Characterization of the mechanical and neural components of spastic hypertonia with modified H reflex

As the H reflex remains unable to assess mechanical changes intrinsic to a muscle, the aim of this study was to modify the H reflex techniques and to characterize the neural and mechanical components of muscle spasticity, relating the two components to clinical observations. Thirty-four patients featuring either a spinal-cord lesion (n = 15) or stroke (n = 19) and 23 neurologically normal subjects were recruited. Soleus H reflex and maximal M response (M_max) were measured with electromyography and mechanomyography (MMG). The motoneuronal excitability was represented with the adjusted ratio of the H reflex to the M_max (H/M_max) and the ratio of the paired H reflexes (H₂/H₁). Muscle mechanical properties were characterized by the amplitude and median frequency of maximal M response recorded with MMG (MMG_Mmax). The results showed that spastic patients exhibited a larger H/M_max, H₂/H₁ and amplitude of MMG_Mmax than the control group. H/M_max and amplitude of MMG_Mmax accounted for 55.7% of the variance in the Modified Ashworth Scale, the clinical hypertonia assessment. The amplitude of MMG_Mmax correlated with functional impairments, as assessed with the Barthel index and Fugl-Meyer motor-assessment scale. It was concluded that spastic hypertonia involved an atypical increase in motoneuronal excitability and muscle mechanical properties, while impairment of functional performance and daily activity was attributable primarily to altered mechanical properties of a spastic muscle.