H-reflex modulations during voluntary and automatic movements following upper motor neuron damage

Objectives: It is not known whether similar mechanisms account for the impa irments of voluntary movement and automatic postural responses of individua ls with spasticity secondary to damage to the sensorimotor cortex and its p rojections (i.e. upper motor neuron syndrome (UMN)). Methods: The present study examined changes in soleus H-reflexes preceding and during voluntary tibialis anterior (TA) muscle contraction of standing subjects and during balance platform induced postural perturbations that el icited similar TA muscle contractions. Twenty-two subjects (12 non-disabled ; 4 with spastic-type cerebral palsy; 6 with adult-onset cerebral vascular accident) participated in the study. Data were analyzed using ANOVAs and Tu key HSD post-hoc comparison tests to assess the timing and magnitude of sol eus H-reflex amplitude changes relative to the onset of TA muscle activatio n. Results: Results indicated that, regardless of the level of TA activation, soleus H-reflexes of subjects with UMN involvement did not demonstrate inhi bition either during voluntary movements or during automatic postural pertu rbations. Conclusions: These findings indicate that postural reflexes, as well as vol itional movements, are impaired following UMN damage and that deficits in n eural pathways subserving reciprocal inhibition contribute to the impairmen ts. (C) 1998 Elsevier Science Ireland Ltd. ALI rights reserved.