act to the limb; x(t): endpoint position; iss(t): supraspinal excitatory commands; F_m(t): muscle force from the contraction dynamics (CD); x_m(t): muscle stretch; : muscle stretch velocity; u(t): motoneuron activation. The muscle spindles (MS) and Golgi tendon organs (GTO) sense the muscle stretch, stretch velocity and force resulting in afferent signals on the I_a(t), I_b(t), and II(t) afferents respectively. These afferent signals have a conduction time τ_I or τ_II, depending on the afferent path, before reaching the BNN. After the conduction time τ_u(t), u(t) reaches the muscle and is converted into the muscle activation a(t) by the activation dynamics (AD). An asterisk (*) denotes the necessity to double the components for the flexor and the extensor muscles