Effect of antispastic drugs on motor reflexes and voluntary muscle contraction in incomplete spinal cord injury

Abstract

Objective: To investigate the effects of antispastic drugs baclofen and tizanidine on reflexes and volitional tasks.

Design: Double-blind, placebo-controlled, crossover, before-after trial, pilot study.

Setting: Research laboratory in a rehabilitation hospital.

Participants: Men with chronic (>6mo) motor incomplete spinal cord injury (N=10) were recruited for the study.

Interventions: Tizanidine, baclofen, and placebo were tested in this study. Agents were tested in separate experimental sessions separated by >1 week.

Main outcome measures: Reflex and strength were measured before and after the administration of a single dose of each intervention agent. Electromyographic and joint torque data were collected during assessments of plantar flexor stretch reflexes, maximum contraction during motor-assisted isokinetic movements, and maximum isometric knee extension and flexion.

Results: Reduced stretch reflex activity was observed after the administration of either tizanidine or baclofen. We observed that tizanidine had a stronger inhibitory effect on knee extensors and plantar flexors whereas baclofen had a stronger inhibitory effect on the knee flexors. The effects of these drugs on strength during isometric and isokinetic tasks varied across participants, without a consistent reduction in torque output despite decreased electromyographic activity.

Conclusions: These results suggest that antispastic drugs are effective in reducing stretch reflexes without substantially reducing volitional torque. Differential effects of tizanidine and baclofen on reflexes of flexors and extensors warrant further investigation into patient-specific management of antispastic drugs.

Keywords: Muscle spasticity; Muscle strength; Reflex, stretch; Rehabilitation; Spinal cord injuries.