Commentaries submitted by the qualified professional readership of this journal will be considered for publication in a later issue as Continuing Commentary on these articles. Integrative overviews and syntheses are especially encouraged.

How does the nervous system control the equilibrium trajectory?

[EB] The classical monkey limb perturbation experiments of Bizzi et al. (1982; 1984) greatly influenced motor control studies. Their findings, as well as those of Asatryan and Feldman (1965) on the unloading of the human forearm, were in good agreement with Feldman's hypothesis that the nervous system controls limb movements not by programming EMG bursts or force pulses for the limb acceleration and deceleration, but by defining a new equilibrium of the “limb/external load” system. In their target article, however, Bizzi et al. reject the concrete neurophysiological mechanism proposed by Feldman for how the system's equilibrium position is controlled.

Alpha-lambda controversy. It is suggested by Bizzi et al. that the X model is a subset of the a model, because the former uses a concrete form of the relationship between joint angle, central control parameters, and the level of a activity. They also reject the necessity of a muscle invariant characteristic for an equilibrium point (EP) model and argue that “the reflex apparatus contributes in a modest way to force generation” (see, however, Gandevia & Burke, this issue).