Impact of Voluntary Muscle Activation on Stretch Reflex Excitability in Individuals with Hemiparetic Stroke

Objective: To characterize how, following a stretch-induced attenuation, volitional muscle activation impacts stretch reflex activity in individuals with stroke
Methods: A robotic device rotated the paretic elbow of sixteen individuals with chronic hemiparetic stroke from 70° to 150° elbow flexion at an angular speed of 120°/s. This stretching sequence was repeated twenty times. Subsequently, participants volitionally activated their elbow musculature or rested. Finally, the stretching sequence was repeated another twenty times. The flexor and extensor stretch reflex activity was quantified as the net torque at 135° and 88°, respectively.
Results: The stretching sequence attenuated the flexion torque (p<0.001), and resting sustained the attenuation (p=1.00). Contrastingly, voluntary muscle activation increased the flexion torque (p<0.001) to an initial pre-stretch torque magnitude (p=1.000).
Conclusions: Stretch-induced reflex attenuation may be nullified when individuals post-stroke volitionally activate their muscles. In contrast, resting may enable a sustained reflex attenuation if the individual remains relaxed.
Significance: Stretching is commonly implemented to reduce hyperactive stretch reflexes following a stroke. These findings suggest that the therapeutic effect of stretching on stretch reflexes may be reversed once an individual volitionally moves their paretic arm.