ABSTRACTThe purpose of the present study was to examine whether increased fusimotor activity prevails in spasticity and Parkinsonian rigidity. Groups of 15 spastics, 15 Parkinsonian rigid patients, and 15 normal subjects were studied by phasic mechanical and electrical stimulation of the ankle reflex. The following three parameters were determined: (1) the stimulus threshold, (2) the stimulus intensity required to elicit a maximal reflex potential, and (3) the maximal amplitude of the reflex response. The effect of the reinforcement manoeuvre on these reflex parameters was also recorded. An experimental selective nerve block was performed in eight spastics, four Parkinsonian patients, and seven normals.—In spasticity all tests showed an increased stretch reflex activity on mechanical but not on electrical stimulation as compared with normals, indicating an increased fusimotor activity of the dynamic sensitive muscle spindles in spasticity. Some, but not all, of the Parkinsonians showed an increase of the mechanically elicited stretch reflex activity.—The effect of the reinforcement procedure was in normals an increase in ankle reflex activity on mechanical but not on electrical stimulation, indicating that the mechanism of the reinforcement is an increased fusimotor drive. In spastics, the manoeuvre added little to the mechanical sensitivity of the reflex, compatible with the assumption that in these patients there already exists an increased fusimotor activity. In Parkinsonians, the reduced, although clear, effect of the reinforcement manoeuvre, compared to normals, is compatible with a moderate increase in fusimotor activity in these patients.—Selective nerve block produced, in all eight spastics, a reduction of the increased ankle reflex activity, without involving the alpha motor or Ia afferent nerve fibres. This result suggests a high fusimotor drive in these patients. A similar but less pronounced effect was obtained in some, but not all Parkinsonian patients and in normal subjects.—The presence also of an increased alpha moto‐neurone excitability in spastics emerged from an increased ratio in spastics, compared to normals, of the maximal reflex to the maximal direct muscular response to mixed nerve stimulation.—The myograms did not reveal any significant difference in muscular properties between the t