Stretch reflexes in the soleus muscle of normal and spastic subjects: Interactive effects of starting positions and stretching velocities

Spasticity figures prominently in the diagnosis of upper motor neuron lesions and its reduction is one of the main goals of therapeutic interventions. However, its diagnosis and reduction rely on the validity of measurement and the distinction of underlying mechanisms between the stretch reflexes of normal and spastic subjects. In the present study, the mechanical parameters of a ramp-and-hold stretch were thoroughly considered and deliberately controlled based on physiological mechanisms of the stretch reflex. The reflex electromyographic (EMG) response was delineated in a comparable fashion, based on a model of the monosynaptic stretch reflex introduced in the study. Instantaneous reflex threshold angle (RTA) and velocity (RTV) were measured at a reflex threshold latency (RTL) and absolute RTA (aRTA), stretch reflex gain (SRG), and absolute RTV (aRTV) were also derived for the stretch reflex of the relaxed soleus muscle in ten normal and eleven spastic ankles. Results were summarized as follows. (1) RTA of normal phasic stretch reflex (PSR) was position dependent velocity independent. (2) In spastic SR, as the starting position was moved toward dorsiflexion, the PSR became dominant over the tonic stretch reflex (TSR). A non-significant velocity effect on the response mode of the spastic SR may be attributed to an overestimate of the PSR at lower velocities. (3) The RTA of the spastic SR was position dependent. As velocity was increased, the SR was transformed from a TSR to a PSR, and the RTA decreased within the scope of the TSR and became steady within the scope of the PSR. (4) The aRTA of the PSR was significantly affected by starting position, across which the aRTA tended to have a wider range and smaller values in the spastic group than in the normal group. (5) SRG and aRTV of PSR were not significantly different between groups. As starting positions exceeded the aRTA, the SRG was diminished steeply or was impractical to calculate and compare, but the aRTV was relatively high. In conclusion, spasticity yields a PSR threshold redistribution, which is lower than the operational limit (aRTA) but cannot be lower than the theoretical limit (aRTV) noted in normalcy.