Does spasticity result from hyperactive stretch reflexes? Preliminary findings from a stretch reflex characterization study

Purpose: To characterize the stretch reflex response of the biceps brachii in stroke patients with elbow spasticity (prior to or within 15 min of treatment with botulinum toxin) and non-impaired volunteers with the aim of quantifying the stretch reflex excitability and observe the differences between the groups. Methods: A cross-sectional study. Stretch reflexes from the biceps brachii were elicited following a controlled elbow extension. The amplitude, latency, rise time and duration, calculated from surface EMG recordings from the biceps brachii, were used to characterize the stretch reflex response. Results: Seventeen non-impaired and 14 stroke patients participated. The amplitude was significantly lower in stroke patients than in non-impaired volunteers (p < 0.05). The latency was significantly shorter in stroke patients than in non-impaired volunteers (p < 0.05). There were no significant differences in rise time or duration (p > 0.10). Discussion: Reduction in the amplitude in stroke patients was unexpected suggesting the stretch reflex is not necessarily hyper-excitable in people with clinically diagnosed spasticity. Latency differences suggest decreased presynaptic inhibition and/or increased motor neurone excitability can occur following a stroke. However, carry over effects from previous botulinum toxin treatment may have confounded amplitude measurements. Further work evaluating the excitability of the stretch reflex independent of Botulinum toxin and its contribution to resistance to passive stretching is being conducted.