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

Purpose: Muscle weakness may contribute to functional problems after stroke , but is rarely addressed during rehabilitation. Functional problems are co mmonly thought to be caused by abnormal movement patterns or possibly disus e atrophy. We investigated voluntary isometric strength, activation and the extent of co-contraction in the knee muscles during the first six months d uring stroke. Methods: Twelve stroke patients (58+/-3 years, mean+/-SEM, 7 female) were s tudied bilaterally on admission for rehabilitation (21+/-1 days after strok e) 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 ma ximal voluntary contractions of the quadriceps and hamstring muscles. Simul taneous measurements were made of agonist force and surface EMG from agonis t and antagonist muscles. Voluntary activation was estimated using the twit ch superimposition technique. Results: Both paretic muscles showed lower (p = 0.01-0.0005) voluntary stre ngth than both non-paretic and control muscles until three months after str oke. Co-contraction of antagonists was similar in all groups and greater du ring knee extension than flexion. Stroke patients showed considerable bilat eral voluntary activation failure (25-40%, p = 0.01-0.001) throughout the s tudy while most control subjects did not (group mean 7%). Conclusions: The muscle weakness and bilateral activation failure in the st roke 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 func tional recovery might be enhanced.