Reflex contribution of spindle group Ia and II afferent input to leg muscle spasticity as revealed by tendon vibration in hemiparesis Antonio Nardone a , Marco Schieppati b, * a Division of Physical Therapy and Rehabilitation, Posture and Movement Laboratory, Fondazione Salvatore Maugeri (IRCCS), Scientific Institute of Veruno, Novara, Italy b Department of Experimental Medicine, Section of Human Physiology, University of Pavia, and Centro Studi Attivita ` Motorie (CSAM), Fondazione Salvatore Maugeri (IRCCS), Scientific Institute of Pavia, I-27100 Pavia, Italy Accepted 19 January 2005 Available online 1 April 2005 Abstract Objective: Foot dorsiflexion evokes a short- (SLR) and a medium-latency EMG response (MLR) in the soleus of standing subjects. SLR is mediated by spindle group Ia, while group II fibres contribute to MLR through an oligosynaptic circuit. We studied the effects of Achilles’ tendon vibration on both responses in spastic patients to disclose any abnormal excitability of these pathways. Methods: SLR and MLR were evoked in 11 hemiparetics and 11 normals. The vibration-induced changes in both responses were correlated to the Ashworth score of the affected leg. Results: There were no differences between normals and patients in the size of control SLR or MLR. Vibration decreased SLR to 70% in normal subjects, but increased it to 110% in patients, in both affected and unaffected leg. Vibration did not affect MLR in normals, but increased it to 165% on the affected and 120% on the unaffected side of patients. Ashworth score was solely correlated with the degree of vibration-induced increase of MLR. Conclusions: While the lack of inhibitory effect of vibration on SLR confirms a reduced inhibitibility of the monosynaptic reflex, the increased MLR indicates a disinhibition of group II pathway in patients, connected to the loss of descending control on group II interneurones. Spastic hypertonia depends on release of group II rather than group Ia reflex pathways. Significance: These findings give a neurophysiological support for the pharmacological treatment of spastic hypertonia and suggest a method for the assessment of its effects. q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Hemiparesis; Spasticity; Hypertonia; Upright stance; Tendon vibration; Spindle input 1. Introduction Despite all the major advances in medical science in recent years, the apparently simple problem of what causes spasticity in the legs of hemiplegic patients is still unresolved (see Mayer, 1997). The muscle reflex response to stretch and the role of Ia afferent fibres from the spindle primary terminations have been the object of many investigations in both normal subjects and patients. Some questions remain open, however, like the relationship between hyperexcit- ability of the monosynaptic stretch reflex and increased muscle tone (Ashby and Verrier, 1976; Berger et al., 1984; Burke, 1988; Lin and Sabbahi, 1999; O’Dwyer and Ada, 1996; Powers et al., 1988; Re ´my-Ne ´ris et al., 1999; Schieppati, 1991; Toft et al., 1993). It has been suggested in previous studies that spasticity could depend on exaggerated monosynaptic stretch reflexes and reduced facilitation of polysynaptic reflexes (Dietz, 1992). Depending on the disease type and stage, muscle tested and contraction condition, the medium-latency reflex response to stretch can be either unchanged, decreased or augmented in size (Berardelli et al., 1983; Ibrahim et al., 1993; Nardone et al., 2001). But how this relates to spasticity is not clear. One would expect that a disturbed control of non-monosynaptic components of the spinal Clinical Neurophysiology 116 (2005) 1370–1381 www.elsevier.com/locate/clinph 1388-2457/$30.00 q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2005.01.015 * Corresponding author. Tel.: C39 0382 592008; fax: C39 0382 592081. E-mail address: mschieppati@fsm.it (M. Schieppati).