Concurrent changes in shortening reaction latency and reaction time of forearm muscles in post-stroke patients

Abstract

The objective was to confirm the hypothesis that shortening reaction (ShoRe), normally occurring on forcibly shortening a muscle, is depressed or delayed in post-stroke patients. Eight post-stroke patients and ten age-matched normal subjects had a 50° wrist extension or flexion induced by a torque-motor in the affected and the non-affected upper limb. Patients were instructed either not to intervene or to assist displacement (reaction-time condition, RT). Frequency of occurrence and latency of stretch reflex (SR) and ShoRe, and RTs were measured from the electromyograms (EMG) of wrist flexor (FCR) and extensor (ECR) muscles. SR had higher than normal frequency in both muscles. ShoRe disappeared in ECR on the affected side but had normal frequency in FCR of both sides. ShoRe latency was prolonged in FCR and ECR, in both affected and unaffected sides. RTs were prolonged in both FCR and ECR, in both affected and unaffected sides. Across all patients, RTs and ShoRe latencies in the FCR were correlated. Neither RTs nor ShoRe latencies were correlated to Ashworth score. RTs were inversely correlated to Medical Research Council scores. The decreased and delayed ShoRe in post-stroke patients supports a role for the cortico-spinal pathway in its production or modulation. Monitoring of ShoRe can give insight into the recovery of the descending control of spinal reflexes.