Although group II afferents from muscle spindles are more numerous than Ia afferents, the role of secondary spindle afferents in spinal motor control was originally largely neglected and then vigorously debated. There are two reasons for this difference in the treatment of the pathways fed by the two types of spindle afferent. First, Ia effects are easier to investigate because they appear first in motoneurones after peripheral nerve stimulation and do so at lower threshold. It is therefore impossible to stimulate group II afferents selectively: (i) group I and group II fibres travel along the same nerve trunks and electrical stimulation of any muscle nerve will necessarily activate low-threshold group I afferents first; and (ii) it is not possible to activate spindle secondary endings by their specific stimulus, i.e. muscle stretch, without also activating the primary endings. Secondly, the asymmetry of group II actions with dominant flexor excitation and extensor inhibition found in investigations performed in the 1940s–1950s in anaesthetised low spinal cats led to their assignment to the ‘flexor reflex afferents’ (FRA) group. Only recently has it become possible to investigate group II pathways in human subjects. Most of the group II effects investigated so far appear superimposed on group I effects, and this has required special attention to identifying criteria. Nevertheless, these investigations have led to the view that they play a major role in the normal control of posture and gait and in pathophysiology of movement disorders.