Biomechanical characterization of the stretch reflex activity as an approach to spasticity measurement and modeling-a pilot study

Spasticity is a clinical condition that may develop in people with central nervous system injuries. It is believed that spasticity results from changes in the excitability of the stretch reflex pathways manifesting clinically as a velocity dependent increase in resistance to passive movement (RTPM) and exaggerated tendon jerks. A biomechanical device was designed to provide a 90 ms torque controlled stretch to the biceps brachii. The stretch reflex response was recorded using surface electromyography and angular displacement with a potentiometer. A pilot study was carried out in four stroke and one non-impaired volunteers. Four different magnitudes of torque were applied to the elbow. Stretch reflex characterisation was done on EMG data collected 150 ms before and to complete 450 ms after the perturbation. The outcome measures were the range of movement, the amplitude of the rectified reflex response and, the latency, rise time and duration of the smoothed reflex response. The reflex in the stroke patients was not significantly different from normals (p>0.10). The mechanical model developed suggest that changes in reflex excitability may not necessarily contribute to joint stiffness (r²>0.9).