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Journal of Neurophysiology, spontaneous deletions occurred exclusively in extensors (Duysens 1977). Why this dominance of extensor deletions? There may be two basic reasons, namely first that the flexor part of the generator is much less affected by absence of afferent feedback and second that the flexor part is most directly controlled by the rhythm generator.

Consider first the issue of afferent asymmetry. In terms of sensory feedback the organization of gait is basically asymmetrical since interaction with the environment is much more intense during the stance phase. This is because during stance one has to act against gravity and the input of load receptors is crucial (for review see Dietz and Duysens 2000; Duysens et al. 2000). In fact, in the past the McCrea group has provided evidence in support of an excitatory feedback system, reinforcing on-going extensor activity during the stance phase of the step cycle …