Knee muscle isometric strength, voluntary activation and antagonist co-contraction in the first six months after stroke

Abstract

Purpose: Muscle weakness may contribute to functional problems after stroke, but is rarely addressed during rehabilitation. Functional problems are commonly thought to be caused by abnormal movement patterns or possibly disuse atrophy. We investigated voluntary isometric strength, activation and the extent of co-contraction in the knee muscles during the first six months during stroke. Methods: Twelve stroke patients (58 3 years, mean SEM, 7 female) were studied bilaterally on admission for rehabilitation (21 1 days after stroke) and then at 1, 2, 3, and 6 months. Twenty healthy controls (61 5 years, 17 female) were tested once on their preferred leg. Subjects performed maximal voluntary contractions of the quadriceps and hamstring muscles. Simultaneous measurements were made of agonist force and surface EMG from agonist and antagonist muscles. Voluntary activation was estimated using the twitch superimposition technique. Results: Both paretic muscles showed lower (p 0.01-0.0005) voluntary strength than both non-paretic and control muscles until three months after stroke. Co-contraction of antagonists was similar in all groups and greater during knee extension than flexion. Stroke patients showed considerable bilateral voluntary activation failure (25-40%, p 0.01-0.001) throughout the study while most control subjects did not (group mean 7%). Conclusions: The muscle weakness and bilateral activation failure in the stroke patients was not explained by either excessive antagonist activity or disuse atrophy. They had potential for increased voluntary strength and if this were addressed during rehabilitation, then the rate and extent of functional recovery might be enhanced.