Stretch reflex adaptation in elbow flexors during repeated passive movements in unilateral brain-injured patients

Objective: To evaluate the effects of repeated, externally imposed, flexion -extension movements of the elbow on the resulting stretch reflex response in hemiparetic spastic brain-injured patients. These effects were compared within a recording session and across sessions for the same subject to dete rmine the impact of movement history on the quantification of spastic hyper tonia using the stretch reflex response. Design: Twenty to 30 sequential, constant velocity flexion-extension moveme nts were applied to the impaired elbow of our cohort, with a 10-second hold interposed between flexion and extension. Movements were applied regularly at 1-minute intervals. Changes in stretch reflex responses were monitored during the applied movements. Participants: We examined a convenience sample of seven hemiparetic brain-i njured subjects between the ages of 26 and 60 yrs, with moderate-to-severe spastic hypertonia of elbow muscles (Ashworth score 2-4/4). Subjects partic ipated in 2 to 9 sessions. Measures: Elbow torque, position, velocity, and electromyograms of the bice ps, brachioradialis, and triceps muscles were recorded for each flexion and extension movement. Stretch reflex torque was calculated by subtracting-pa ssive torque from total elbow torque, recorded over large amplitude movemen ts. A linear regression analysis quantified both the initial torque respons e of the stretch reflex and the ensuing adaptation of the stretch reflex du ring sequential movements. Intersession variability was characterized both for spastic hypertonia measures and for stretch reflex adaptation. Results: Repeated, externally imposed, sequential flexion-extension movemen ts of the elbow decreased the elbow flexor stretch reflex in six of seven s ubjects. The mean reduction in reflex torque after 30 movements was 50% of the initial torque values (p = .001, t test vs 0% change). Intersession str etch reflex responses for each subject were found to vary greatly (SDs of r eflex torque ranged from 0.1 to 4.0Nm), and there were also significant var iations in the degree of adaptation between subjects. Conclusions: Stretch reflex adaptation must be taken into consideration whe n spastic hypertonia is quantified using repeated joint motion, as is often the case. The magnitude of intersession variation in spastic hypertonia me asures suggests that ideally, such measurements should be made across multi ple sessions before conclusions are made regarding the efficacy of spastic hypertonia interventions. This study provides quantitative evidence that re peated joint movements may have a significant short-term beneficial effect on spastic hypertonia.