In the absence of support (air stepping), the rhythmic activity continues, which for Sherrington was an argument for stating that “the extensor thrust cannot therefore be an indispensable factor in the reflex step” (pp. The extension during the stance phase of gait could be provided as some type of “extensor thrust,” evoked by “the weight of the animal applied through the foot against the ground” (pp. In his view, stepping was basically a series of flexion reflexes, with extension occurring merely as the “rebound” following the flexion. He proposed that the flexor reflex is a basic building block of the central nervous system and that “the flexion reflex is in reality the reflex stepping of the limb” (pp. Reading his work, it is clear that for him the flexor reflex was the mother of all modules and synergies (in the broad sense, not in the sense of the mathematical synergies defined recently). One of the first authors to point out the modular organization of the motor control system was Sherrington (1910a, b). Meanwhile, many of these physiologic features have found their way in the control of very flexible walking bipedal robots. This activation is controlled by afferent input (facilitation by a broad range of afferents, suppression by load afferent input). The latter structure forms the core of a new asymmetric model of the CPG. More specifically, it is suggested that the responses in that period relate to the activation of a flexor burst generator. It is proposed that this is linked to the activation of circuitry that is responsible for the generation of locomotor patterns (CPG, “central pattern generator”). In particular, the end of the stance phase is a period when the flexor synergy is facilitated. In addition, it will be shown that there is a great flexibility in the expression of some of these modules during gait, thereby allowing for a phase-dependent modulation of the appropriate responses.

It will be argued that there is large overlap between these notions on modules and the older concepts of reflexes. Similarly, the relation between the flexor reflex and the withdrawal reflex modules of Schouenborg and Weng (1994) will be discussed. In this paper, we question how these finding build upon the original work by Sherrington, who proposed that the flexor reflex is the basic building block of flexion during swing phase. One of the basic modules involves the flexion of the leg during swing and it was shown that this module is already present in neonates ( Dominici et al., 2011).

Recently there has been a growing interest in the modular organization of leg movements, in particular those related to locomotion.

3Department of Mechanical Engineering, KU Leuven, Heverlee, Belgium.

2Department of Research, Sint Maartenskliniek, Nijmegen, Netherlands.

1Department of Kinesiology, KU Leuven, Heverlee, Belgium.

Jacques Duysens 1,2* Friedl De Groote 3 Ilse Jonkers 1

Flexor synergy pattern lower extremity