Starting position and stretching velocity effects on the reflex threshold angle of stretch reflex in the soleus muscle of normal and spastic subjects.

Background And Purpose: Although both starting position and stretching velocity play an important role in reflex response, their interaction with stretch reflexes has not been thoroughly investigated. This study examined the interaction effect of starting position and stretching velocity on the reflex threshold angle (RTA) of the stretch reflex in the soleus muscle of normal and spastic subjects.

Methods: The spastic group included 11 ankles from 7 subjects with a history of upper motor neuron lesions. Their ages ranged from 22 to 40 years. Another 10 healthy subjects served as the control group. RTAs of the stretch reflex in the soleus muscle were measured for a matrix of starting positions and stretching velocities. The matrix design enabled the use of a 2-way analysis model to investigate the effect of starting position and stretching velocity on the RTA.

Results: No interaction effect was found in either the phasic stretch reflex (PSR) or the tonic stretch reflex (TSR) in the spastic group, or in the PSR in the normal group. The RTA of the PSR in both spastic and normal groups was significantly affected by starting position but not by stretching velocity. In contrast, the RTA of the TSR in the spastic group was affected by both starting position and stretching velocity. Stretching velocity and starting position played independent roles in determining the RTA of both the PSR and the TSR of the spastic group and in the PSR of the normal group.

Conclusions: The lack of interaction between length-sensitive and velocity-sensitive muscle spindles in both normal and spastic subjects supports the hypothesis that they independently modulate the stretch reflex. Results for the RTA of the TSR demonstrated that spasticity results in disinhibition of motoneuron excitability with different thresholds.